Ice, Sleet, Snow Coming Monday

Below is the Free version of the forecast for a long-duration winter weather event beginning tomorrow and continuing through the day on Monday.  Premium subscribers received a more detailed forecast outlining travel and school impacts and mentioning other future storm threats as well.  You can purchase the individual fore case here or sign up to receive all Premium content emailed to you daily by subscribing to our Premium service here.  Otherwise, we will present the relevant parts of the free forecast here:

The forecast is for 3-6 inches of snow south of the Merritt Parkway/Hutchinson Parkway, with 4-8 inches expect between the Merritt and Interstate 84.  Then, the forecast north of I-84 is for 5-10+ inches of snow, as they look to see the heavier precipitation.  The further north you go the larger the accumulations will be.  Additionally, I expect up to .1-.15 inches of ice accretion primarily in coastal areas, with amounts dropping off to near nothing by I-84.  Around and north of I-84 I would be surprised if they saw any freezing rain, with sleet and snow being the predominant precipitation types.  Even in coastal areas, I think sleet dominates a little more than freezing rain through the storm, but at least a glaze to minor ice accumulations look likely.  I would be quite surprised if any location saw more than .15 inches of ice accretion.  The NWS here is actually more bullish than the ice than I was expecting, especially as the last storm did not play out with the icing as they had expected, but the general gradient remains the same.  I just don’t see a scenario in which precipitation is heavy and there is not enough cooling aloft for more sleet/snow than freezing rain.  But the icing potential is certainly there, and that is what will make roads bad Monday.

Through most of the day tomorrow there looks to be a lull in precipitation.  It is late tomorrow afternoon into the evening that precipitation begins to pick up.  It is important to note, however, that at no point do I find precipitation with this being “heavy.”  There is no very strong low pressure center creating a large precipitation shield.  Instead, atmospheric instability across the region supports drizzle/flurries and more patchy precipitation.  That drizzle/light precipitation moves in between 2 and 5 PM across the region, and right now it looks to mainly be in the form of snow.

Overnight tomorrow night into Monday is when precipitation will pick up and could become moderate, but this is not a guarantee.  My best guess is that the steadier precipitation moves in sometime between midnight and 3 AM, and thus overnight on Sunday road conditions go downhill.  Model agreement is strong that through the day on Monday the precipitation is steadiest, with the mix line hanging out near the region and potentially changing us over to sleet/freezing rain in coastal areas.  Coastal Westchester is most at risk to turn to sleet/freezing rain first, with coastal Fairfield County next in line.  Almost all models agree then that the mix line pushes inland through the evening and into the overnight hours, with a turn back to snow early Tuesday morning, with the remaining light snow winding down between 5 and 8 AM on Tuesday.  Thus, sleet can be expected up close to I-84 sometime between 9-11 PM at the latest Monday, but cold air swings back through by 2-5 AM and moves all the way back down to the coast presumably by around 6 AM for any final last patches of precipitation to be all snow into Tuesday morning.

One major concern with this storm is for that extremely sharp precipitation gradient, that can be seen on the RGEM weather model here.  If you look at the bottom right, you can see how the heavier, steady precipitation is to our north, and coastal areas see less precipitation than inland areas.  The SREFs show a similar gradient here too.  Models have backed off it being quite as steep, but the idea is that even without a changeover to sleet/freezing rain at the coast (which is very likely) the coast should see less snow than further inland areas.

So thus we have a huge range of variables, with models showing anything from 1 to 8 inches of snow being possible at the coast depending on the scenario.  I like the 3-6 inch range as a good compromise, and because I think this could be a little more snow than most models show, but I do think that there is at least some sleet at the coast that keeps amounts down.  There’s a chance I may have to slightly raise snowfall totals, primarily in the slightly inland region, but most models currently show liquid equivalents within the range I am expecting, and with snowfall ratios close to 10:1 I am fairly confident now that I have covered my bases with the going forecast.  Of course, there could be some tweaks going into the day tomorrow, but overall confidence is decent in the accumulation forecast despite huge model discrepancies.

A free revision to the forecast will be made available around this time tomorrow night, so please be sure to check back in for any forecast revision in this extremely complex storm.

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