Wednesday, 11:45 a.m.
We're a month away from the winter solstice, but we're about two months away from reaching the bottom of the curve in terms of average temperatures. Already this month, arctic air has made its presence known, and we're about to see the coldest air mass of the bunch get loosed on the northern Plains and Midwest, then points downstream heading into the coming weekend.
Several pieces will come together to make this all happen. The first of these will be an upper-level trough of low pressure coming through the Northwest into the northern Rockies this afternoon and tonight. As features go, it won't necessarily have a lot of moisture to work with. Nevertheless, as the arctic air drains out of western Canada into the northern Rockies and is forced into the mountains on the back side of an arctic front, there will be some snow.
This snow will reach Denver late tonight and spread out into parts of northwest Kansas and Nebraska during the day tomorrow. The cold front attached to this upper-level disturbance will continue to advance along, reaching the eastern Great Lakes Friday morning. The southern end of the front will also be pushing into northern Texas at that time, as seen here on the 12z Nov. 20 GFS model surface forecast for Friday morning:
By Saturday morning, this front will be reaching the New England and mid-Atlantic coast in the fast westerly flow aloft. However, just as we saw with the strong cold front that raced from west of Chicago at midday Sunday to off the mid-Atlantic coast less than 24 hours later, the air immediately behind this front won't be bitterly cold - colder, yes, and certainly colder than we saw in the Northeast and mid-Atlantic states on Monday, but the core of the cold will be held up behind a trailing feature. Examine the GFS 500mb forecast for the wee hours of Saturday morning:
That feature is coming through the Midwest into the Great Lakes Friday night and early Saturday, and it will have an area of low pressure tied to it, along with a secondary cold front. You can see that fairly easily when you look at the surface forecast for Saturday afternoon:
THEN the arctic air will be fully unleashed across the Great Lakes and Ohio Valley, and Saturday night and Sunday into the East. Look at the 850mb forecast for Sunday morning:
Now, if you do the math and assume you get the absolute most out of this air mass with a full complement of sunshine, you'd still not get temperatures out of the 30s anywhere in the mid-Atlantic, and maybe to about 40 in parts of southern and eastern New England. There, however, the cold is still drilling in during the day Sunday, which means the 850mb temperatures will fall, and that should wipe out most chances of surpassing 40 in these areas.
To put that into proper context, those are afternoon high temperatures lower than on any typical, or 'normal,' January day from the Carolinas to New England back into the Ohio Valley and Midwest! In other words, this will an arctic outbreak stolen from the middle of January and dropped into the last third of November - and before Thanksgiving!
But wait! The evidence is mounting that another arctic outbreak is on tap for the latter half of next week behind a potent storm coming out of the Southwest! It's too far out to know for sure just how cold that second blast of arctic air will be, but suffice it to say it will much below normal in a lot of places from the Plains on east around the Thanksgiving Day holiday!
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