Joe Lundberg

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Bipolar Weather Pattern May Turn More Polar

April 22, 2011; 9:48 AM

Friday, 11:30 A.M.

A stark contrast in air masses remains in place across the country, one that will be reinforced this weekend as the chilly high over the Pacific Northwest slides over the Rockies into the northern Plains. This will lead to another chilly couple of days into the start of next week across the northern Rockies and northern and even portions of the central Plains. In contrast, another surge of warmth emanating from the southern Plains and Gulf Coast regions will take aim at the Ohio and Tennessee valleys, then spread at least into the mid-Atlantic states, if not into portions of upstate New York and New England.

To give you an idea of what this contrast might look like, here is a look at one of the computer models' projections of expected temperature departures for tomorrow across the country:

This extends into next week. The same model forecast for next Tuesday looks like this:

At least if you live east of the Mississippi, you're going to get a break from the chill! Notice, however, that I said a break, not the actual elimination of the chill. No, that would be far too nice after all of the pretty lousy spring weather in most places from the northern Rockies to the mid-Atlantic!

Just as a sneak peek at what MIGHT be coming, here's the very same GFS models' projection of temperatures for Tuesday, May 3:

Geesh! That suggests that even the South can expect a cooldown by the time we open up the month of May!

The sad part is that there is a fair amount of support for it being clearly below normal over a large area of the country once past the warm period of next week (warm, that is, from the eastern and southern Plains to the East Coast). The latest European weeklies are just flat-out chilly in all but Florida and the southernmost Rockies for the first week of May. In fact, their week three and week four forecasts look just about as cold, with maybe a little less of it across the Gulf Coast to the Southwest.

Of course, it should be duly noted that these same weekly forecasts have been all over the place, and there's not been much week-to-week continuity on them. It certainly wasn't forecasting the kind of warmth for the East that we're likely to see much of next week when the previous forecasts were issued on April 14th. But by Monday, it was growing pretty clear the warmth was coming, and coming to stay for at least four days next week.

Another longer-range tool we look at is a product from our neighbors to the north, the Canadian NAEFS forecast. Here you see the probability of cold or warmth for week two (days 8 to 14), which extends from next weekend through the following week:

This will all get started this weekend as the chilly high nosing into the Northwest edges into the Rockies, then out into the northern Plains. There were several locations in the interior Northwest that dipped into the teens this morning, and if you look at the region as a whole, the entire month has been persistently colder than normal.

Even places such as Minneapolis that were about 7 degrees above normal for the first 13 days of the month have turned dramatically colder, and are now just a tad more than a degree above normal for the first three weeks of April, with nowhere to go but down for the rest of the month.

Anyway, as this high migrates eastward, the existing chill in the Northwest will come with it, and any upper-level disturbances embedded in the jet stream will be forced underneath a fairly weak upper-level ridge in place over the Northwest right now. That means clouds and scattered showers will move across California into Nevada, and then Utah and Colorado this weekend, with one of these features spurring on the development of low pressure out in the central Plains by late Sunday and Sunday night.

This, in turn, will pull the warm, moist air across the southern Plains and Gulf Coast regions this weekend back to the north. It will be best classified as a 'dirty' warm-up, in that there will be a lot of clouds the farther north you go, and probably some scattered showers and thunderstorms stretching from the southern Plains northeastward to the Ohio Valley, then eastward toward the mid-Atlantic and southern New England. It might be best termed a 'ring of fire,' though not in the classic meteorological sense. Still, it will be along the boundary that will be separating this warmth from the chillier air to the north and west of it. And this is the very place where we can expect multiple disturbances, or waves of low pressure. And as each one moves along in the flow, there will be plenty of clouds, showers and thunderstorms, and some of these are going to be severe and/or produce a lot of rain.

To give you an idea of what that might look like, here's the accumulated precipitation off the 0z run of the GFS through next Thursday:

Yeah, just what this region needs--more rain! Ugh. What a spring! And, of course, with such bull's-eyes of precipitation showing up on the map, and the prospects of the bipolar pattern continuing, you have to believe some of that rain will be tied to severe thunderstorms--and that means the very high potential for more damage from hail and high winds, not to mention the likelihood of more tornadoes.

It may take a while, but eventually an upper-level trough will push east of the Mississippi, likely reaching the East Coast by the end of next week or possibly the start of next week. I say eventually, as there are still some differences amongst the various models as to the exact timing. But by next weekend, most of the warmth will be gone, and it will allow cooler air to expand in a more meaningful way into the South and East as we go into May. There may be one last attempt at bringing warm air back to the Ohio Valley and mid-Atlantic Saturday into Sunday before the door is shut, and the upper-level ridging near the East Coast is finally vanquished for a while.

What that ultimately means is that the jet stream will again go back into a mode of being pushed farther south than usual. In turn, I would think that means an expansion of the cool air farther south and east. And with disturbances/storms rolling along in the flow at a steady clip, you can almost count on seeing more rain and strong to severe thunderstorms parading across the country in the first week or so of May. The air across the northern half of the country won't really be polar in nature, but given where it could be or even should be for the time of year, it might feel polar!

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

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Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.