UPDATE 11:10 PM, 7/18: No need to change the blog tonight. Friday will have similar temperatures to Thursday — mid/upper 80s west of Roanoke, low/mid 90s Roanoke Valley southward and eastward — with scattered afternoon storms. Perhaps a click or 2 greater on areal coverage of storms Friday afternoon than Thursday. Pattern starts to change over the weekend. END UPDATE
Same song, next verse on Thursday — another hot day with scattered afternoon and evening thunderstorms (photo at left shows distant storm over Appomattox, as seen from downtown Roanoke early Wednesday evening). With a disturbance having pushed west (flooding downpours in spots from southern West Virginia to Georgia) and somewhat weaker and drier upper-air flow overhead, I suspect showers and storms might be a little more spotty on Thursday, especially from the Roanoke Valley eastward. It may also be perhaps a little hotter, with low-mid 90s Roanoke Valley and east of the Blue Ridge (Roanoke hit 92 on Wednesday, down a bit from Tuesday’s storm-punctuated 94) and mid to upper 80s to the west (Blacksburg’s high of 87 on Wednesday was also down 2 degrees from Tuesday). But leftover outflow boundaries from Wednesday’s storms and subtle disturbances not picked up by models could prove me wrong, so keep an eye to the sky and radar. Wherever a storm does pop up, it can deliver torrential rain, gusty winds, frequent lightning and some hail for a short period of time, while folks a few miles away just hear some thunder. This is what we typically expect of summer, but the storms seem a little magnified, given the preceding sogginess of the summer.
The summerlike pattern we’ve had this week, with hot days and scattered afternoon storms, is about to crash again this weekend, back into the familiar cooler but sticky, showery, rainy at times pattern this summer has become known for. Wednesday morning’s initialization of the Global Forecast System model (essentially, an assessment of current conditions at that time) at 500 millibars (roughly 18,000 feet) clearly shows the large, stagnant high pressure system centered over the Ohio Valley, bringing hot weather to much of the Midwest, East and Northeast. The placement of that high to our northwest, circulating clockwise, is why we’ve seen storms moving in an odd southwest direction the last couple of days. Also of note on the linked map is the disturbance in New Mexico — it was over eastern Kentucky on Saturday and, as expected, gotten pushed far to the southwest by the growing high pressure system, bringing much needed rain and some unseasonably cool temperatures to drought-stricken Texas. But run the model ahead to next Tuesday, and it depicts a return to the pattern we’ve seen so much, with high pressure centered in the West and upper-level winds diving southeast over the East, carving out a low-pressure trough. (Other models are in general agreement with the GFS.) This pattern will allow somewhat cooler air to filter in from the north, though in July, cold fronts almost always hang up, stall out, wash out or even reverse when they get just below us, and this one appears likely to do the same this weekend. It should be enough to pull us out of the upper 80s-mid 90s high temperatures back to more like upper 70s-mid 80s for several days. Perhaps more importantly, though, surface low pressure systems and disturbances moving through that low-pressure trough will be well placed to pick up Gulf of Mexico and/or Atlantic moisture and squeeze it out over us in rounds of showers and storms. I’m sure this prospect doesn’t excite almost anyone in Southwest Virginia, given the soggy summer we’ve already had. Flooding potential will exist with any rounds of heavy rain that occur over the weekend into next week.