Major Storm Threat Saturday, But Southern Trends Possible

The below post is part of a Premium update that was just sent out to subscribers ahead of the storm threat Friday night into Saturday.  They continue to receive more detailed blog content emailed to them along with forecast summaries daily ahead of major storms to see what parts of the forecast are known, what aren’t, and what trends are expected from here.  To sign up for Premium, subscribe here and learn how schools, businesses, and communities will be impacted by winter weather before anyone else.  Free updates will continue regularly as well ahead of tho storm threat:

There’s a lot to cover with this upcoming storm.  Models show anywhere from a couple inches to close to 2 feet of snow being possible for the region, and it is still too early to rule out any specific scenario.  What I am very confident in is at least some very strong winter storm forming along the East Coast, but beyond that the details are quite fuzzy.

Let’s begin with this image of the 12z GFS ensemble members for 1 PM on Saturday.  I use this to demonstrate just how little of this storm we have nailed down, and why I continue to remain skeptical of some of the forecasts calling for the wildest snow amounts near the area.  We see that some GFS ensemble members do have a major snow storm for the region, while others have it just starting around this point, and others have it generally sliding to the south and missing.  A couple even show the threat of rain mixing in (relatively unlikely given the storm track and setup).  Each of these scenarios does remain on the table, and though some are more likely than others I do not feel confident ruling out either the major snow scenario or the storm sliding to our south and sparing the region scenario.

The end result is this.  These are each of the GFS ensemble member’s total accumulated precipitation expectations for the storm.  8 of the 12 GFS ensemble members show more than 1 inch of liquid equivalent, which would be about 10 inches of snow.  Only one other member besides the operational GFS shows more than 1.5 inches of QPF across the region, indicating it is actually relatively likely that the SWCT/NY region sees more than 14 or so inches of snow.  This also means that the operational GFS may pull back in future runs on just how much snow it prints out across the region.  Of note also are the three GFS ensemble members that give the region almost nothing.  We are seeing a very, very tight snowfall gradient setting up, and it is likely that we are right on the northern edge of it.  A scenario thus where the I-84 corridor sees 8 inches and the coast sees 14+ inches is one that could very well play out, though it is far too early to know what exact amounts would be likely to be.

This threat of most of the storm sliding to our south is quite realistic.  We can see that here on the most recent operational ECMWF for Saturday 7 AM.  On the top left we see the 500mb upper level low centered over the Southeast.  A weak shortwave moving through central Canada may try and keep the storm slightly more progressive, and the closed upper level low means it is relatively unlikely to be pulled quite as far north as a classic Nor’ Easter would be in this scenario.  The closed low means that it will be slow-moving, however, so wherever the storm does stall will see some crazy snowfall amounts.  We can see that by Sunday 7 AM the ECMWF shows the storm gradually getting kicked out to sea here.  The end result on the model was about 2-4 inches of snow across the region during the day on Saturday with almost all the impacts further south.  The winds would not be as bad, nor would the storm surge, and essentially we would be spared as the southern Mid Atlantic gets rocked.

It is worth noting, however, that besides the most recent ECMWF operational and the few GFS ensemble members shown, most other models do give the region a relatively direct hit with the storm, bringing heavy snow and gusty winds.  The end result is likely to be a blizzard.  The operational GFS, seen here, has snow start between 7 and 10 PM on Friday.  The consensus generally has been towards a slightly slower storm (actually relatively in-line with the ECMWF operational) and this should keep any impacts during the day on Friday to a minimum.  The storm will be incredibly slow-moving, so snow would not even get heavy until the 3-7 AM window on Saturday, as seen here.  The model actually has the heaviest snow for the region falling Saturday afternoon and evening, as seen on this output on 4 PM on Saturday.  The model would have wind gusts peak late Saturday morning, with sustained winds to 30-35 mph and gusts to 50 mph certainly possible.  With the heavy snow, this could easily cause power outages if it were to verify, along with storm surge.

The GFS operational keeps snow going all the way until 4 AM on Sunday, as seen here, before the storm finally moves out.  The operational CMC also keeps the storm going until 1 AM Sunday, as seen here.  However, the model should not be taken seriously due to this output for 7 AM on Saturday.  As seen on the top right, the model is suffering severe convective feedback issues, as there are a large number of “low pressure centers” at the surface under the upper level low.  The model is struggling to figure out exactly where the largest pressure falls will be, and from there is unable to best determine exactly how the precipitation shield will be oriented.  Granted, the ECMWF and GFS operational models are also going through severe convective feedback, due primarily to the rapid intensification that will take place over land ahead of the normal timeframe.  This leads to increased volatility in forecasts, and that combined with the sharp snowfall gradient that could set up across the region leads to very low confidence for snowfall and overall impacts.  That said, I do want to summarize what we know.

WHAT WE KNOW: If a major winter storm were to hit, the worst impacts would be early Saturday morning through later Saturday afternoon, when gusty winds could cause power outages at the coast and heavy snowfall could make roads impassable.  A sharp gradient should set up, increasing the risk that coastal areas will see more snowfall than inland areas.  Temperatures should be cold enough for the entire storm to be snow, with ratios around 10:1 due to shattering dendrites with heavy winds.  Any snow will pile and drift very quickly due to the strong winds.  Storm surge could be an issue if the storm comes close enough as well.  School impacts on Friday will not be an issue, with businesses/towns only seeing the real impacts on Saturday (the only day with significant impacts expected).

WHAT WE DON’T KNOW: Whether the upper level low will close off early enough to keep the storm system from moving more significantly to the north.  If the storm slides further to the south, the winds, snow, and storm surge all become non-factors as the worst impacts remain to the south as well.  The exact snowfall ratio may be hard to determine due to both colder temperatures aloft but very strong winds at the surface.  Essentially, almost everything from late Friday night into early Sunday is uncertain.

The first preliminary snowfall outlook/percentages for the upcoming storm have been published for Premium members.  It factors in the possibility of further southern shifts in guidance that could spare the region the worst wind, storm surge, and snowfall accumulation impacts.  The official Free snowfall accumulation forecast will be released on Thursday, though another update on storm progress will be published tomorrow evening.

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