Tuesday, 11:30 a.m.
The snow has all but ended in my backyard after a slushy inch or less on the grassy surfaces. About 2 to 3 inches of snow has fallen over portions of south-central and southeastern Pennsylvania, and there will be a relatively narrow band with a 3- to 5-inch snow fall when all is said and done across portions of Pennsylvania and New Jersey and even portions of southern Connecticut and Rhode Island.
The air behind this weak storm is clearly cold. Look at the early-morning lows today:
Where it was clear a good bit of the night, temperatures plummeted into the teens all the way to western Oklahoma and over half of Missouri. Where skies are clear tonight, it will drop toward freezing or lower in the Tennessee Valley and the interior South. That will occur tomorrow night throughout the interior Southeast and up into the mid-Atlantic states as high pressure, now over Kansas, ends up over Virginia and North Carolina by Thursday morning:
If you look beyond the high, though, what do you see? Notice the thickness (dashed) lines, a loose correlation to temperature? There's nowhere near the 'packing' of those thickness lines out on the Plains and in the Rockies, and the numbers get higher and higher, equivalent to a warmer air mass. Contrast that to the thickness packing over New York and New England late Thursday night:
In this latter case, we see a piece of arctic air flying through Ontario into Quebec, and some of it will ooze into New York and New England to wrap up the week.
Back to the Plains and Rockies, the first cold air mass associated with the large high on the Plains kept Denver cloudy and around freezing yesterday. With a southwest breeze and sunshine today, it's already 50 at 9 a.m. local time, headed toward 60 or better. Yes, the air mass quickly moderates behind this high. Part of the reason for the quick warmup is the lack of snow cover across the Plains. However, another part is due to the lack of moisture in the broad west-southwest flow aloft over the region, as well as where the upstream upper-level trough is positioned. Look at the GFS ensembles for Friday evening:
Since the upper-level trough is so far offshore and the surface storm associated with it is also well northwest of Seattle, it will be hard to get cold air to come into the region, either from the south side of the complex, or from it draining in from north of the border, where there is a ton of cold air. That leaves the West warm, and when you wring out whatever moisture there is in the atmosphere as it passes over the mountains of the West and the Rockies, then flow it downhill out onto the Plains, it only gets milder. And that's why the anomalies are through the roof on Saturday:
Furthermore, with the downstream blocking breaking down or moving more east of Greenland this weekend and beyond, the flow becomes much more progressive across the country. It thus becomes only a matter of time before at least some of that warmth gets into the East. It won't really get there Saturday, as the return flow around high pressure over New England offers some resistance to the warming. That will mean a lot of clouds Saturday and some very light precipitation - drizzle, a touch of rain or maybe even a little snow in upstate New York and northern New England, enough to keep temperatures in the tank for one more day. That warmth will gradually overwhelm the region Saturday night and Sunday with rising temperatures at night, and temperatures rising above normal with ease from southwest to northeast across the region Sunday into Monday.
To put it all into perspective, here's the projected temperature departures for the period Dec. 1-7 across the country:
There can be little argument against that as the warmth will dominate in most of the country heading into early December. The next real question will become when will it change to colder again, and where will the attack be?
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