UPDATE 1 AM, 8/21: Carry this over another day, as Tuesday is likely to be similar to Monday — highs mid 70s to perhaps some low 80s after morning lows in the upper 50s and low 60s, scattered showers and storms. Sunday/Monday highs of 72 and 76 at Roanoke were the coolest back-to-back highs since 72 and 66 on May 13-14. Keep an eye on the tropics with 4 areas of interest -- one of which may be near Florida by early next week. I will try to get a larger update focusing on possible tropical troubles up sometime Tuesday. END UPDATE
I’ve often called fall in our region a battle between the tropics and the tundra. Though it’s too early to really call it “fall” yet — summer will almost certainly make another push or two our way — that battle has been fully joined. Through the past week, a mass of cooler air has been working south over the Canadian prairies, originiating from near the Arctic Circle. That air mass arrived Saturday behind a cold front — significantly warmed from its original chill, but still cooler and drier than typical August fare. Meanwhile, a surge of tropical moisture arrived early Sunday, pulled northward by a wave of low pressure that developed on that cold front. The result was widespread rainfall in Southwest Virginia — the first date since May 14 when both Roanoke and Blacksburg topped an inch of rain — and atypically cool temperatures, shaded by rain and clouds, with a high of 69 at Blacksburg (13 degrees below normal) and 72 at Roanoke (14 degrees below normal). Without the cooler, drier air mass from the tundra that condensed the moisture, it would have been a sticky, partly cloudy day with highs in the 80s and scattered afternoon storms. Without the tropical surge, it would have been a relatively mild, dry sunny day with highs in the 70s to low 80s and lows in the 50s. The two air masses intersecting is what caused Sunday to turn out like it did. Though we’re going to miss out on what could have been some unusually early crisp morning lows that would have occurred if the tropics didn’t join this battle, what occurred Sunday is indicative of what often happens in autumn, especially early in the season. So in a way, the rainy, cool Sunday was a taste of fall.
The tropics-tundra battle will continue this week. As is typical in late summer, the tropics will ultimately win this battle, likely leading to a return of warm to hot, sticky weather, possibly as early as the coming weekend. The result in the short term, however, will be rather stable temperatures, mostly mid 70s to low 80s for highs and upper 50s to mid 60s for lows — highs a little below normal, lows near normal — through at least midweek, perhaps slowly warming by Friday. There will be intermittent chances of showers and storms, especially today (Monday) and Tuesday as some daytime heating, weak disturbances and perhaps another wave along the front to our south squeeze out some of the moisture. It’s not likely we’ll see a repeat of Sunday morning’s widespread downpours — projected amounts through the next 5 days total no more than half an inch across our region. But some showers moving up the Blue Ridge from North Carolina early this Monday morning might top that projection in a few spots even before the sun rises. (Latest radar linked here)
Long term, we need to be watching the tropical Atlantic, especially the two disturbances west of Africa that may have some chance of affecting the U.S. by next week. It’s a long way out and a lot can happen, but it is the season for the tropics to have a big upper hand it will be sure to lose to the surging tundra by October and November.