The Fall Forecast Looks Much Drier in the DC Metro

Well, how's this for unfortunate timing? Now that meteorological summer is over and the kids are back in school, our weather pattern is turning much more pleasant. The long-range forecast looks sunnier and drier, in stark contrast to the soggy summer weather we had to deal with.
As I talked about in a previous post, the summer was a bit cooler than normal, and much wetter. If we're just considering meteorological summer- which is the months of June, July and August- we had 5.3" more precipitation than in a typical 3-month summer period. Statistically, the summer didn't look that cool on paper- our average temperature was 0.57 above normal- but that number takes both the high and the low temperature for the day into account. Since it was so humid this summer, our overnight low temperatures were much milder than average. The humidity not only prevented temperatures from cooling down overnight, but it also kept us from getting too terribly hot during the day. The humidity is partially responsible for our small number of days with 90+ temperatures (only 23 days in 3 months). As a result, most of the summer was very warm and uncomfortably humid.
Of course, summer is typically more humid than fall, but this pattern shift in the forecast is more than just a seasonal variation! We're also finally seeing a breakdown of the huge ridge out west, which gave portions of the Rocky Mountains one of the hottest and driest summers on record. An upper-air map from September 1st shows the ridge in place out west, and our hot and humid pattern from the Labor Day weekend:

Model data, such as the chart below from the European model, suggests that the ridge will subside a bit and slide to the east. Here's a look at the forecast upper-air map for this upcoming Sunday: 

Notice how the ridge is now in the center of the country! When a ridge is situated just to our west, it blocks strong and/or moisture-laden cold fronts from moving through the DC Metro. A general northwest flow will keep the humidity nice and comfortable, and should lead to some nice, crisp fall nights. The ridge is expected to slowly progress eastward during the month of September. If that happens, we can expect a period of dry weather with temperatures above normal. I think we'll see lots of 80s for high temperatures in September, with only a scant few days with highs in the 70s. The 90s that we had to begin the month might be it for this year! Let's hope I didn't speak too soon. :-)