Tuesday, 11:20 a.m.
The unofficial end to the summer season is this weekend as we come to the Labor Day weekend. I know, I know. The calendar says it is not officially fall until 4:44 p.m. EDT on Sunday, Sept. 22, the autumnal equinox. On the other hand, most kids are already starting school, from kindergarten all the way up through college! (By the way, whatever happened to starting school after Labor Day? Bring back those days!) Then, as if to really get your mind spinning, we consider Sunday, Sept. 1, as the beginning of 'meteorological' fall. It's all so confusing!
Regardless of your viewpoint, the atmosphere has the final say on the matter, and from everything I'm looking at from now until Labor Day, it still looks a LOT like summer. Feast your eyes on these highs from Monday afternoon in the nation's midsection:
Look at that area in Nebraska and South Dakota where it reached 100. How about the 97 in Minneapolis after the morning 'low' of 80?? That's autumn??? If it is, then I might have to rethink my ideas on global warming!
All that aside, most places from the mid-Mississippi Valley south and east remain cooler than normal for August, so the surge of heat to end the month is simply, as my friend Todd always tells me, 'things balancing out.' And that's what's going on right now. There's no downstream blocking in the Atlantic, so the cool air that moved in late last week has come and gone, and some of that heat in the middle of the country IS coming east, though not to the same extent as is being felt out on the Plains.
There are a couple of other things that tell me it is still summer:
1) MCCs. Short for Mesoscale Convective Complexes. Typically these form near the apex of an upper-level ridge and run southeastward from there. Most often they form over the northern Plains and dart across the Midwest. They are an integral source of moisture for the region. Right now, though the ridge axis is over the Mississippi Valley, so the launching point for these thunderstorms is farther east, largely over northern Minnesota, northern Wisconsin and the Upper Peninsula of Michigan. The start point is not the thing I am focusing in on here. The mere fact they are forming is far more indicative of a summer weather pattern rather than a fall-like pattern.
An interesting little side note should be inserted here. These thunderstorm complexes are going to move in the direction of the mean flow. If you superimpose that on the thunderstorms we have seen this morning, it will take them more through Michigan and across Ohio into southwestern Pennsylvania and West Virginia, rather than across Lake Erie into northern and central Pennsylvania and upstate New York. The same is likely to happen tomorrow with next one forming over Michigan.
2) Monsoon circulation still well intact. It's now farther west than it has been, but it's still very present. Here's the late-morning water vapor image:
The evidence of that is the 12z August 27 NAM model forecast of total precipitation through Thursday afternoon:
Note also the band of heavier precipitation stretching from the Great Lakes southeastward to the Chesapeake Bay, all of which will come through tomorrow night and be gone by Thursday morning.
3) Summer dew points. Just look at the model projection of surface dew points this evening:
It's MUCH more humid across the Midwest and Ohio Valley right now that it is in the Southeast and parts of the Deep South!
And, of course, there's still the heat, which goes without saying. To give you an idea of the heat anomalies I'm talking about, here's the 0z Aug. 27 GFS ensemble temperature anomalies for this coming Sunday:
You don't see too many blue areas on that map, do you? Parts of California and Arizona, then Manitoba and Ontario into Quebec. Some of that cool air will follow this heat and humidity into the Midwest and Great Lakes to finish out the Labor Day weekend, and then move across the Ohio Valley into the East on Tuesday.
For now, though, it's still summer.
Heavy rain will soak drought-stricken areas of the mid-Atlantic over the next couple of days. Focus will then shift to Matthew and its potential to impact the Eastern Seaboard with more heavy rain later next week.
Summer has ended astronomically, but from a meteorological standpoint, there's plenty more warm weather heading into October from the Plains to the East.
Two strong cold fronts will charge across the country in the next week, eventually taking out the current hot and humid air mass from the Plains to the East Coast.
Over the next three days, hot and humid air will expand across the Mississippi Valley all the way to the East Coast. This will be followed by even more heat and humidity leading into the weekend.
Hermine will head across the Florida Panhandle late tonight, then cut across the coastal Carolinas and become a headache for the mid-Atlantic and southern New England over the Labor Day weekend. It will be followed by a heat wave later next week.
The heat and humidity will be erased from much of the East later this week, but warmth will spread from the Plains eastward over the weekend. The tropics could still play an important role in the weather along the Eastern Seaboard this weekend.