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Hello everyone!

Today's weather will be dominated by two storm systems, one in the Northeastern states and one in the Pacific Northwest. Each system will bring mostly rain, though ice is expected in interior parts of Northern New England and snow is expected in the high mountains of Washington and Oregon, both areas that will see travel disruptions due to the wintry weather. Farther south, the tail end of the Northeastern storm will bring some showers and thunderstorms to parts of the Mid Atlantic, though no widespread issues are expected there. Meanwhile, the center of the country will enjoy a day of quiet weather after a strong storm brought blizzard conditions, severe weather, and dust storms Sunday and Monday. 



Here's a simulated radar look at the Northeastern storm from weathermodels.com valid at noon. This map gives a pretty good idea of what to expect from this storm as warm air surges north east of the Appalachians. The only region where cold air will hold out against the incoming warmth will be parts of Maine and New Hampshire between the mountains and the immediate coast. Even there, a change to rain is expected this afternoon, but not before appreciable ice accumulation potentially reaching 1/4". Farther south, bands of heavy rain are expected in advance of an approaching cold front. Heavy rain will cause some flooding concerns, primarily of the urban/small stream variety though rivers like the Housatonic and Connecticut that remain impacted by ice jams could see larger scale flooding issues. 



The tail end of this system will move through parts of SE VA and Eastern NC today in the form of a cold front, with showers and thunderstorms developing ahead of it. This simulated radar image shows an area of strong but poorly organized thunderstorms in the Chesapeake Bay area around noon. Most of these storms should remain below severe limits, but a few damaging wind gusts can't be ruled out. Storms will move offshore this afternoon. 



Out west, Water Vapor satellite imagery (what's that?) shows a strong storm moving into the northernmost part of Vancouver Island this morning. Tropical moisture surging north on the south side of this system will move into parts of Washington and Oregon during the day today. This moisture will get wrung out by the Cascades as well as an approaching cold front, and the result will be a steady rain for the valleys and a steady snow for the higher elevations. The other important feature to note is the speckled pattern in the imagery west of this storm. This indicates lots of cold, Arctic energy. As that energy gets swept into the storm system, it will provide a supply of colder air that will be enough to lower snow levels a bit, down below the passes though still well above sea level. 



Here's the ECMWF's take on the results of this interaction between the colder Arctic energy and the plume of tropical moisture. Notice the indications of heavy precipitation, as well as the relatively wide expanse of snow stretching from the Cascades all the way east to Idaho. 



As far as accumulations go, this map shows how much QPF is expected to fall as snow through tomorrow morning (for more on QPF and why it's useful for snow forecasting). The jackpot area will be the Olympic mountains, which will see 2-3 feet of snow. 1-2 feet is expected in the higher elevations of the Cascades, with lighter amounts at lower elevations east of the mountain range. 



Elsewhere across the country, the ECMWF's overview map shows that a relatively quiet weather day is expected. 

For more information on your local forecast: https://weather.us/

For more information on the local forecast for ME/NH: https://forecasterjack.com/2018/01/23/freezing-rain-transitions-to-rain-today-as-cold-air-damming-slowly-erodes/

-Jack
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Hello everyone!

After a few quiet days, our weather pattern is about to ramp back up as a storm system develops and intensifies in the lee of the Rockies tomorrow night before moving east on Monday. There are two sides to every storm, and because this one is developing in the center of the country, we'll have to deal with impacts from both heavy snow and strong to severe thunderstorms.

Overview


Here's a look at water vapor satellite imagery from this morning (what's that?) showing several key elements of our storm beginning to organize. The most important feature is a strong disturbance located in Southern California. This energy is what's going to trigger storm development east of the Rockies tomorrow evening. A strong stream of Pacific moisture is noted out ahead of the disturbance, and as this moisture encounters the Rocky Mountains, rain and high elevation snow is expected today from Northern Arizona through Utah to Wyoming. Downstream of this disturbance, a large ridge of high pressure is noted across the Southeast. This is important for a couple reasons. First, ridges favor warm weather, and this will be no exception. Warm, moisture laden air is already surging north from the Gulf of Mexico, a process that will continue through the next couple days. This will serve two purposes for our sensible weather discussion. First, it will add a strong supply of moisture to our storm, even as the Pacific moisture tap dries out, supporting heavy winter precipitation for a wide swath of the Northern Plains. Second, it will open up discussions of severe thunderstorms as a dryline sets up before the storm pushes a cold front into the warm, moist airmass.

Because it's January and cold air is still the dominant force in storms like these, the winter weather aspect of the storm will be much more impactful than the severe weather, so that's what I'll discuss first. Snow will stretch from Arizona to Maine with this system, and some parts of the Plains could even see near-blizzard conditions as winds begin to pick up in bands of heavy snow.

First, though, I'd like to look at the overall setup through the lens of the ECMWF 300mb wind forecast for tomorrow evening.



I've noted some of the important features on this map that will play a part in both the storm's track, and its impacts. Two jet streaks, one located over Texas, and the other over Southern Canada, will play the biggest roles in shaping our storm. The first jet is curved cyclonically (counterclockwise) while the second is curved anticyclonically (clockwise). The left entrance region of the first jet and the right entrance region of the second jet are both known to be favorable for large scale rising motion, and when they overlap, the effect is accentuated. This will result in an area of rising motion from Western Kansas/Eastern Colorado up through Nebraska and into NW Iowa/southern Minnesota. This is the general area favored for heavy snow. I'll dig into that in more detail below. The other features noted on the map give us important clues as to the storm's track. There's a ridge downstream of the storm, so it won't move due east, but there are three elements that will work together to make sure the storm doesn't just track NNE right into Central Canada. An upper level low is poking a hole in the aforementioned downstream ridge, which will weaken its steering influence. A zonally (W-E) oriented Pacific jet is noted moving into the West Coast, which means this storm will be a relatively quick mover, and will continue to move more east than north. Finally, an upper level low in Labrador is exerting a bit of a suppressive influence on the jet stream over Eastern North America, a process that will be important to New England's forecast as it will help drive a cold wedge into the middle of the advancing warmth.

Map from weathermodels.com.

Now let's dig into each impact in a little more detail.

Southwestern Snow


Here's a simulated radar loop from weathermodels.com.  that shows bands of moderate-heavy snow and lower elevation rain moving through Arizona today and tonight as the upper level disturbance discussed above moves east through the area. Snow from this system is already impacting parts of Utah and SE Nevada, as seen on our HD radar products (click to zoom to county level for an even closer look). Snow will continue in those areas as well, with moderate accumulations in the higher elevations.



Here's a look at how much QPF the ECMWF model expects to fall as snow through tomorrow afternoon (more on QPF, this map, and how to interpret/use it). With strong dynamics aloft ahead of the upper level disturbance, and cold air surging south, snow ratios will be a little higher than normal, especially at elevation. This means that areas seeing between .2 and .4" of QPF (blue shading) will see 3-6" of snow, areas seeing between .4 and .6" of QPF (dark blue shading) will see 6-8" of snow, and areas seeing .6-1.0" of QPF (green shading) will see 8-12" of snow. Snow ratios will be lower at lower elevations, so adjust those numbers down a little if you're looking towards the valley floors where warmer temps will hang on the longest.

Great Plains Snow/Wind
After the disturbance exits the Southwest, it will emerge on to the Great Plains, where it will drive cyclogenesis (the process of storm formation) tomorrow afternoon.



The ECMWF's synoptic composite tool (what's that and how do I use/interpret it?) is a great one for analyzing the dynamics behind cyclogenesis. We need two basic ingredients for cyclogenesis: a thermal gradient to provide a supply of baroclinic potential energy, and some sort of disturbance to act as a spark, igniting that potential energy and transforming it into kinetic energy in the form of wind. The synoptic composite map above shows that we have both of those criteria fulfilled. A warm and moisture laden airmass from the Gulf of Mexico will be moving north, running into a southward moving Polar airmass. The bitter cold Arctic air we've been dealing with in the past couple weeks will be bottled up well to the north in Canada, so this storm won't be nearly as strong as some of those that have developed off the East Coast in the past few weeks where we've had an even sharper thermal gradient between tropical and Arctic airmasses. However, the tropical/polar gradient will be sufficient for a decently strong storm system to develop as the strong upper level disturbance we saw on WV satellite imagery above moves east and acts as the spark to ignite development.



As cyclogenesis occurs, we'll see a familiar set of events unfold in the mid levels of the atmosphere that will result in the development of heavy snow bands. The wind forecast for 10,000 feet tomorrow afternoon highlights this process, known as frontogenesis, that results in the creation of a front. This process is important for the development of heavy snow bands, because it results in converging airstreams in the low/mid levels. Not only does this contribute to rising air that leads to precipitation, it also strengthens the thermal gradient due to the warm airmass coming in closer proximity to the colder airmass. As each airstream intensifies due to potential energy being converted to kinetic energy (energy of motion, in this case wind), the frontogenesis dynamics and banding will also intensify.



This simulated radar map shows the result of the frontogenesis dynamics discussed above. Notice the band of heavy snow wrapping around the back side of the storm system as it moves east through the Plains early on Monday morning. The storm will be located in NE Kansas, but because the mid level frontogenesis dynamics are located to the NW of the storm system, that's also where to expect the heavy snow bands. These dynamics will be strong, and snow growth should be strong, but the airmass will only be marginally cold enough for snow so as a result snow to liquid ratios should be right around the typical 10-12:1 range. Snow in the frontogenesis band will fall at rates of 1-3" per hour, with visibilities dropping to near zero, especially once the wind picks up.

Map from weathermodels.com.



Speaking of wind, here's the ECMWF's wind gust forecast for Monday morning. Notice the widespread area of wind gusts above 35mph from NW Kansas through Central Nebraska and into NW Iowa. This is the same place we're expecting the heavy snow due to the frontogenesis dynamics discussed above. The combination of heavy snow, and blowing snow due to the strong winds will result in blizzard conditions that will reduce visibility to near zero at times. Watch for blowing and drifting snow to continue to cause problems for travel even after the snow stops falling Monday afternoon, as strong winds will linger before tapering off overnight Monday into early Tuesday morning.



Here's an idea of how much snow to expect in the Plains from this system. As the storm develops, the snow shield will contract and intensify, meaning that there will be a narrower band of heavier snow in places like SE MN and NW IA compared to NE, KS, and CO. There will be sharp cutoffs in this area both to the north where total QPF will be limited, and to the south where snow will begin as rain or mixed precipitation. These areas at the edge of the heavy snow zone are areas of higher than normal forecast uncertainty, as a track change as small as 30 miles would mean the difference between 2" and 12" of snow. There's higher confidence to the southwest where even if the snow shield shifts around a little bit, the same general ideas still apply.

Severe Weather
While areas north and west of the storm's track deal with heavy snow and blizzard conditions, areas south and east of the storm's track will deal with the threat of strong to potentially severe thunderstorms as a dryline moves east and interacts with a warm and moisture rich Gulf of Mexico airmass.



The ECMWF's thunderstorm composite shows a dryline setup in Texas tomorrow evening. A dryline is a boundary between dry and moist airmasses, and often acts as a focus for thunderstorm activity, similarly to a cold front. Remember for severe thunderstorms we need three ingredients: instability, a trigger, and shear. We have 2/3 criteria taken care of with this setup, where the severe threat is conditional upon the sufficient presence of the third. The dryline will provide the trigger and an intense jet streak located over the region (discussed above) will provide the shear. The only thing we may be missing is the instability. The ECMWF forecast shown above indicates a narrow corridor of >500 J/kg of CAPE from the Austin area up towards far SE OK and far SW AR. 500 J/kg is enough to get thunderstorms to develop, but is near the low end of what's considered supportive of severe weather. However, despite the marginal instability, there's so much wind energy aloft that even weak thunderstorms may be able to mix down wind gusts capable of low end wind damage.



Here's a short animation of what one model expects the radar to look like overnight Sunday into Monday. The strong forcing that develops as the dryline transitions to a cold front will promote the development of a line of storms that could extend as far north as South-Central Missouri. Damaging winds will be the greatest threat, though a weak tornado or two can't be ruled out. Storms will weaken as they move east on Monday, though another weaker severe threat is possible in the New Orleans area. More on that tomorrow evening/Monday morning.

Map from weathermodels.com.

Northeast Impacts


Eventually this storm will reach the Northeast, as nearly all storms that impact the US do. While the track west of the region means the area will be on the warm side of the storm, an intrusion of cold air will sneak in just before the storm arrives, and is likely to cause problems for parts of Northern New England on Tuesday. The phenomenon responsible, known as cold air damming (CAD) occurs when a spindle of high pressure settles in on the east side of a mountain range, such as the Appalachians. Warm air moving in from the southeast, perpendicular to the mountains, will rise up and over the cold air which is too dense to be pushed up the mountains. As a result, the cold air is not displaced, even as warm air is transported into the region. What we end up with is low level cold air that stubbornly remains locked into place as warm air surges in aloft, a recipe for mixed precipitation.

Map from weathermodels.com.



It's not a surprise then that the ECMWF's precipitation type forecast for Tuesday morning looks like this. While New York and Southern New England see rain in the warm sector, the cold air is locked into place across Maine, New Hampshire, and Eastern Vermont. Areas farther NE will see cold air deep enough for snow, but that will quickly transition to sleet and freezing rain as warmer air works in aloft. Depending on the exact timing of each changeover, from snow to sleet, then from sleet to ice, then from ice to rain (if that even happens at all) will determine how much snow and ice falls, but at the moment the storm looks to remain more of a nuisance event as opposed to anything capable of serious ice damage. Models are notorious for overestimating temperatures during CAD events, so if you're in VT, NH, or ME (with the exception of the ME coast NE of Brunswick) take any forecasts of plain rain with a large grain of salt!

Map from weathermodels.com.

I'll have more details on this part of the storm as we get closer!

Track the storm's evolution not only with our model data at weather.us and weathermodels.com, but also with our HD radar products and Super HD satellite imagery (more information on how to access/interpret our satellite products) from the brand new GOES-East (formerly GOES-16) satellite. You can click any map to zoom in either to state or even county level for more details! Also be sure to watch current observations to see various aspects of the storm develop. Use this data to keep track of wind gusts in the Northern Plains, moisture return across Texas, and the progress of cold air moving into New England as the storm evolves into the early part of the coming week. To see the latest data, press the refresh button at the top left of each map.

Next blog update will be either tomorrow evening or Monday morning.

-Jack
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Hello everyone!

The Arctic front we talked about on Sunday morning has evolved more or less as expected through this morning. Light snow has fallen across the Plains with higher amounts in the Midwest/Great Lakes area. The front is now bringing widespread wintry precip to the Deep South as it encounters a little bit of Gulf moisture. If you're interested in this aspect of the storm, read back to that Sunday morning post for a breakdown of the setup, and watch it unfold on HD radar maps, Super HD satellite imagery, and current observations. This post will focus on the next phase of the storm, which will bring a moderate snow event to parts of the Northeast tonight and tomorrow.



GOES East water vapor satellite imagery (what's that?) shows all the pieces falling into place for a moderate snow event in the Northeast tonight/tomorrow. A strong ridge is located over the Rockies, stretching north into Western Canada. However, a strong Pacific jet is beginning to eat into this ridge, and its eventual collapse will play an important part in our storm's evolution. If that Pacific jet wasn't breaking down the ridge, this would likely be a major winter storm as opposed to a moderate event. Meanwhile, the specific disturbance that will be responsible for the developing storm is diving SE around the base of an upper level trough oriented SW-NE from Kansas to Michigan. This orientation, also known as "positively tilted" is forced by the collapsing ridge, which is oriented in the same direction. However, while this orientation is unfavorable for a major storm, a decent moisture plume is now becoming established from the Pacific as the jet begins to break down the ridge. Additionally, offshore thunderstorms are boosting downstream heights (more on that process, how it works, and why it's important) near Bermuda, which will prevent the whole system from moving offshore too quickly.

Here's a quick look at the dynamics that will fuel a band of heavy snow tomorrow morning from NNJ up through coastal ME.



The most important dynamics at play will reside right around 5,000 feet, the level shown here. As low pressure begins to develop at this level over or just east of NYC, a process known as frontogenesis will occur to the north and northeast of the low. Frontogenesis is the process that results in the creation of a front. This process is important for the development of heavy snow bands, because it results in converging airstreams in the low/mid levels. Not only does this contribute to rising air that leads to precipitation, it also strengthens the thermal gradient due to the warm airmass coming in closer proximity to the colder airmass. As the low level jet runs into this increasingly strong gradient, it will promote an increasingly heavy band of snow. Because we're dealing with frontogenesis on the front side of a weak storm system as opposed to the back side of an intense storm, there's likely to be a single band of heavy snow that develops as a result of these dynamics. Confidence in the placement of that band is fairly high, but adjustments of a few miles in any direction will make a big difference for eventual snowfall totals.



Here's another way of visualizing the upcoming clash of airmasses and 850mb dynamics that will result in the band of heavy snow. This map is one we just added to weather.us, and shows 850mb theta-e (a measure of atmospheric heat/moisture content) as well as 850mb geopotential heights. We can see a closed gph contour over Southern NH, signaling the development of a new low pressure system. Ahead of that low, we can see the frontogenesis process wrapping up, with a well defined frontal structure beginning to emerge along the coast of Maine. As the low level jet continues to push warm, moisture laden air parcels towards this front, they will be forced to rise up the front, cooling and condensing as they go.



Here's a look at the 500mb energy map, which shows why this storm won't be a major one. Our main trough energy is strung out from Labrador to Alabama, and isn't consolidated. It's also positively tilted (NE-SW) meaning winds ahead of the trough axis are out of the WSW, which pushes any developing storm system ENE instead of NNE or N. There's also the problem of an Arctic kicker disturbance that instead of dropping into the trough and amplifying it, will act to push the whole system offshore even more quickly than if it were left to its own devices. The energy pinwheeling around the base of the trough through the Southeast/Southern Mid Atlantic will be a little too late for our lower level systems, which will develop well NE of the upper level support. As a result, none of the dynamics here will be explosive, unlike the past couple systems on the East Coast, however a long enough period of moderate snow will produce some decent totals even without 2-3"/hour snowfall rates.



Here's how much snow I'm expecting from this system. There are lots of complicated dynamics at play, but this map should work out pretty well. In areas under the frontogenesis band from NNJ through NW CT, much of MA, and into Coastal ME, over 6" of snow can be expected, with a few isolated areas of 8-10". As the dynamics of the coastal low itself take over, heavier snow bands will expand in coverage and intensity over parts of Downeast Maine. The front itself is bringing light-moderate snow to parts of the Appalachians/Ohio Valley this morning, and as that front sweeps east today, rain will change to snow in parts of Virginia with an inch or two of accumulation possible. Snow will wrap up tomorrow from SW to NE as the system moves into Canada.



While storm total snowfall will be far from record setting, this simulated radar map from weathermodels.com shows moderate to heavy snow falling at 7 AM tomorrow from NYC to Boston. This will cause big problems for the commute, as moderate/heavy snow and the morning rush don't work well together. Don't be fooled by the low totals, especially in places like NYC and Boston where a brief period of rain could hold totals down a bit. 4" may not sound like much, but if it's falling right as everyone's headed to work and school, major problems will ensue! Plan for delays and use extra caution if you do have to head out tomorrow morning.

Track this storm with the array of tools we have at weather.us, including HD radar imagery, Super HD satellite imagery, and current observations (click any map to zoom in for more detail, all the way to county level), as well as the model data both at weather.us and weathermodels.com.

-Jack
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Hello everyone!

After our large Ohio Valley/Northeast winter storm wrapped up yesterday morning, we've had a 24 hour stretch of generally quiet weather across the US. That's coming to an end this morning, as attention turns to a strong Arctic cold front dropping south through the Northern Plains. This front will move all the way to the Gulf Coast by Tuesday morning, bringing yet another threat for wintry precipitation to the Deep South.



Water vapor satellite imagery (what's that?) shows several features that will be important to the evolution of this front and its impacts. First and foremost is the disturbance itself, currently diving southeast through the Northern Plains. This will provide the energy needed to produce snow as the front travels southeast today. However, despite the strong nature of the available energy, snow totals are expected to be fairly light. This is due to the lack of moisture available to the system. There are three primary sources of moisture for an Arctic disturbance in the Northern Plains: the Pacific Ocean, the Gulf of Mexico, or the Atlantic Ocean. Each of those sources is blocked off today, which will prevent this system from bringing heavy snow and major accumulations. A large ridge stands between the disturbance and the Pacific Ocean, very dry air has moved in behind a front over the Gulf of Mexico, so even air tapped from that region won't be rich in moisture, and a strong front is holding the Atlantic's tropical moisture several hundred miles off the East Coast.



The result of the lack of moisture will be a lack of heavy snow. This map from the NAM model shows light to moderate snow along the front this afternoon, and as the front is pushed quickly southeast ahead of the swiftly moving energy, accumulations should be light.



Here's how much snow I'm expecting in the Plains through tomorrow afternoon, using the ECMWF's QPF snow forecast (what's that?). Coating-2" amounts will be most common, with some parts of KS and MO seeing those amounts fall on top of some light snow that already fell this morning. Some parts of WY, MT, and SD will see some localized Pacific moisture enhancement, as well as a boost from terrain features, resulting in slightly higher totals.



Here's a simulated radar image for tonight as the system begins to move into the Midwest. The front will continue to move south through the Plains, bringing the accumulations I discussed above. However, an area of low pressure will develop on the eastern edge of the front, over Wisconsin. As a result of this low, and of a little bit of moisture left from a system that moved through the Southern Plains this morning, snow will begin to break out ahead of the front itself across parts of Indiana and Illinois. Additionally, the cold air swirling around the Great Lakes will result in lake enhanced bands of snow, which, given their access to abundant moisture, will be heavy at times.

Map from weathermodels.com.



Here's an accumulation map for the Midwest showing the effects of the enhancement due to the low pressure formation, the pre frontal snows, and the lake effect. All in all, it will still be a fairly minor event, but even a couple inches will be enough to make the roads slick, and visibilities will drop significantly in any of those lake effect bands.



As we move towards tomorrow night, our storm system will split into two parts. The first part will be the low that formed over Wisconsin and moved east through Michigan. This system will weaken as it traverses the Appalachian mountains, and will eventually go on to form a storm that brings snow to parts of Canada. Minimal accumulations are expected east of the Appalachians due to this system. The second part of the storm will continue to drive the cold front south through the Plains. As it reaches the Gulf Coast on Tuesday morning, the cold air will encounter some Gulf of Mexico moisture which, while not rich enough for heavy precipitation, will be enough for a wider swath of light-moderate precipitation to develop.

Map from weathermodels.com.



The ECMWF's synoptic composite map (what's that?) shows each of the ingredients behind the southern snow on Tuesday morning. By looking at the white isobars (lines of equal surface pressure), we can see a surface cold front draped along the Gulf Coast from Brownsville to New Orleans. Arctic high pressure can be seen crawling south into Texas as cold air pours south east of the Rockies. Meanwhile aloft, an area of mid level moisture is seen from Central Texas through Northern LA, NW MS, and SE AR. A jet streak centered over Memphis will result in rising air from Austin NE to Shreveport, even though the forcing associated with the surface front is displaced hundreds of miles southeast.



The results of these dynamics can be seen in this simulated radar animation showing a band of light-moderate snow developing late Monday night and continuing through Tuesday morning. Cold air will be a bit shallower in parts of south-central Texas, and as a result an icy mix of snow, sleet, and freezing rain is possible. Mixed precipitation will cause issues all the way to the Mexican border SW of San Antonio. Amounts of snow, sleet, and ice will be far from impressive, however it only takes a quick coating to make roads, bridges, and sidewalks extremely slippery! Snow and mixed precip will depart Texas on Tuesday afternoon, but will continue across parts of Georgia into Wednesday as that upper level energy moves east.

Map from weathermodels.com.



After bringing snow to the south, that second part of the disturbance will continue to march east, eventually finding the East Coast by Wednesday. Any time strong upper level energy hits the East Coast in the cold season, it's important to watch for potential storm development. However, there's one important feature that will prevent any major snowstorm along the East Coast this week. That feature is located several thousand miles away, in the Gulf of Alaska. A strong storm developing in the GOA will knock the West Coast ridge on its side, and will eventually lead to its collapse. Developments like this in the atmosphere are mirrored downstream, so it's to be expected that the East Coast trough will become "positively tilted" or oriented in the same NE-SW plane as the West Coast ridge. This trough orientation favors SW winds aloft, which will push our developing storm well offshore. A weak disturbance well removed from this system will bring light-moderate snow to parts of New England, however due to the large scale pattern, a major storm is not expected.

I'll have more updates on that New England snow event in the next couple days.

You can follow each one of the features I've discussed here, from the Northern Plains to the Great Lakes to the Deep South, using our wide variety of tools at weather.us including HD radar, satellite imagery, and current observations. Click any of our maps to zoom in to state and even county level, or use our custom zoom function to pick your own domains.

-Jack

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Hello everyone!

I've been discussing a winter storm in the Ohio Valley and interior Northeast for a couple days now. Back on Tuesday, I highlighted the potential for a winter storm in this area, and talked about how the forecast was still fairly uncertain. Yesterday, I talked about how the forecast was slowly becoming more confident, and briefly highlighted some of the impacts expected as the storm develops and moves northeast. I also discussed some of the meteorology behind the overall setup, including what trends I was seeing that supported my forecast track/intensity for the storm. Today, I'll leave the overall setup behind a little bit to dig deeper into the impacts expected in specific areas.



Our Radar Composite map (click the map to zoom in to see HD data for a specific area) shows two areas of organized precipitation this evening, with scattered showers spread out over a much wider area. The two areas of organized precip are associated with our the two storm systems we've been discussing, the first that is now moving into Canada, and the second that is developing over Eastern Texas and Western Louisiana. The scattered precip between the storms is a reflection of the tropical moisture streaming north from the Gulf of Mexico. There's not much rising motion in this area, but when you have so much moisture, you don't need much lift to generate scattered precip. The final feature of note on radar is the Arctic front that connects the two storms. As this front plows east, it will clash with the tropical airmass currently streaming into place on either side of the Appalachians. The result will be heavy snow, sleet, freezing rain, and rain, despite the core of the storm being fairly weak.



I want to take a closer look at this front for a minute, because its structure will play a key part in the weather expected across the Ohio Valley and Northeast over the next few days. Fronts are not two-dimensional lines on a map, but instead can be thought of more accurately as three-dimensional circulations. The maps displayed here are the wind speed and direction forecasts from the ECMWF model at different levels of the atmosphere tomorrow afternoon On each map, I've marked the location of the front at that level. The map farthest to the left is the surface map, and in addition to the location of the surface front (blue), I've also marked the location of the front at the other levels. This is to help illustrate that this frontal circulation slopes westward with height, at varying slopes. The bottom part of the front has a very steep slope, and there's hardly any horizontal location change in the first 2,500 feet of the atmosphere. However, as you move towards 5,000 and 10,000 feet, the slope of the front becomes much shallower. What does that mean for the sensible weather underneath the frontal circulation?



Here's the NAM model's simulated radar map for the same time tomorrow afternoon (1PM EST/12PM CST) as the maps above. Notice the distribution of precipitation types near the front. For a wide swath of the Ohio Valley, surface temperatures will be cold enough for frozen precipitation, but there will be a warm layer somewhere in the mid levels to prevent snow, at least early on. Once we have this setup established, precipitation type depends primarily on the depth of the near-surface cold layer. The deeper that layer, the more likely raindrops falling out of the warm layer will refreeze into ice pellets (sleet). The shallower that layer, the more likely that liquid raindrops will make it to the surface, where they will freeze on contact with cold surfaces (freezing rain). The steep frontal slope in the bottom 2,500 feet of the atmosphere suggests a fairly deep near-surface cold layer. This would support more freezing rain than sleet in the mixed precipitation zone. Eventually, as you head far enough west, the front reaches so high into the atmosphere that even those on the "warm" side have temps below freezing, allowing for snow.

Map from weathermodels.com.



Here's an animation of the ECMWF's precipitation type forecast showing that mixed precipitation zone shifting east during the day tomorrow as cold air pours in behind the Arctic front. Ahead of that mix zone, heavy rain will cause some flooding problems, especially in parts of Southern New England. Behind the mix zone, accumulating snow is expected, with some parts of far western NY possibly seeing up to a foot of snow, not including any lake effect behind the system. In the areas impacted by the mix zone (including much of the Ohio Valley and interior Northeast), expect ice accretion of between a tenth and a third of an inch, depending on the exact structure of the front as it crosses your area. This is more than enough to cause extremely slick travel, but should fall just short of what's needed for widespread power disruption, though scattered issues of that nature are possible. In addition to the ice, also expect sleet, which could accumulate to a very dense inch or so, on top of the ice, and underneath the final coating of snow that falls after all the warm air has been pushed southeast.

Map from weathermodels.com.



Speaking of snow, here's an overview of how much to expect. The heaviest snow will fall in Western New York and Pennsylvania, with amounts over 6" also possible in the far northern parts of New England. Much of the rest of the southern/central Ohio Valley will see moderate accumulations of 3-6" while coatings could fall as far SW as Memphis. There will be a sharp cutoff in terms of snow amounts just NW of the I-95 corridor as the cold air will be racing the back edge of the precipitation to the coast. For more detailed snow forecasts for your area, check out the ECMWF map I used above, which is how much QPF the model forecasts to fall as snow (click for more information on QPF and using it to forecast snow totals) and click the map to zoom in. You can also explore our Forecast Ensemble product which will help give you a sense of the uncertainty in the snow forecast for your town.



While mixed precipitation causes problems along the front, and snow has a similar effect behind the front, tropical moisture surging north will bring heavy rain and flooding concerns into the picture for parts of Southern/Coastal New England tomorrow. A tropical low that has been gathering moisture over the Gulf of Mexico for the past few days will get swept up into the developing storm, decaying along the way. The remnants of this system are visible in ECMWF precipitable water forecasts for tomorrow evening. Precipitable water is a measure of how much moisture is stored up in any given part of the atmosphere. PWAT values above 1.5" indicate an extremely moist airmass for this time of year, and that's what's forecast to move through New England.

Map from weathermodels.com.



Given the strong front in the area, and the deep supply of tropical moisture, it's no surprise that the Swiss Super HD model is forecasting some very heavy rain tomorrow afternoon. Downpours will impact areas from New Jersey to Maine, and in areas with snowpack still left over from the blizzard, flooding will be a concern due to the combination of heavy rain and snowmelt, as well as the added effects of snow clogged storm drains. Total rainfall of 1-3" is expected before the front arrives and changes precipitation to freezing rain and/or sleet on Saturday.



Just because most of the precipitation in Southern New England will fall as rain, that doesn't mean they're out of the woods in terms of icy conditions. Swiss Super HD model forecasts for Saturday morning show an impressive temperature gradient across the area as the front moves through. Temps will drop around 30 degrees in just a couple hours as the cold air rushes in. This sharp drop from the mid 50's to the mid 20's will result in all the puddles that formed as a result of the rain freezing up in just a couple hours. These "flash freeze" events are notorious for producing very slick conditions, even after precipitation has ended. Add the potential for some snowflakes as the storm departs and you have a recipe for very slick roads all across New England on Saturday morning.

Model guidance has precipitation moving out of New England Saturday night, however there are hints that perhaps we could be in for a little bit of a "surprise" snow where the departing precipitation shield slows down a bit more than expected.



Here's the ECMWF's jet stream forecast for Sunday morning. Notice the very strong jet streak (area of especially fast winds embedded within the larger jet stream) stretching from Eastern NC through the Canadian Maritimes. This jet streak bends to the right, meaning it's anticyclonically curved (winds bend clockwise, or anticyclonically, as they traverse the jet). The right entrance region of anticyclonically curved jets are known to favor rising motion. With the Arctic front still lingering not too far offshore, we could have just enough moisture to get squeezed out by that lifting and produce snow. The setup isn't right for a major storm, and accumulations would total a couple inches at best, but a couple inches of snow on top of flash-frozen roads would make an already slippery situation much worse.

Map from weathermodels.com.

Quiet weather will return (albeit probably briefly) Sunday afternoon into Monday.

Our next storm to watch will likely take shape in some form or another during the middle part of next week.

-Jack
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Hello everyone!

In last night’s update, I discussed two winter storms forecast to bring snow and ice to parts of the US this week and into this weekend. The first system was covered in more detail, because the forecast was more certain. Read the first part of last night’s update for info on that storm, and watch it evolve on our Satellite and Radar products at weather.us. This update will focus on the second storm, which wasn’t discussed with as much detail last night due to the higher uncertainty. While the forecast isn’t a sure thing even now, model guidance has come into clearer agreement in the last 24 hours regarding what to expect with this system, so I’ll dig a little deeper into it here.



Here’s the ECMWF’s mid level energy map for Friday morning showing all the key players for our storm. The system itself will be located over the Lower Mississippi Valley, and the disturbance noted in that region will drive the development of the storm over Northern AL/MS. To the east of the storm, tropical energy will be lifting north between the storm and a highly amplified ridge of high pressure to the storm’s east over Bermuda. This energy will carry with it a lot of tropical moisture, helping the storm produce lots of precipitation despite being fairly weak overall. Meanwhile to the storm’s west, a large amount of energy will be diving SE through the Rockies while a flattened area of high pressure is located along the West Coast.

Map from weathermodels.com.



The two ridges of high pressure can tell us a lot about the behavior of our storm. The amplified downstream ridge will keep the storm from going too far east, and taking a track offshore. This will not be a coastal storm with snow for the I-95. That being said, the flattened upstream ridge will deny the storm too much amplitude, thus preventing a track farther to the west. If the ECMWF’s idea from yesterday ended up panning out, the mid level energy would be able to dig enough to form a cutoff low, which would pull the whole system back to the northwest by about 150 miles. This does not appear to be the case, which means we can hone in on a track from N MS/AL through the Central Appalachians (WV) and towards the Southern New England coast.

Map from weathermodels.com.

So what does that track mean for impacts? I’ll go through a general impacts overview now, and will be back tomorrow evening with a much more detailed look at the expected impacts as well as some of the science behind the forecast.



Here’s the ECMWF significant weather forecast for Friday morning. Notice the area of heavy rain over parts of Virginia, Pennsylvania, and New York. This is associated with that tropical disturbance I mentioned above. Watch for some localized street/small stream flooding due to this area of rain, especially as it heads into New England where storm drains are likely to be clogged with snow. Farther west, freezing rain will be in progress from Arkansas to Indiana as cold air will be quicker to rush in at the surface than aloft. Ice accretion will likely end up being the main story associated with this storm, as significant ice accumulations of a quarter inch or more are possible from the Memphis area all the way up through the Ohio Valley and Northeast to New England. The farthest NW fringe of the precipitation shield will change to snow, with bursts of moderate to heavy snow possible on the back side of the developing storm.



By Friday night, the storm will be moving across the Appalachians. Rain and freezing rain will change to snow across the Ohio Valley as the storm moves out. Bursts of moderate to heavy snow are once again expected before the storm drags the moisture away from this area. Rain and fog will impact the I-95 corridor from Virginia up through New England as warm moist air rushes north over a deep snowpack. Meanwhile, freezing rain will continue near the border between rain and snow, with potentially significant ice accumulations in parts of Central PA and NY.



By Saturday morning, precipitation will be ending for everyone but New England as the storm moves ENE near Long Island. Far interior parts of New York and New England will see a complete changeover to snow, with most of the rest of the area away from RI and SE MA see freezing rain and/or sleet. The system will move out Saturday afternoon, though light to moderate snow could linger into Sunday for Eastern New England due to a sneaky little setup I’ll discuss more as we get closer (I’ve already mentioned it a couple times on Twitter if you follow @WeatherdotUS and @JackSillin).

I’ll have a more detailed look at each impact (rain, snow, and ice) as well as some of the finer points of the science behind the forecast tomorrow afternoon/evening.

-Jack
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Hello everyone!

While recent winter storms have brought snow and ice to parts of the East Coast, this week that pattern will be shifting. We’ve already seen the bone chilling cold retreat from the East Coast at the beginning of this week, now the end of the week will feature changes in the pattern of wintry precipitation. Instead of snow and ice being focused on the East Coast, we’ll turn our attention to the middle of the country, where snow is expected to fall from Montana east to Maine, and from Minnesota south to Mississippi.



Here’s the ECMWF Ensemble Prediction System (EPS)’s forecast for how much snow is expected in the next 7 days. I wouldn’t focus all that much on amounts, but I like how this map shows who’s expecting snow from each of the two storms we’re watching through this weekend. The first storm will be the farther west of the two. It will bring snow from the mountains of Arizona and New Mexico through the Northern Plains into the Lake Superior area. Snow from this system will generally be light to moderate, with most amounts staying under 6″ not including terrain enhanced totals in the Four Corners region. The second storm will form a little later in the week, and will track farther east than the first one. The Ohio Valley will be the epicenter for this storm, with some areas likely to receive over 6″ by the time things wrap up early in the weekend. Impacts won’t be limited to the Ohio Valley, however, snow and ice will cause problems in parts of the Mississippi Valley, as well as in parts of the interior Northeast.

Map from weathermodels.com.



Water Vapor satellite imagery (what’s that?) highlights all the components of storm number one. A strong upper level low moved onshore today in Southern California, bringing tremendous flash flooding and mudslides to parts of the LA region. I discussed this storm yesterday, and unfortunately the concerns I outlined there did indeed pan out. This system will move east tonight, emerging out onto the Plains tomorrow. Due to the strength of this system, as well as its Pacific moisture feed, moderate snow bands can be expected from Colorado to Minnesota tomorrow and Thursday, with some of the stronger bands producing locally heavy snow. However, the upper level low will be getting sheared apart as it moves NE Energy now moving onshore in Washington State will move east tonight as well, emerging out onto the Plains at a similar time as our upper level low, just a bit farther north. This system will be weaker, so expect only light/moderate snow to be associated with it.



Here’s an animation of simulated radar from the NAM model showing snow impacting parts of the Plains tomorrow and Thursday as the two elements of the first storm move through. Notice the lighter snow forecast in the Dakotas with locally heavier bands in parts of Colorado, Kansas, and Nebraska. To the northeast, across parts of Iowa, Wisconsin, and SE Minnesota, warm air may hold on longer aloft than at the surface, resulting in a brief period of sleet and/or ice. While ice accretion amounts will generally be light, it doesn’t take anything more than a quick glaze to result in extremely slippery roads.

Maps from weathermodels.com.



Here’s a look at how much snow to expect from the first system, through Thursday evening. Totals in the Rockies will depend highly on elevation, so I haven’t drawn in each contour there. However, some of the peaks in Montana could see over 2 feet by Thursday evening, with more on the way through the weekend. The heaviest non-elevation snow will be found from parts of Eastern Nebraska through SE Minnesota, where 4-6″ is expected in a narrow band of moderate-heavy snow right on the NW side of the remnant ULL. For more info on this type of map and how to use it, check out this video.

Snow will move out Thursday evening, with a lull developing Thursday night before our next storm on Friday.



Here’s a look at the ingredients for storm number two beginning to line up Thursday night as shown by the ECMWF’s synoptic composite product (what’s that). A deep upper level trough will be moving east through Texas, and will begin to close off into an upper level low by Friday morning as it moves into Louisiana and Mississippi. Tropical moisture will be streaming north ahead of this system, as a strong ridge builds downstream over Bermuda. This ridge will act as a wall, stopping the storm from moving east and developing offshore. Instead, the storm will turn north much sooner than we’ve seen in the past month. As a result, most of the I-95 corridor south of Boston can expect a plain rain event from this system. The heavy snow focus will shift west into the Ohio Valley, where over 6″ of snow is possible. The bitter cold Arctic air that has helped to fuel explosive storm development off the East Coast recently is bottled up in Canada, meaning this storm won’t set any records in terms of its strength, but the clash of deep tropical moisture with polar air just cold enough for snow will bring an impactful snow/mix event regardless of the relative weakness of the storm system itself.



While the forecast for storm 1 is fairly confident, give or take a few miles, the forecast for storm 2 remains much more up in the air. I’ve taken the GFS and ECMWF precipitation type/amount forecasts for Saturday morning and placed them side by side above, marking where each model is forecasting heavy snow, and the rain or mix/snow line. Notice how the two models are over 100 miles apart with each feature. For parts of Indiana, the ECMWF forecasts near blizzard conditions, while the GFS indicates nothing but flurries. This discrepancy highlights the uncertainty in the forecast, something we have plenty of tools at weather.us to make sense of.

Maps from weathermodels.com.



Here’s our Forecast Ensemble product for the snow depth in Indianapolis, Indiana through the end of the storm. Notice the very wide spread among ensemble members, from 0-11″. The deterministic model we compared to the GFS above is at the very top of the ensemble guidance envelope. Not a single one of the 51 ensemble members forecasts more than the operational model. This is a sign that perhaps that solution is a high outlier, and that actual amounts may be much lower. A range of reasonable best and worst case scenarios is given by the 10th and 90th percentiles respectively. There’s roughly a 90% chance of getting more than .2″ (a coating) in Indy, while there’s only about a 10% chance of getting more than 7.3″ (a solid storm). The ensemble mean, while far from perfect, can give a good ballpark estimate at this range: ~3″. You can get this data for any town in the world at weather.us, just select our Forecast Ensemble product and type in the location you’re looking for.



As the storm moves ENE, it will run into a low level cold airmass brought south by the first storm. The result will be an expansive area of icy mixed precipitation from New England through New York and into parts of Pennsylvania. I’ve picked an old ECMWF run to highlight this, because I believe recent runs are too warm and underestimate the power of low level cold air in these areas. It should be noted that there are several areas of especially high uncertainty with this forecast, where very small changes in the storm’s track will make huge differences to the sensible weather forecast. These include the western fringe (who sees moderate snow and who sees nothing), the mix/snow line (who sees 6″ of snow and who sees 1″ of sleet), and the mix/rain line (who sees significant icing, and who sees a cold rain). While everyone in the path of the storm should keep up to date with the latest information, staying informed and being ready for multiple potential outcomes is especially important in these areas.

Map from weathermodels.com.

The storm will depart into Canada on Sunday leaving cool and dry weather in its wake.

Much more on this system as it gets closer!

-Jack
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Hello everyone!

Rain is already beginning to fall across California this morning as a storm system approaches from the West. This system will impact the area through tomorrow, with the main threat being mudslides as a result of torrential rains falling on the burn scars from earlier wildfires. The Thomas Fire burn scar is of especially high concern as 6 to as much as 10″ of rain is possible there.



Water Vapor satellite imagery (what’s that?) shows a setup favorable for heavy precipitation today. Deep plumes of Pacific moisture are noted both directly in front of a strong upper level trough, and a little farther to the east. A disturbance diving SE through the core of that upper level trough will help it “cut off” into a low pressure system of its own, which will slow its forward progress as it approaches the coastline.



Here’s an animation from the NAM model (maps/GIF from weathermodels.com) that shows rain and mountain snow associated with this system evolving from this morning through tomorrow night. Notice the heaviest precipitation falls along the coastal mountain range that runs from just north of LA to just north of Santa Barbara. This is right where the Thomas Fire burned with such ferocity just a few weeks ago.



Here’s the Swiss Super HD model’s forecast for total precipitation through the end of the event on Wednesday. Notice the mountains north of Santa Barbara where the Thomas Fire burned are expecting over 6″ of rain, falling in a relatively short period of time. Prolonged downpours and steep slopes that lack vegetation are a dangerous combination, and mudslides are becoming a near certainty in these areas. Elsewhere, watch for urban/small stream flooding in places like Los Angeles and Santa Barbara themselves where 2-4″ of rain are expected.

Rain will move out early Wednesday morning.

-Jack
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Hello everyone!

Arctic air that has been in full control of the Eastern third of the country since Christmas is finally retreating this week. While this will lift the spirits of many who are tired of the bitter cold, the transition between weather patterns is often a timeframe ripe for storms, and this will be no exception.



Water Vapor satellite imagery (what's that?) shows several important changes to our pattern. Off the West Coast, a series of Pacific storms are breaking down the persistent ridge of high pressure that has been sitting over the Western part of the country for the past few weeks. This will result in an eastward expansion of warm Pacific air that will be chasing out the Arctic cold in the East. This process will occur faster aloft than it will at the surface, and freezing rain is expected as a result due to warm air aloft melting snowflakes into raindrops, which then promptly freeze onto cold surfaces at ground level.



Here's a map showing surface temps and 2500ft temps this evening. I've marked the location of the surface freezing line and the 2500ft freezing line, according to the ECMWF model (which I suspect may be a little too quick to warm surface temps in Southern Indiana). I've also marked the area where the surface freezing line is farther south than the 2500ft freezing line. It's in this area that we'd look for freezing rain due to the shallow cold layer near the surface. Temps will be warming through the rest of this afternoon and through tonight, so areas that are above freezing in the image above valid at 7 PM may see ice earlier in the day, and areas below freezing may see temps rise above 32 later in the night.



Here's an animation from weathermodels.com that shows freezing rain spreading east from where it's ongoing now (in Missouri) through the Ohio Valley tonight and eventually down into parts of the Deep South by tomorrow morning. For most areas, ice accretion will be light enough to avoid issues beyond very slick travel, though some parts of Southern Indiana, Northern Kentucky, and Southern Ohio may see just enough ice (.25-.35") to start worrying about scattered power outage issues. Areas farther north will see all snow due to the deeper cold airmass, while parts of the Gulf Coast will see all rain with the chance for some thunderstorms.

The whole system will clear out to the east tomorrow afternoon leaving a day of dry and cool weather on Wednesday before our next storm develops in the Plains towards the end of the week. More on that system later.

Jack
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Hello everyone!

Today will feature brutal cold up and down the East Coast as gusty NW winds on the back side of yesterday's blizzard carry air straight from the Arctic southeast. Wind chills will become dangerous in the Northeast this afternoon, and below zero wind chill values will expand as far south as the Carolinas by tomorrow morning. Meanwhile, tranquil weather will be enjoyed by folks in the middle of the country as well as across the intermountain west due to a large ridge of high pressure. On the West Coast, expect rain to develop today for all but the highest peaks of western Washington, Oregon, and northern California, where snow will be expected.



Here's the ECMWF's overview map for this afternoon highlighting who's expected to see active weather today. The one thing that doesn't show up on this map is the cold, which as I mentioned above will be an issue for much of the East. If you're in the East, bundle up, if you're in the center of the country, enjoy a lovely winter day, and if you're on the West Coast, pack the umbrella. After a couple days of extreme weather, today will be pretty mellow.

For more information about your local forecast: https://weather.us/

For more information about the local forecast for ME and NH: https://forecasterjack.com/2018/01/05/falling-temps-today-as-arctic-air-pours-in-behind-our-departed-blizzard/

-Jack
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