UPDATE 11 AM, 9/18: As expected, a tornado watch has been issued to the east of the Roanoke and New River valleys, affecting Staunton, Lynchburg, Danville and points east, through 7 p.m. Extreme atmospheric shear will combine with some instability to our east to produce a chance of rotating storms that could produce damaging downdraft winds, and possibly a few tornadoes, though they are likely to be sporadic, brief and weak. Meanwhile, a large area of moderate to heavy rain is expanding back over the valleys after a morning lull. Expect another 1 inch of rain in most areas, possibly 2 or more in some localized spots. There is some chance of gusty winds with a few isolated storms even though we are outside the severe weather watch. A flash flood watch continues through today for Roanoke and Floyd westward. END UPDATE
We have a springlike storm system in a winterlike large-scale pattern that will lead to some fall-like temperatures later this week. If we had summerlike temperatures, we’d have a big problem Tuesday — atmospheric shear (winds changing direction with height) is extremely ramped up with the storm system passing just west of us, and that could lead to some storms rotating, increasing the risk of strong downdraft winds and a few tornadoes. If it were hotter, we could have a major severe weather outbreak — it may be bad enough over eastern Virginia, through the D.C. area and much of the Delmarva peninsula, the area marked by hatched lines in the Storm Prediction Center map that is inset. Expect to see several reports of wind damage and a few reports of tornadoes over that way Tuesday. The yellow area on the map marks the western edge of the “slight risk” of severe weather, basically following the Blue Ridge. It’s close enough that even the Roanoke and New River valleys should pay attention for a few morning storms with strong winds. The risk will be greater to the east, where it may get a bit warmer during the day, as the bigger storm risk arrives later. Expect tornado watches east of the Blue Ridge on Tuesday, with the Roanoke area on the margin of potential. Follow the Storm Prediction Center for the latest outlooks and watches for severe storm potential.
Flip the severe map to see where the greatest risk of flash flooding exists. That will be greater the more you go west. Locations farther west of Roanoke are more likely to be affected by a large area of rain moving out Tennessee and Kentucky, while locations along the Blue Ridge, including the Roanoke and New River valleys, are likely to get rounds of heavy rain and storms out of North Carolina overnight and early Tuesday. Farther east, the storms will be a bit more spotty on Tuesday, but could cause locally torrential rain. The actual rainfall never turns out as smooth and even as the projection map shows, but some spots will likely get 2-plus inches in the Roanoke and New River valleys westward. Areas from Roanoke westward are under a flash flood watch for the potential of storms and heavy showers moving over the same locations repeatedly, flooding roads and some ditches and creeks. Flooding of large rivers is not expected with this round of rain, as we enter it fairly dry.
This round of rain, combined with some cool mornings ahead with a series of cold fronts (lots of 40s by Thursday morning, maybe some 30s, and likely even a little colder by early next week) may help get some trees changing color a bit earlier than we’ve seen most autumns recently.