UPDATE 7:45 AM: The Storm Prediction Center’s slight risk of severe weather extends as far south to cover most of western Virginia north of Roanoke, as storms that give states north of us a potentially scary day of severe weather may sink southward overnight. They are expected to run out of the best wind flow and gradually weaken, but it’s close enough that we should keep an eye on it. Meanwhile, warm air aloft has moved in comparable to that of late June when Roanoke hit 104. If high clouds are limited today and downslope westerly winds are sufficient, we may again crack triple-digits in the Roanoke Valley and many locations to the south and east. Expect at least upper 90s in Roanoke, with widespread low-mid 90s over most of the New River Valley and other Southwest Virginia locations below 3,000 feet in elevation. END UPDATE
UPDATE 10:45 PM, 7/25: No changes. Still expecting a shot of heat on Thursday, possibly making a run toward 100 for the Roanoke Valley and locations to the south and east. Well into the 90s most other places in Southwest Virginia. And then more storms Friday and cooler weather into the weekend. Speaking of storms — the “d-word” is in play for Ohio and Pennyslvania on Thursday. END UPDATE
I wrote a somewhat tongue-in-cheek column (linked here) about the “d-word” that’s graduated from weather geekdom into the popular lexicon (at least in the East — it was already there in other parts of the country) this summer.
What passes for a cold front will slide southward through our region by Wednesday morning. The only way you will really be able to tell is that Wednesday’s high won’t go up much above what it was on Tuesday (90 for Roanoke, 82 for Blacksburg) and showers and thunderstorms are likely to be focused more to the west and south of our region than toward the Roanoke and New River valleys (which, if you think about it, isn’t much different than Tuesday, either). The weird twist is what is expected to happen late Wednesday into Thursday. The cold front is expected to turn northward as a warm front, which may drag some more showers and storms northward, and lead to a surge of hot, dry air on Thursday from the southwest. With westerly downsloping winds setting up Thursday as well, leading to compressional heating and drying of the air, temperatures are expected to shoot up close to 100 degrees from the Roanoke Valley southward and eastward, with widespread 90s elsewhere in western Virginia. Going from a high of 90 one day to 100 the next would be an unusual event for Roanoke. Extreme heat usually builds over a few days. Of the 48 100-degree or higher days since the establishment of the current Roanoke Regional Airport weather station in 1947, only 2 have followed a day with a high cooler than 95, and only 9 have followed a day with a high cooler than cooler than 97. Thursday also presents the possibility of a fifth 100-degree day in 2012 for Roanoke, which would be the most in any year since there were 8 in 1977.
With another cold front and upper-level disturbance arriving from the northwest Friday, and the accompanying increase in storm chances, Thursday will likely be only a 1-day hot spell and not the start of a prolonged period of extreme heat.