Joe Lundberg

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Progressive Flow Leading to Steady Changes

September 5, 2013; 10:06 AM ET

Thursday, 11:30 a.m.

The jet stream flow across North America is quickening these days, and that's leading to a steady parade of fronts we move away from summer and head toward the colder season. One cold front sailed off the Northeast and mid-Atlantic coast Tuesday night, ushering in a cooler, much drier air mass to the region. A second cold has been sliding across the region over the past 24 hours, and it will clear the Northeast and mid-Atlantic coast by this evening. Between the two, it warmed nicely yesterday, and much of the mid-Atlantic will enjoy sufficient sunshine this afternoon to boost temperatures into the 80s.

Behind this front, however, there is a very cool air mass moving out of central Canada. Temperatures fell into the 30s in much of central and northern Ontario this morning, and with high pressure setting up camp tonight over Pennsylvania and New York state, the combination of a clear sky, dry air and little wind will allow temperatures to slip into the 30s in the normally colder valleys from central and northern Pennsylvania into northern New England, low enough for frost in some locations.

The day tomorrow will remain cooler than average throughout the region, with many places ending up more than 10 degrees below average for the day as a whole. Look at the 0z Sept. 5 GFS ensemble temperature anomalies for Friday:

Don't look now, but the heat is quickly expanding out of the central and northern Rockies and across the Plains behind this shot of chilly air! By tomorrow afternoon, temperatures are back up in the 80s around Chicago and in the 90s across much of the Dakotas and parts of Minnesota and Iowa. Yes, a few spots could hit 100, too, from southeastern Montana down into western Nebraska!

With the surface high moving to the mid-Atlantic coast Friday night, the southwest flow will carry warmer air right back across the Ohio Valley into the Great Lakes and parts of the East. A check of the 12z Sept. 5 NAM 500mb forecast shows the upper-level ridge still going strong over the eastern Rockies and Plains, while an upper-level low rolling through the Northwest brings some cooling through Washington and Oregon into Idaho, along with some showers and a couple of thunderstorms.

Meanwhile, another disturbance will be poised to drive the next cold front southeastward toward the Great Lakes and Northeast. By Sunday afternoon, that front will be darting across New England and down into the mid-Atlantic states, stretching back across the Ohio River Valley into the Plains:

The surge of warm air ahead of the front will push temperatures to near 80 in southern New England Sunday and into the 80s in the mid-Atlantic states and the Ohio Valley. But note the surface high moving by the northern Great Lakes Sunday afternoon, a feature that will drive another shot of cool air by the Great Lakes into the Northeast Sunday and Sunday night into Monday.

Even as that is taking place, though, another disturbance, this one tied to the upper-level low rolling through the Northwest, will pull hot and relatively humid air back up into the northern Plains later Sunday into Monday, with that warm and increasingly humid air moving into the Great Lakes and Ohio Valley by Tuesday. Look at the 0z Sept. 5 GFS surface forecast for Tuesday evening:

It's a pattern that has a lot of back and forth in it due to the progressive nature of the flow pattern, with the bulk of the heat over the Rockies and Plains, and most of the chilly air aimed at the Northeast.

I'm looking forward to some downtime this weekend and early next week, as I'll be leading a team of riders from the State College area up to southern New England. We'll all be riding in The Flattest Century in the East on Sunday, and it looks like a pretty nice day with some sunshine and highs in the 70s to near 80. I'm also looking forward to spending some time with my dad, who has been dealing with some right leg issues of late that has had him away from home in various facilities. While I'm there he should be able to head back home, though with some needed assistance. Still, it will be nice to see him there, as well as my one brother and some other friends. It's a great time of the year for vacation, and I'm going to take full advantage of it!

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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.