Winter is Back

Evening Update 2/20: While I try hard to avoid bouncing my 3 day or longer forecasts in response to one day's worth of model runs, I thought today's trend in computer guidance for Friday was worthy of a quick update to this discussion. This morning's guidance hinted at plenty of mid-level, and even low-level, warming during the height of the storm Friday night. This idea would lead to a mix with or change to sleet and perhaps rain after an initial burst of snow. However, the midday runs of nearly all the national and international models paint a slightly colder and snowier scenario here on Cape Cod, with less mixing. The trend continues in the 18z models, with the NAM now producing .8" of precipitation (melted equivalent) at Chatham in the form of snow. This might be a bit overdone, but barring a major shift southward, a solid 4 to 6" snow event may be in the cards late Friday into Friday night.

In addition, with a non-phasing storm, a piece of energy trails the initial storm and helps a secondary surge of moisture move in on Saturday and Saturday night. This could keep clouds and light snow going into Saturday night. More updates tomorrow morning.

Wednesday 2/20:

Winter is making a comeback on Cape Cod this week as temperatures head below normal for several days and snow returns to the region by Friday and Saturday. Given the relatively tame nature of most of January and February thus far, this will be a nice late winter reality check.

While somewhat colder air built into New England late Monday night and during the day on Tuesday, it was offset by a combination of Tuesday's strong sunshine and leftover warmth from Monday's balmy conditions. The result was a relatively mild day with highs in the low to mid 40s. While the air temperature was mild, a secondary cold front was approaching the region during the day and really helped to kick up winds. These winds, which at times were sustained at 30 mph, added a chill to the air, taking some of the fun out of the mild February weather. Still, this cool feel was nothing compared to what our neighbors to our west were experiencing behind the secondary cold front. This front meant business, as temperatures behind it over Upstate New York were in the 10s and 20s Tuesday - some 20 to 25 degrees colder than here on the Cape.

With the passage of the aforementioned front last night, readings dropped into the low to mid 20s and will mean hold in the low to mid 30s on Wednesday - some 10 degrees lower than Tuesday and putting us near or slightly below normal. However, take a moment to look at the current temperatures across the country. It's hard not to see the growing area of bitter cold temperatures dropping into the Northern Plains. This represents yet another surge of cold arctic air oozing down the Canadian plains and into the Dakotas and Minnesota.

This air, though certainly modified, will be ours to deal with as it moves eastward into the Northeast behind a clipper system tonight. By Thursday, 850 mb temperatures will be approaching -18C and 925 mb readings will be sitting at -12C. A cold north to northwest wind will aid in the drainage of the chilly air into the region and should result in high temperatures in the 20s on Thursday. A quiet night Thursday night should allow temperatures to head well down into the 10s Thursday night.

While the coldest core of air will be lifting out by week's end, low level cold air will remain trapped Friday thanks to a very cold start, light winds and advancing clouds ahead of our next weather system. The combination of clouds and a dry, cold air mass should hold readings at or below freezing through the day on Friday.

That brings us to our next situation - a snow storm to end the week? For several days now computer guidance has been flirting with the idea of a moderate sized snow event for the Northern Mid-Atlantic and Southern New England, as a low pressure area tracks northeastward from the Southern States to off the Mid-Atlantic. The jet stream has shifted around just enough to allow this further south low track this go around (when compared to what we've seen much of this winter.) Such a track allows cold air to remain in place over the Northeast and means the snow/rain line is suppressed southward. But just how much further south? Unfortunately, the the exact track remains a question mark and hinges on the track and phasing of jet stream energy. This, of course, will play a role in just how much moisture actually works into the region and more importantly, just how much mid-level or low-level warming we experience. Rising critical thicknesses during the event suggest that mild air will move in aloft during the middle of the storm, which could allow snow to mix with or change over to sleet or rain at times. Just how much that happens will depend on the storm's ultimate track.

Given the current set-up and the available solutions for a variety of computer models, my current thinking is several hours of steady (perhaps heavy for a time?) snow later Friday and into Friday night, allowing for several inches of accumulation. The air will be plenty cold at the onset to support snow and that initial front wall of snow could be decent. However, after that passes, it could be followed by a period of mixing as that milder air tries to move in for a time. It's a bit early to think total accumulations, so check back for updates.