UPDATE 8 AM, 7/17: Tropical heat builds into midweek; desert heat stays west, helps push through a cold front by late week
UPDATE 8 AM, 7/17: In general, a repeat of Monday on this Tuesday — perhaps a bit hotter (mid 90s for Roanoke, maybe upper 90s Southside, upper 80s-near 90 New Rive Valley) and fewer afternoon showers and storms with weak downsloping westerly surface winds heating and drying things up a bit. A cold front approaching Wednesday and Thursday will increase the chance of storms again before turning it cooler for late week. END UPDATE
There are two kinds of hot spells we get in Virginia. One comes from the tropics; the other comes from the desert. Two and a half weeks ago, an air mass built over much of the central and eastern U.S. from the Desert Southwest, intensifying over the Plains and spreading eastward. That eventually gave Roanoke 12 straight days of 90-plus temperatures, including 10 97-plus and 4 100-plus. Heat is again building in the early to middle part of this week, but it is the other kind, from the tropics. High pressure over the western Atlantic — what we often call a “Bermuda high” — has been pumping in southerly to southeasterly winds, bringing moisture off the Gulf and Atlantic Ocean. While the surface winds may actually turn westerly with a center of the high located to our south in the Southeast U.S., the tropical humidity has already been laid down, and the heat will build into that. This kind of hot spell usually doesn’t produce widespread 100-degree temperatures — a couple of such readings can’t be ruled out, especially in Central and Southside Virginia (map at left is NOAA projection for highs Tuesday)– but it does produce humid conditions and lots highs in the low to mid 90s over most locations from Roanoke eastward, and mid-upper 80s to the west. So while the actual temperature probably won’t challenge the hottest days of the recent extreme heat wave, the feeling outside may be similar, or a little worse at times, because of dew points in the upper 60s to low 70s. With the heat and humidity will come daily chances of showers and thunderstorms, the kind that dump an inch or more of rain in one spot and sprinkles or nothing a mile or two away.
The good news for folks who don’t like the heat (most of you, according to my feedback — there are some exceptions, though) is that it appears very unlikely that the next round of “desert heat” spreading into the central and northern Plains with triple-digit temperatures this week will spread eastward to our region, at least through the next 7 days or so. In fact, the clockwise spin around that “heat dome” high pressure will help push a decent cold front through from the north by late in the week, perhaps Thursday or early Friday. That front will knock the top off this hot spell, likely driving high temperatures back into the upper 70s/lower 80s by the coming weekend. The front pushing into the heat and humidity will raise the risk of storms sometime in the Wednesday-Friday time frame, with severe weather always a possibility in such a scenario.