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    Finally Turning the Corner

    4/24/2013, 7:40:04 AM

    Tuesday, 11:30 a.m.

    In a follow up to Monday's post, I thought I would toss out a couple of other items of interest as it relates to the easing of the cold, stormy, snowy weather pattern we've been stuck with for a long, long time. Two main things jumped off the page to me this morning in terms of the big picture, and while they won't necessarily mean we flip a switch and go to summer, they do portend a much warmer pattern overall.

    The first of these is the slow but very noticeable retreat of the jet stream. Look at how it appears across the country this morning:

    The disturbance seen over Missouri is helping to drive a potent cold front south and east. After reaching the low 80s in Houston yesterday, essentially a normal day for this time of the year, temperatures dropped into the upper 40s early this morning, and the afternoon, with limited sun, will barely reach the lower 60s - nearly 20 degrees below the norm.

    There's a nice surge of warmth ahead of the front in and east of the Appalachians today after a several-day stretch of below-normal temperatures, but the front will blast through in the next 24 hours. In the process, much cooler air will once again push eastward over the mountains and into the East, resulting in a significant cooldown tomorrow.

    Now look at the jet stream Sunday evening:

    Let me point out several things off the image:

    1) The main branch of the jet stream is now much farther north, coming through the Northwest and across the northern Rockies, then east-northeastward into northern Quebec. That is a dramatic turnaround from this morning! And it has huge implications on the pattern going forward.

    2) The upper-level low over the western Atlantic, and 3) The upper-level high over the northwest Atlantic. These two features combined are really going to force change upstream, slowing down the normal west-to-east movement of features.

    4) The weak feature coming into the Southeast. Yes, it is hard to see, but it's what remains of the feature that will bring some showers from the southern Plains on Friday, through parts of the South and Tennessee Valley Saturday, and into the Southeast and maybe the southern mid-Atlantic on Sunday.

    The remains of this latter feature will have a hard time exiting the playing field because of numbers 2 and 3 above. That means its remains will still be in play early next week to generate clouds and some form of rain or showers in the East, slowing any warm-up.

    Rather than focusing on the day-to-day weather concerns, let me go back to the big picture and show you the other reason why I'm more optimistic for a pattern change. That would be the location of the polar vortex. It's more easily seen by looking the GFS ensemble Northern Hemisphere mean heights and anomalies. Let's start first with this morning:

    The polar vortex is basically north or north-northwest of Hudson Bay. The northwest flow on the western flank of it continues to drain very cold air across Canada, which is still buried in deep snow. Every time there's been a strong buckling of the jet stream, a large piece of that very cold air has been loosed on the country in one form or another.

    Now look at ensemble forecast for late next week:

    The heart of that vortex is not only significantly weaker, but also much farther west. When you combine that with the northward retreat of the jet stream, you will pull the bitter cold back to the north and northwest and, more or less, cut off that sort of source region from the rest of the U.S. That doesn't mean it can't be cool, but rather that we're not likely to see anything close to the kind of extreme cold and snow that has remained so prevalent so late in the season.

    In fact, with that moisture left behind on the playing field early next week, another disturbance will move into the northern Plains. In recent weeks, that would have brought very cold air right back into the country. However, with far less cold air available to be tapped, it means a far more modest cooling into the Plains and Midwest initially. Eventually it may link up with that eastern moisture to produce a broader upper-level low over the Ohio and Tennessee valleys, a feature that should at least generate a lot of clouds and some rain. If that's correct, then any place near or underneath it will likely be cooler than normal, but probably not anywhere near the same kind of intensity as we see now across the eastern Rockies, Plains and Midwest.

    So, in summary, it appears as if we are finally about to turn the corner and get more typical weather for the middle of spring, or, at least, a lot less chill than we've seen so far this spring overall.

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


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