UPDATE 12:40 PM: The storm cluster that has weakened some but maintained some strong winds in western Ohio is sinking south-southeast and may affect far southwest Virginia west of I-77 this afternoon. Other storms will continue to develop farther south and east, some of which may become severe with locally damaging winds. END UPDATE
UPDATE 10:20 AM: Here is some good news for folks not wanting a derecho today: The current damaging-wind storm line in Indiana is expected to weaken over the next couple of hours hitting rain-cooled air in western Ohio. The bad news: There are other storms in West Virginia that may strengthen and affect parts of Southwest Virginia, some possibly as early as the next couple of hours (in fact, one such cluster is about to enter Craig and Giles counties now), and the storm line in Indiana may kick out an outflow boundary that helps develop strong to severe storms later when it runs into more unstable air than that in western Ohio. Short summary: A long-lived derecho system moving in from the Ohio Valley is NOT likely, but smaller storm clusters and possibly a re-developing squall line later may pose a high-wind risk to at least some parts of Southwest Virginia through the remainder of the day. END UPDATE
UPDATE 8:30 AM, 7/24: Radar this morning shows a series of storm clusters from West Virginia northwestward through the Ohio Valley into Minnesota, generally moving southeastward. The strong line in the Chicago area is perhaps the most eye-catching — that of course is where the June 29 derecho started. The Storm Prediction Center is closely monitoring this area to see if it develops into a more organized bow echo that could produce widespread damaging winds. As the day heats up, severe storms may be possible just about anywhere in the region, though locations west of I-77 have the greatest risk. END UPDATE
The battle lines between the central U.S. heat dome and cooler air out of Canada will shift southward over us on Tuesday. The result is likely to be that a disturbance will ride southeastward over the heat dome toward Southwest Virginia. With strong northwesterly winds aloft, it is possible that one more clusters of storms may develop in the Ohio Valley and move rapidly southeastward, building strength in afternoon heat and humidity. So there is a threat of severe storms, with damaging winds as the main threat. That is why we are in a slight risk of severe weather for Tuesday. The “d” word — derecho — has been mentioned by a few online weather outlets, and that word makes folks very nervous after June 29, rightfully so. A derecho is within the realm of possibility Tuesday — though probably not as widespread or powerful as June 29 — but the more likely outcome is a bow echo or squall line that produces some damaging winds but doesn’t meet the longevity requirements of a derecho (6 hours/240 miles of continuously damaging winds, according to different interpretations ) or multiple storm clusters capable of some damaging wind gusts over smaller areas. There may also be storms that develop in Southwest Virginia ahead of anything that might move at us from the northwest — such pop-up storms will have the potential to become severe with damaging winds themselves, and would somewhat reduce instability available for a storm cluster moving in later from the northwest. June 29 became so ballistic largely because of the historic heat (104 at Roanoke) that built up throughout the day remaining “capped” by warm air aloft until it could all be used at once to fuel the bow-echo that developed near Chicago and continued into the Atlantic. The heat is not expected to be nearly as intense (upper 80s-low 90s mostly) nor as bottled up as it was on June 29. That said, there will be plenty of fuel for severe storms, and it’ll basically be a matter of tracking on radar Tuesday how things develop. Damaging winds are possible in storms that develop Tuesday, along with large hail, torrential rain and frequent lightning. So be aware when out and about on Tuesday.
Somewhat cooler air moves in for Wednesday, before the front retreats and brings a surge of hotter air for Thursday, perhaps a brief taste of extreme heat well into the 90s. The overall long-range outlook still suggests most of the extreme heat to stay west of us, with storms developing in humid air most afternoons, occasionally stirred up a little more by passing cold fronts and/or disturbances from the northwest.