For what I believe is the first (or maybe second) time this season, a true Arctic airmass is pushing into the region from the northwest this morning. As that cold air arrives today, expect breezy NW winds and a classic upslope/downslope pattern in terms of clouds and winds. The one exception this morning appears to be Portland and surrounding parts of York County where convergence from Mount Washington is producing a persistent layer of low clouds. If you’re curious why those clouds form, I’ve included a longer explanation below.
Temps today will be falling steadily as the colder air moves in. While temps currently range from around 5 in the north to around 25 in southern NH, by this evening northern areas will be in the 10’s below zero and the coast will dip into the mid/upper 10’s above zero. Bundle up!
If you look closely at the image above, you’ll see a swath of grey shading that’s just a bit brighter than its surroundings. That’s the band of low clouds forming in the shadow of the Presidentials. How does it form? As air approaches Mt Washington, it can either go around the mountain or over it. If you’ve ever hiked to the summit, you’ll know that going over the mountain is a hassle (though lots of fun!). Air parcels agree with the less exercise inclined among us: better to stick to lower elevations.
About half the air parcels decide to hang a left and go up through Gorham/Route 2 while the other half decide to hang a right and head down through Crawford Notch/Route 302. On the other side of the mountain, an area of low pressure exists just downwind of the summit. Remember (almost) all the air went around the mountain either through Gorham or Conway and there’s not much left for Pinkham Notch. This area of low pressure draws the two airstreams back together. This is known as convergence. When two airstreams converge in the low levels of the atmosphere, some air must rise because too much air is attempting to fit into too small a space near the ground.
This rising motion downwind of Mt Washington happens any time there’s wind in New England. What’s unique about today is that there’s just enough moisture present to allow for the weak upward motion produced by that convergence zone to produce clouds. Unfortunately for those in Portland, those clouds happen to be right overhead this morning. By later today, we should see drier air arrive in the area so those clouds should dissipate.