Monday/Tuesday Winter Weather Threat: The Latest

Below we post the latest blog that has been sent to Premium subscribers.  As this is the first winter weather threat of the season, we wished to share this latest post publicly as well to demonstrate what kind of content we will be publishing to subscribers around winter storm events.  Similarly, subscribers receive detailed emails every day or every other day explaining weather forecasts through the next 2 weeks, including when winter weather could threaten, what temperatures are expected to be, and what days feature any significant chance of precipitation. These emails are easily summarized and bolded to allow quick takeaways of the important weather forecast changes, allowing subscribers to easily plan their day-to-day lives and potentially make important decisions regarding school closings, travel, etc.  An example of the latest email can be seen here.  During major winter events, subscribers will receive daily or even more consistent blog updates on the latest expected conditions and impacts.  This blog below is relatively standard for a storm still 5+ days out.  To subscribe to our Premium service, please click here.  Any questions about bulk discounts or tailored products for school districts, towns, etc. can be sent to jacob AT swctweather.com.  Please note that free updates will also continue as usual through the winter season detailing our forecasts for storm timing, precipitation type, and snowfall accumulation/ice accretion, but that school delay/closing forecasts and detailed impact analysis will be for Premium subscribers, along with more advanced storm analysis.

First, the threat: anytime from Monday evening into Tuesday afternoon evening will remain at risk for wintry weather disrupting travel.  Inland portions of SWCT/NY will be more at risk than coastal areas, as surface temperatures there could stay colder for longer.  For a storm 5-6 days out confidence is around average, as weather model agreement is surprisingly strong on at least some winter weather impacts across the entire region, though details about exact storm structure and timing remain up in the air.

Here I show the GFS operational weather model output for early Monday morning.  From here I can break down what we are looking at.  We see on the top left at 500mb that an upper level low is sitting right along the Texas/Oklahoma border, which is the disturbance to note.  To the right of that we see clearly the low pressure center stacked almost perfectly below that upper level low (though off slightly to the east as is common for a storm cutting up to the north).  On the bottom right we see that temperatures 5,000 in the atmosphere are slightly above freezing across SWCT/NY as some showers are lingering from a previous cold front that is moving through Sunday night.  Already, we see that the low pressure center is elongated off to the east as high pressure up to the north of the storm is blocking it from cutting up to the north like previous storms have.  That high pressure center there will help lock in some of the fresh cold air that moves in on Monday, and will also shunt the low pressure center off to the east, potentially helping a secondary low pressure center form offshore which could help keep some colder air aloft as well as at the surface.  There are a large number of moving parts here, including that high pressure to the north, that low pressure center across the South, ridging extent over the Southeast that will guide the primary low, and the cold front-turned-stationary front that will allow the entry of colder weather across New England.  If these align correctly, we could be dealing with heavy snow, sleet, or freezing rain.  Should one of these deviate from the current forecast, the entire forecast could fall apart, cold air damming (the trapping of cold air at the surface as it gets “dammed” under advancing warm air) does not occur, and almost the entire region sees rain with minimal impacts.

Now let’s get into what the latest weather model output shows.  Let’s begin with the GFS, where we have the most detailed graphics.  New 6-panel charts will help me show exactly what is going on at various layers of the atmosphere.  This image is for Monday night at 10 PM.  As seen on the top middle graphic, precipitation is just starting to move into the SWCT/NY region.  The solid black line in that image represents the freezing line at 850mb in the atmosphere (near a key snow growth level), and it sits to our south indicating that we could actually start as light snow.  On the top left we see the pink surface freezing line slightly away from the coast, with temperatures across the region generally hovering around freezing.  Fast forward 3 hours to 1 AM here and things are beginning to change.  In the top center image we see light to moderate precipitation moving in as that freezing line aloft is moving closer to the region.  Meanwhile, on the top left the freezing every is now right down to the coast as inland areas drop down into the upper 20s.  The atmosphere at this point will be warming at different rates at different levels, likely resulting in widespread sleet, especially at coastal areas, as snow would likely be short-lived.  Further inland we could still be seeing some snow as the colder air hangs out.  Fast forward another 3 hours to 4 AM here and we see that surface temperatures are still below freezing across all of SWCT into New York and the 850mb freezing line is approaching the region rapidly.  Sleet would be widespread, and at the coast we would be turning to heavy freezing rain as moderate to locally heavy precipitation overspreads the area.  Roads would quickly become ice-covered and travel would be treacherous.  3 more hours takes us to 7 AM here, where we see that temperatures aloft are now above freezing across almost the entire state of Connecticut, but only the immediate coast has risen above freezing.  Anywhere north of Interstate 95 is still seeing steady, driving freezing rain that is coating the majority of surfaces in ice and creating very dangerous travel.  Though the remainder of the morning we would gradually warm up, and by the afternoon travel would be safer, but early Tuesday morning the GFS is showing a travel nightmare.

I must emphasize that this is just one of a wide range of weather model scenarios, and in fact at this time this is pretty much the worst-case scenario.  This is one of the few models that starts the storm out as snow for the region, and it also has some of the most sustained freezing rain, especially for those close to the coast.  Still, it is worth taking serious as other guidance has begun to trend more towards this scenario.  Canadian CMC guidance for 1 AM Tuesday now shows more of a potential for snow, as seen on the bottom right here.  That 850mb freezing line is across central New Jersey as the first precipitation begins to move in.  By 7 AM we are seeing that freezing line move north of the region aloft, but surface temperatures would still be near freezing, resulting in ice threats away from the coast through the morning, as seen here.  Yet, by 1 PM, rain would be ending and coastal areas would be approaching 50 degrees as a much warmer flow would take over, seen here.  This is some of the warmest output, and it shows how stubborn cold air will be in the morning but how once the cold air damming finally gives way, we could rapidly heat up and melt the ice late Tuesday morning into early Tuesday afternoon.

ECMWF operational guidance is the slowest.  It does not have precipitation start until Tuesday morning, and again it would likely start as light snow, not accumulating more than inch (and in some coastal areas potentially not even sticking as it stays light and surface temperatures hover near freezing).  Then, ECMWF guidance shows very gradual warming along the coast as heavier precipitation moves in through Tuesday morning, with areas south of I-95 likely seeing almost all plain rain, those between I-95 and Route 15 seeing a mix of sleet, freezing rain, and rain, and those north of Route 15 most at risk for significant ice accretion through Tuesday morning before the region warms up slightly more in the afternoon.  ECMWF guidance is by far the slowest with the storm, though it does have a bias to over-amplify the flow across the Southwest and not handle cutoff lows as well in that region, which leads us to believe it may have overly biased output despite being historically one of the best models.  In coming runs we can expect the ECMWF to gradually speed up its output.

The first thing to again emphasize is that these are just three operational weather model solutions for a very complex winter weather storm system that sits 5-6 days out.  Much remains on the table, though as you can see they are actually in decent agreement at this point.  There remains the potential for more warm air influence or a slower cold front progression ahead of the storm (or even a slightly western track of the low pressure center) to ensure that we see all or almost all rain from the storm.  Similarly, this could ensure that by 4-6 AM the entire region is turning over to rain, so even with some light snow/freezing rain overnight travel impacts on Tuesday are minimal.  However, we could also see a GFS or slightly colder scenario verify with more sustained icing, especially away from the coast, which could bring significant impacts, including treacherous roads, scattered power outages, and numerous business/town closures.  As is the case with any storm where mixed precipitation is expected, a huge amount of uncertainty remains, and future updates will focus on identifying exactly what we do know and what is still entirely unknown, using that to figure out what impacts SWCT/NY should begin to prepare for.

I want to end with one final note.  We can see the GFS ensemble 500mb output for Monday evening at 7 PM here.  Here we see the main pattern drivers; the storm over Oklahoma, the ridge across the Southeast, a large upper level low south of Greenland, a ridge beginning to build into western Alaska, and a weak trough diving down into California.  Part of the uncertainty stemming from this storm is the dual-trough we see, with one over California and the other being the upper level low in the center of the country.  South that trough over California trend stronger, it will help hook the storm in the center of the country further into the block overtop (the higher 500mb heights), or at least allow a quicker warm air flow across the East Coast.  There is a very real potential of that happening, which is why I am favoring more of a scenario where coastal areas do not see much sustained icing and instead see more plain rain for this system.  More complicated is what inland regions will see, as there will be sufficient cold behind the cold front swinging through, as seen by how tightly packed together the 500mb pressure bars are just offshore of New England and how deep the upper level low is over Greenland.  Models are very impressive with how much lower-level cold air they have across the region, which is particularly why I am so concerned about the ice threat here.  I do not expect that upper levels will remain cold enough for much snow, and maybe not even enough for all that much sleet as we continue to have a strong southerly flow that could aid warming in the 700-800mb layer.  It is at the surface that I see a very typical cold air damming signal from that high pressure centered over Quebec, and if that were to verify at least inland areas could see an extended period of freezing rain (and further north across northern Massachusetts, Vermont, and New Hampshire would get the snow).

Either way, there remains a significant amount to watch with this upcoming storm, as a large number of scenarios remain on the table, and despite model agreement now that could all vanish within the next 24 hours.  Be sure to stay tuned for the latest, and consistent updates will be starting on what could be the first storm to bring winter weather impacts to the SWCT/NY region.

To begin receiving consistent email updates on the latest weather threats and to receive all detailed Premium blogs that we will send out, please subscribe here.  Free updates, as always, will come before storms as well detailing our latest snowfall total forecasts and timing.  However, Premium members will receive in-depth impact analysis, focusing primarily on school closures/delay percentages and travel impacts.  They also will receive email updates every day or every other day detailing all the latest forecasts through the next 15 days along with 7-day forecasts and Storm Notes updated daily on the Premium page.  Any major threats to travel will additionally be outlined in free posts to ensure that all of SWCT/NY is prepared for disruptions and can safely prepare.

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