Thursday, 11:30 a.m.
The normal progression of weather systems is from west to east in the Northern Hemisphere for much of the year. During the summer season as the jet stream retreats way to the north, it does allow more east-to-west movement in the low latitudes. There, we find the low-level circulation dominated by the strong Atlantic high, a feature that is seemingly always there in one form or another. That's how tropical waves move across the Atlantic Basin, steered by this fairly strong lower-level flow out of the east and northeast all the way across the Atlantic toward the Caribbean or the Southeast U.S.
Every once in a while, though, the normal pattern is stood on its head, for lack of a better term. Late last week, it was becoming quite clear that an upper-level trough coming into the Northeast would not just pass through and move on but instead develop into an upper-level low that would come to a grinding halt over the central Appalachians, then roll southwest and then westward across the country. At the same time, an upper-level ridge over the Atlantic then redeveloped farther west over Maryland and Pennsylvania over the weekend, a ridge that quickly expanded westward into the Ohio Valley early this week.
The clockwise flow around that high has meant an upper-level flow out of the east throughout the Carolinas and Georgia into Florida and points west. Look closely at the 12z Thursday, July 18, NAM 700mb forecast for this afternoon:
If you examine that image carefully, you'll see the remaining center of the decaying ridge over Indiana and Ohio. You will also note a weak feature over the east-central Gulf of Mexico, an upper-level low that is helping to trigger showers and thunderstorms over the eastern Gulf at this hour:
Note also in that 700mb image the presence of an inverted trough over Tennessee down into Mississippi, a feature that has had its own share of convection over the past 24 hours as it has rolled westward underneath the upper-level ridge to its north. And lastly, look at the forecast wind barbs, virtually all of which are oriented from the east from Virginia to the Mississippi Valley on south!
Interestingly, if you tracked that upper-level low from over the weekend, it ended up burying itself over Texas, then the southern Rockies. It has helped to drag a fair amount of moisture through the Lone Star State into the southern Rockies over the past several days. This helped to bring a fair amount of rain to areas from Houston south and east the past 48 hours. That area of rain and thunderstorms is now much farther west and southwest and is bringing clouds, showers and thunderstorms to the lower Texas coast back into the Hill Country. It stretches right into parts of New Mexico.
One of the byproducts of this east-to-west movement underneath the ridge in recent days is a general lack of heat in the South. It's tropical, yes, but that's not unusual in July! With limited dry air, there's not quite as much sunshine around, and temperatures are being held in check as the source region is clearly tropical in nature.
In contrast, the ridge that backed westward for the better part of four or five days is no longer doing so. In addition, with the ridge being relatively flat, there's a pretty strong flow over the top of it, especially in light of the strong vortex smack dab over Hudson Bay this afternoon:
Just underneath that jet stream, the air is hot and, while humid in the low levels, is pretty dry aloft, dry enough to promote a lot of sunshine from the central and northern Plains eastward across the Midwest into the Great Lakes and northern Ohio Valley:
With good mixing, the boundary layer is getting deep enough to mix out the warmth above, translating into mid-90s sort of heat in a lot of places. The air aloft over the Northeast is not quite as hot, but it's really close. But in these latter areas, the effect of the air flowing downhill from the Appalachians is contributing to compressional warming, more than making up the minimal difference in terms of ambient temperatures aloft. Absent any sea breeze, it is just plain steamy from New England to the mid-Atlantic, and it will be again tomorrow. Even on Saturday ahead of a cold front, it is going to be hot and humid one more day before any kind of cooling relief can move into position. By then, though, the unusual pattern of east-to-west moving weather systems will largely be gone, with the winds aloft much more likely to be blowing from a more traditional west or southwest direction deep into the South over the weekend.
Much above-normal warmth is in store for the next week from the Plains to the Northeast and mid-Atlantic states, while it turns much cooler throughout the Northwest.
Despite the current chill, warmth is becoming more certain this weekend and beyond from the Plains to the Northeast, even as Erika threatens Florida Sunday night and Monday.
While Danny has been declared an open wave, the low pressure area trailing it will likely have at least a passing impact on the weather along the East Coast next week.
It may be hot now in the Northeast, but much cooler air is drilling into the Rockies and is heading eastward. An even cooler air mass this weekend and early next week will be followed by a stronger warming trend.
The retreat of the jet stream will lead to more warmth than not in much of the country heading into the second half of August.
More concentrated areas of active weather will be the rule into the early part of next week across the country as weak storms move steadily eastward.