Thursday, 11:30 a.m.
Record high temperatures have been falling each day this week. Yesterday, some of those records were in the northern Rockies and northern Plains, where there is virtually no snowcover (yet), as well as in Arizona, where the 82-degree reading in Phoenix tied the record for the date from 1965. A few more records will undoubtedly be erased between now and Monday as two more surges of warmth spread out of the central and southern Plains to the East Coast between now and then.
Despite all of this warmth, there will a wintry look to more and more of the country by Monday. And when I say wintry, I mean snow and cold. Will this be everywhere? No, not really. However, it will turn MUCH colder across the Rockies this weekend, and that colder air mass will charge into Texas Sunday and Sunday night. Look at the Sunday evening NAM surface forecast:
That's a classic setup for a 'blue norther,' with the cold air draining out of high pressure across the northern Plains and eastern Rockies and charging southward across Oklahoma and Texas. This front will blow right through the Valley by early Monday and continue barreling across the Gulf Coast states and off the East Coast Monday night and Tuesday morning. A peek at the GFS forecast map for Tuesday morning:
One of the differences between this front and some recent ones is that the source region of the air mass behind it is straight from the frozen hinterlands of Alaska, the Yukon Territory, northern sections of British Columbia and Alberta. To give you an idea of just how brutal it has been there, Fairbanks, Alaska, reached a high of 16 below zero early this morning, the 'warmest' it has been since last Tuesday! During that eight-day stretch, they have AVERAGED 27 degrees below normal, with the average temperature itself at 29.4 below zero F!!! Granted, you will NOT get all of that air mass to come bodily southeastward and into the northern Rockies this weekend, but make no bones about it, Sunday will be dramatically different in Denver than yesterday's springlike, 69-degree warmth!
The other notable difference is that this will be a full latitude trough that sweeps across the Rockies and Plains to the East Coast. By that, I mean a deep trough that will stretch all the way into the Gulf of Mexico. You can see that by looking at the Monday morning 500mb forecast off the GFS:
It is for this reason that I believe this front will have no trouble ushering much, much cooler air to all of the East by Tuesday and Tuesday night, resulting in a couple of days of below-normal temperatures throughout the South into the Southeast. The departures from normal will be less farther downstream across the mid-Atlantic into New England, though it will still go from quite balmy Monday afternoon into Monday night ahead of the front to much colder Tuesday afternoon into Wednesday behind it.
That, of course, is the cold part of the wintry look. There is also the matter of snow. Well, that should begin to appear over northeastern Washington and into Montana tomorrow night and early Saturday. As the wave of low pressure tracks eastward across the Dakotas Saturday, it should lay down a nice swath of snow, particularly across North Dakotas and into northern Minnesota into Saturday night.
Meanwhile, a second piece of upper-level 'energy,' if you will, will dig southeastward and cut across the Rockies later Saturday and Saturday night. The combination of these two features will drag the arctic air southeastward, and behind the arctic front there should be a 'burst' or period of snow across Wyoming and Colorado, the exact amounts of which are yet to be determined, and will no doubt vary greatly from place to place thanks to terrain influences. Regardless, it will be greatly welcomed by skiers and ski resorts that have been all but snowless in recent weeks!
This latter feature should round the base of the larger-scale, upper-level trough Sunday night and Monday, causing rain and even some thunderstorms to expand northeastward from eastern Texas and the lower Mississippi Valley into the Ohio and Tennessee valleys with and ahead of the hard-charging cold front. There may be enough moisture lingering on the back side of the front for rain to end as snow in northern and western sections of the Ohio Valley and portions of the Great Lakes on Monday.
Then, of course, with at least some arctic air in the mix pouring across the Great Lakes, there will certainly be a period of lake-effect snow off virtually all of the lakes, starting Monday in the northern and western Great Lakes and lingering through Tuesday off the eastern Great Lakes.
Now, the bigger question is this: Is this a pattern change? Probably not. If you refer back to the GFS 500mb forecast for Monday morning, you may also note that feature in the lower right-hand corner, the center of an upper-level ridge over the Bahamas. That upper-level high is something that tends to diminish the sting of arctic air masses coming into the East, as well as their length of stay. It seems to hold its ground in the week leading up to Christmas on most of the ensemble packages I've looked at. That would suggest to me that while there will be much more cold in the pattern overall during the middle of the month, it may be more notable north and west rather than south and east when push comes to shove. And it reeks of a storm track cutting from the southern Plains toward the eastern Great Lakes. If you like snow, that's potentially good news if you live in the central Plains and Midwest, but no so good in the mid-Atlantic and southern New England.
Still, it is only early December, after all. There's still plenty of time for the pattern to change, and I suspect it will.
A building upper-level ridge of high pressure in the West will promote hot, dry weather in the Northwest, while a downstream trough brings cooling through the Midwest into the Great Lakes and Ohio Valley.
A pattern more typical of late July and early August is shaping up around the country, one with heat and humidity, but with fewer incidents of severe thunderstorms and flooding.
There will be plenty of heat and humidity from the southern Plains to the East Coast this week while much cooler air prevails for a time over the Northwest to the northern Plains.
Severe thunderstorms raked across the Midwest and Ohio Valley in the past 24 hours, with more on the way this afternoon. The pattern will repeat itself over the next week.