Joe Lundberg

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A Weaker Cool Press, and More Moisture This Week

September 24, 2012; 9:44 AM

Monday, 11:30 a.m.

Fall arrived Saturday at 10:49 a.m. Eastern Daylight Time, and a very cool air mass set record low temperatures yesterday from parts of Minnesota and South Dakota to northern Missouri. Even Waterloo, Iowa, tied a record low this morning at 29 degrees, set in 1989. If you look at the daily temperature departures for Sunday, you'll see the core of the chill Sunday was from the Appalachians to the middle and upper Mississippi Valley:

A broad look at the 500mb chart for this morning shows the eastern trough is not as deep as was expected, but it's still plenty deep enough to bring the cool, dry air into the South and the East:

You'll also note the position of the downstream ridge over the Atlantic right up into Greenland, a classic negative North Atlantic Oscillation (NAO) setup, and one that leads to some form of blocking. Notice how that changes by Wednesday morning:

The heights are much lower over the North Atlantic, and the ridge over Greenland is gone. The heights have come up dramatically over the East, so there will be some warming, not just aloft, but also at the surface. Some of that surface warming will come from the southern Plains, where it never really cooled down over the weekend, and temperatures should have no trouble jumping back to normal, if not above, across the mid-Atlantic, if not into southern New England.

You will also note a couple of other features that tie directly into the title of today's post. One is the feature slipping into the Upper Midwest, ostensibly another cold front moving across the Great Lakes into the Ohio Valley and the Northeast at midweek. The other is the upper-level low over the Rockies.

If you look at the surface weather map behind the front, another decent sized surface high follows into the Upper Midwest and Great Lakes by Thursday:

Underneath that high, it'll probably be frosty cold again in the Midwest, as much of the last of the chilly air to our north is drained out of central Canada. Look at the 2-meter temperature forecast off the 12z September 24 NAM model:

This cool air will then fan out Thursday and Thursday night across the Great Lakes into New England and down into portions of the mid-Atlantic and Ohio Valley, but the farther south one goes, the more clouds there will be and the smaller the impact of the front on the actual weather. Still, after a period of warming into Wednesday, it will turn cooler in most of the areas for the second half of the week.

Another fascinating thing about that map is the lack of cool air south of the boundary, and how much milder it is over the western Dakotas and eastern Rockies, where it appears as if there will be a decent amount of clouds and perhaps even a little rain.

The other main story this week will be a steady moistening of the air mass over the Rockies and the central and southern Plains, tied to the presence of the upper-level low over the Rockies. That's the same feature that rolled through the Northwest late last week and into northern California over the weekend. What remains to be seen later this week into this weekend will be whether or not Miriam throws even more moisture into the mix late this week and this weekend. No, I'm not talking about my friend in Charleston, S.C.! I'm talking about the major hurricane about 650 miles south-southwest of the southern tip of Baja California. The computer models have been waffling back and forth as to the strength of this system as it recurves and heads across Baja California and into the southern Rockies, and how much moisture is still left if and when this happens. But suffice it to say, at least some moisture will be siphoned from Miriam long before it actually 'arrives' or makes landfall, and that should mean a somewhat cooler and decidedly wetter pattern for at least the southern Rockies, and, eventually, the southern Plains by this weekend.

Once beyond this weekend, there will be another shift in the pattern as a whole, one that should favor a lowering of the heights over central and western Canada and the Northwest, and that should allow these areas to at least cool toward normal, and perhaps even below it in early October. In turn, that might well translate into warming downstream across the Great Lakes and Ohio Valley into New England and the mid-Atlantic states.

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


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