Joe Lundberg

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Warming Over the Top

August 15, 2013; 9:43 AM ET

Thursday, 11:30 a.m.

The typical pattern of temperatures across the country is one that has the highest readings in the southern tier of states and the lowest readings to the north. There are times, of course, when deep upper-level troughs and strong upper-level ridges develop in the atmosphere that send warm air to the north while driving cool air to the south. So, on some occasions, you'll see these weird variations in temperatures, where it's colder in New Orleans than it is in Caribou, Maine, for instance.

Sometimes, though, the departures from normal become more of an issue. It may not be quite as warm in the absolute sense in, say, Minnesota as compared to Mississippi, but in relative terms when compared to what it might normally be in both locations, the seemingly small difference can mean vastly different things!

Such is the patten we face going into the weekend and beyond. On the playing field now, we have a lot of cool air from the Plains to the East Coast. Look at the early-morning low temperatures:

As expected, record lows were set in the Midwest yesterday morning, and today a number of record lows were broken across the Ohio Valley into the interior mid-Atlantic. With a sprawling high pressure area still controlling the weather through tomorrow, temperatures will again be below average over a wide area of the country. The 0z Aug. 15 GFS ensemble mean temperature anomalies for tomorrow:

Let's look at the upper-level pattern right now. Here's the 12z Aug. 15 NAM 500mb forecast for this evening:

Note the strength of the western ridge that will promote increased heat and less shower or thunderstorm activity in the West and the depth of the trough that goes all the way to the Gulf Coast. Compare that to the same model forecast of 500mb heights for Sunday evening:

The western ridge, while still very prominent, is now considerably flatter, and that will have some implications going through the weekend into early next week. Note also the upper-level high showing up near Bermuda, more or less 'pinching' the trough between the two ridges. This, too, will have some important ramifications in the weekend forecasts across the South and Southeast.

Focus on the 582dm height line, which goes from near Portland, Ore., to International Falls, Minn., then to Bangor, Maine. The height lines north of there are pretty tightly packed, indicative of a strong jet stream flow across southern Canada. This will take the warmth over Alberta and Saskatchewan down into the northern Rockies and fan it eastward, while it turns cooler in western Canada. Notice the changes in the temperature anomalies for Sunday:

Now look at the projected height anomalies for next Tuesday evening:

As the heights lower in western Canada and along the Northwest coast, they will respond by rising somewhat downstream. This will have the effect of pushing the warmth even farther downstream, and the result will be a surge of warmth heading over the top of the lingering and shrinking cool pool to its south. Here are the projected temperature anomalies for next Thursday:

Now in the end the numbers probably won't work out quite like shown, but it should give you the idea that the warming that will come to the Northeast later next week will come over the top, not from the southern Plains or the Deep South. In fact, this more direct approach means there will be less moisture involved, and could still send temperatures to or above 90 in parts of the Northeast during the second half of next week.

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

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About This Blog

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