Clouds and pockets of light rain and sprinkles early this morning are associated with a weak storm center sliding out to sea well south and east of the region. This system’s moisture will vacate the area this morning and leave us with a partly to mostly sunny and quiet December day. Temperatures will peak in the lower 40s and winds will blow from the west and northwest at 10 to 15 mph…overall not an atypical day for this time of year.

However, today’s relatively pleasant weather is about to change in a big way.

A blast of pure arctic air is diving southward across Canada and will plow into the Great Lakes and New England tonight and move directly over the area Thursday night and Friday morning. This arctic blast will bring a variety of hazards to the area, including bitter cold, dangerous wind chills, a period of damaging winds and likely some snow showers and/or snow squalls.

Temperatures: The worst of the brutal air arrives Thursday night and lingers into the day on Friday, but the deep chill starts taking hold tonight. Temperatures will bottom out in the 20s overnight and struggle to rise on Thursday – likely failing to break freezing (if we can even manage to get out of the 20s) – before tanking into the 10s by evening. Readings will subsequently drop to the single digits and lower teens Thursday night and may fail to reach 20F on Friday. The coldest of the cold will be on the Upper Cape where temperatures may dip under 5F in spots. On the far Outer Cape, readings will likely hold closer to 10F.

GFS Midday Temperatures

Wind Chills: While the “official” wind chill forecast for the area holds values above Wind Chill Advisory thresholds for the most part, this may prove a bit deceiving as winds Thursday night and Friday morning will be HOWLING and frequently gusting over 45 mph…and potentially as high as 60 to 65 mph in spots. A Wind Chill Advisory is hoisted for “Wind chill index between -15°F and -24°F for at least 3 hours using only the sustained wind”. We will be flirting with this criteria throughout the overnight period and into the first few hours of Friday morning as it is…but the wind gusts will easily create dangerously cold values. Considering a temperature of 10F with a wind of 40 mph creates a wind chill of -15F, it’s very likely wind chill advisory “criteria” will be felt.

NAM Wind Chill Values Thursday Night

Winds: Behind the arctic front winds aloft will strengthen markedly and the atmosphere will become deeply mixed and turbulent, allowing strong winds aloft to frequently mix downward toward ground level. The strongest of winds pass through the region Thursday night, timing concurrently with the most unstable environment as the deepest of the cold passes through the region. It’s during this period – beginning around dinnertime +/- Thursday evening and continuing through dawn +/- on Friday that really just about all of Southern New England will gust 40 to 45 mph and numerous locations will very likely gust over 50 mph. Some of the typical high wind locations in the region have the opportunity to reach 60 mph at times…and remarkably, there is even a probability – albeit lower – that a few spots go higher than that. Thankfully all of the leaves are down and there won’t be much, if any, precipitation to contend with at the same time. Even so, limb and tree damage and resultant utility issues are a good bet. Given the severity of the incoming cold, power outages will be of greater concern than usual.

Deep-Mixing / Strong Winds Thursday Night

Snow Showers / Squalls: There are a couple of opportunities for a brief snow shower or snow squall around the area. The first opportunity comes first thing Thursday morning as a weak disturbance slides by the region…the second will come Thursday night as the true arctic front crosses the region and the final opportunity comes late Thursday night and into Friday morning as ocean effect snow showers develop over Cape Cod Bay and move onto the Outer Cape. While none of these 3 episodes are a big concern, the sudden, “unpredictable” nature of each implies the good old “nowcasting” method of weather prognostication will be in full effect. Despite a mainly offshore wind, the extreme instability created by the arctic air over still very mild (relative) ocean waters will yield some ocean effect flurries (at a minimum) and perhaps some steadier / heavier snow squalls from Provincetown to the Lower Cape (Brewster, Harwich, Chatham area). The set-up is not too dissimilar, in fact, to last winter’s arctic outbreak near Valentine’s Day which yielded quite the observation from Provincetown:

“7:15 AM

Temp: -0°F

Wind Chill: -24°F

Visibility: 0.2 mi

Wind: NW 24 mph G 33 mph

Weather: Heavy Snow”.

This isn’t to say expect “heavy snow” Friday morning on the Outer Cape but that even a 320-330 wind direction can yield ocean effect on the Outer Cape in cases of extreme instability driven by pure arctic air. The 12z sounding from Chatham on 2/14/16 showed most of the surface to 850 flow at about 330…in this case there is a window of time Friday morning when winds between 900 and 800 mb bend to that level. And, of course, it’s “good” to see the 4km and parallel 3km NAM showing some ocean effect clipping P-town to Brewster at times.

NAM Simulated Radar

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