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Joe Lundberg's Weather Blog

Not Your Normal Blocking

Feb 2, 2012; 11:01 AM ET

Thursday, 11:30 A.M.

We're in the midst of a pattern change these days, one that has been dominated by a tremendous amount of warmth in most of the country, to one that won't be nearly as warm next week. That doesn't mean or translate into a cold and stormy pattern, as some would probably interpret that statement! In fact, I think I'd rather call this a pattern shift. It's a 'softer' term, and doesn't exclude some cold days or even some snow.

In fact, snow is front and center in the next 48 hours or so from a storm that is coming through the Four Corners area and will soon redevelop over the southern Plains. As it does, colder air will be drawn southward into the circulation of the storm, while at the same time a lot of low-level moisture will pulled from the Gulf of Mexico and wrapped back across Oklahoma into Kansas and eastern Colorado. Where it is cold enough for snow, which will be the case over western and portions of northern Kansas into Nebraska back into Colorado, there will be plenty of snow. Here's our latest projection at AccuWeather.com:

Not only will it snow a lot, but there will also be quite a bit of wind, too, enough to warrant blizzard warnings for eastern Colorado and portions of western Kansas.

As we make this pattern shift, it will become more meridional with time, or high amplitude. In other words, there will be a lot of north and south undulations in the jet stream. And the end result is a pattern than will have some blocking to it, just not your typical blocking. Normally when that term is thrown out there for general consumption, it's assumed there's some sort of upper-level high over over very near Greenland, a feature that forces cold air southward through Canada and into the central and eastern U.S., and pulls mild air northward across the Atlantic. Quite often it leads to an upper level ridge over the West, where it will be warm. And you can see this when you look at the projected 500mb forecast for next Tuesday evening:

Now I won't go into all of the ramifications of the temperature pattern next week across the country. I'll try to look at that more in depth tomorrow. I will say this much, though. It won't be as cold as the pattern might normally dictate.

Anyway, this pattern shift will have direct results on this southern storm. The growing support from the operational runs of the models, as well as in the ensembles, is for the storm to only come so far north, then run into a veritable road block, and be forced to slide underneath this block.

This unusual block will be aided in its formation by the storm passing off the East Coast today. As it swings farther out to sea, it will deepen and turn northward, then hook around Newfoundland Sunday and Sunday night. In so doing, it will back things up across the Northeast and mid-Atlantic states, and turn the upper level winds into the northwest from the Great Lakes to the mid-Atlantic states. So while the storm will move across the Mississippi Valley toward the lower Ohio and Tennessee valleys, it should go no farther northeast, and instead begin to weaken and slide underneath this upper level low.

On last comment. The amazing mildness across Canada into the northern Plains isn't done yet, and it will be pulled eastward across the Midwest into the Great Lakes Monday into Tuesday, and into the Northeast on Tuesday in advance of the next cold front. The farther east you go, the smaller the departures will be with respect to normal, and will in no way rival the warmth of the past two days across the mid-Atlantic and southern New England, but it will easily be well above average. Then front will move through, it will turn cooler, but even that chill down will only be to near normal before it tries to moderate again rather quickly.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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