KEVIN MYATT The Roanoke Times

The rounded base of a rotating thunderstorm gathers strength east of downtown Roanoke minutes before severe thunderstorm and tornado warnings were issued Wednesday afternoon.

Kevin Myatt is The Roanoke Times' weather columnist.

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On the large scale, conditions were not conducive at all to rotating thunderstorms or tornadoes Wednesday afternoon across Southwest Virginia.

Upper atmospheric winds across the region as a whole were not particularly strong, nor were they shifting much with height. Surface winds were also very light, typical for the summer months, and not obviously converging anywhere.

But something on a finer scale allowed one storm, just east of Roanoke, to spin Wednesday afternoon.

The spin became so pronounced and tight that Doppler radar detected it as a potential tornado, and the National Weather Service office in Blacksburg responded with a rare tornado warning for the eastern half of the Roanoke Valley.

Towering cumulus clouds, starkly gray against a drab white background of high cirrus clouds, drew my eye more than an hour before any warning was issued.

There was something a little different this time. The cloud features looked more precise than usual. The base of the clouds looked more rounded, clearly separated from any other developing storm cells.

Early on, I thought the clouds brought to mind the embryonic stage of Plains supercells rather than the quick-hitting mountain pulse storms that pepper our summer.

From the roof garden of The Roanoke Times, I watched the storm gather to the east, partially blocked by some of the city's downtown skyline. I could see rising cloud tags, often a sign of rotating updrafts. The storm base became more rounded and exhibited a broad, slow rotation.

Back down in the office, I was not surprised to learn a severe thunderstorm warning had been issued. I was much more surprised to see a small red box go up on the radar screen a minute or two later, signaling a tornado warning.

That got me back on the roof. A few of my colleagues followed. The rounded base was much tighter, and obviously rotating.

It didn't take long for the storm's rotation to be disrupted by outflow winds. Once I felt easterly winds blowing out from the storm, rather than westerly winds blowing into the storm, I knew its tornado threat was just about finished. Still, the reports of large and copious hail from the Vinton area were highly consistent with a rotating thunderstorm that, at least for a time, had the potential to produce a tornado.

So what spun the storm?

It's somewhat of a mystery, but here are a few thoughts.

• A Storm Prediction Center midafternoon map revealed a tiny patch of 40-knot (46 mph) shearing winds from the surface to about five miles high over and just east of Roanoke. Over just a few counties for a short time, the modest winds were changing direction and speed with height just enough to reach the minimal level considered necessary for the likely development of rotating storms.
• National Weather Service meteorologist Phil Hysell and I independently came to the conclusion that outflow boundaries from Tuesday night's storm cluster to the east may have played a role. Such outflow boundaries -- or the cool air pushed outward by storms -- can bump against a different air mass, or even a terrain feature like the Blue Ridge, to create a horizontally spinning tube of air that could be pulled upward into a storm updraft, translating into vertical rotation.
• Hysell said weather service forecasters were impressed with how rapidly the storm exploded Wednesday afternoon, which could have stretched any slight spin rapidly upward. Virginia Tech meteorology instructor Dave Carroll noted that short-range models Wednesday were indicating quite high potential storm energy in the lowest layer of the atmosphere, which would have been fuel for rapid growth of storms.

So while the large-scale pattern was not present for an outbreak of violent thunderstorms or destructive tornadoes, it's the features on the small scale that can take one thunderstorm in a different direction.

Some tornado-favoring details -- converging low-level wind flow and increased helicity, or the ability of air to spiral upward -- were not present in the Roanoke Valley on Wednesday. That probably spared us from a meteorological novelty becoming a disaster.