2009.06.10
Tornado-warned storm was definitely swirling
Click here for a Weather Journal column about why this storm may have developed rotation
Right place, right time. I was watching the storm build east of downtown Roanoke, and noting some rotation in it, even before the tornado warning was issued once National Weather Service Doppler radar began picking up tight rotation within the storm just before 4 p.m. This photo (bigger version here), taken from the roof of the Roanoke Times building, captures the counterclockwise circulation I observed, with the arm of clouds coming in from the left wrapping toward the center of the circulation back behind the Wachoiva tower. Some hail up to an inch in diameter was reported in Vinton, very indicative of a storm with rotation lifting water droplets higher in the atmosphere for a more sustained time.
Here is one clue as to why this storm might have exhibited rotation for a period of time: A midafternoon chart from the Storm Prediction Center shows a small pocket of 40 knots of wind shear up to 8 kilometers high located over the region where the storm developed. We typically look for 40 knots of shear (wind changing speed and direction with height) up to 6 kilometers high as the baseline for likely supercell development. This storm may not have had rotation long enough to be a supercell, but it certainly started out with a supercelluar-type structure. Another possibility I am wondering about is some kind of atmospheric boundary produced by the storm cluster over central Virginia last night. An outflow of cooler air from previous storms can produce horizontal rotation where it bumps into a different air mass or a terrain feature (the Blue Ridge?), and that rotation can be pulled into a storm's updraft, producing vertical rotation. That is just conjecture on my part at this point.
Click here and here for additional photos I took of the storm organizing just before the tornado warning was issued.
Ron Bailey of Vinton sent in several photos from the storm as it passed overhead. The lowered cloud structure on pics 6 and 7 definitely has the appearance of a possible wall cloud, or lowering with circulation that has the potential to drop a tornado (though it appears that no tornadoes actually touched down.)
David Gray sent this photo of rotating storm clouds over his backyard in Vinton.
A reader only identified as Kelly sent in this photo from over Cardinal Glass in Vinton.
A small photo of hail accumulation in Vinton from Karenna Glover, who works in marketing at the Roanoke Times.
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We had an inch and a quarter of rain here in the last 30 minutes or so...no wonder there is standing water in the intersection!
Comment by Betsy — June 10, 2009 @ 5:41 pm
Yes, the same storm that affected eastern Roanoke County/Vinton is near Smith Mountain Lake now moving eastbound ... not severe-warned at the moment but still pretty stout on radar ... looks like additional waves of rain and storms moving in from the west ... a wet night in store for most of Southwest Virginia!
Comment by Kevin Myatt — June 10, 2009 @ 5:48 pm
This looks like the Roanoke area (mainly Vinton) had a really close call today. Wow!
Comment by Brandon R. — June 10, 2009 @ 10:38 pm
There wasn't much in the way of low-level wind flow or shear ... that could have helped get something spinning on the ground. The storm also started producing outflow pretty quickly, so that would have cut off any further rotation. We didn't have the outrageous helicity (ability of winds to spiral upward) numbers we did for last year's small South Roanoke tornado. If that kind of helicity and some low-level shear had been present, this might have been a really bad day.
Comment by Kevin Myatt — June 10, 2009 @ 10:52 pm
Your excellent posts match my experience with this storm exactly! I was on the east side of the Wachovia tower with a great view through a big window (I really need to start carrying a camera). The storm seemed like a textbook example of a tornado-producing cell, exactly like the ones I've seen on storm chaser shows. It was scary because my family and my house were potentially in the path.
Comment by Danny — June 12, 2009 @ 10:40 am
Yes i had another angle of the funnel cloud whic another pic in here had the direct image i had from another angle, As a weather spotter and weather enthusist this is the best structure i have seen . I will use this as an investigation for the paper i am writing for the NWA.
Dean D Davison Bedford County SKYWARN
Comment by Dean D Davison — June 12, 2009 @ 11:44 pm