[Update, 10:15 p.m.: The National Weather Service (NWS) rated the Suffolk tornado a "3" on the Enhanced Fujita Scale. It has put together an excellent page with imagery, video, photos and its storm assessment.]
While rain-cooled air created an atmosphere too stable for severe thunderstorms in the D.C. metro area, a bit of morning sunshine helped raise temperatures above 70 in Norfolk, Va. And it wasn't just warm, but it was also humid with dewpoints in the sticky upper 60s. The warm, sticky air and its interaction with the strong approaching cold front fueled the development of storms in southeast Virginia.
But the clash of the warm and cool air alone didn't cause the tornadoes.
The other key ingredient was wind shear; that is, winds changing in direction as you go up in the atmosphere. A lot of wind shear was present yesterday due to fast winds 15,000 feet up blowing in a different direction than winds closer to the surface. And this wind shear served to turn the winds inside developing thunderstorms. National Weather Service meteorologist Bryan Jackson, on-duty in southeast Va. during yesterday's storms, told me "the Suffolk tornado was from a classic supercell -- a thunderstorm with a rotating updraft [upward movement of air]."
The rotation within the tornado-producing thunderstorm is evident from radar. Radar can measure the velocity of wind blowing towards and away from it. When wind blowing away from the radar is adjacent to wind blowing towards the radar, that's an indicator of spin and, in many instances, a funnel cloud or tornado. The image to the right shows such a velocity couplet. The tornado, denoted by the "x," is located at the intersection of green and red shading, which shows wind blowing towards and away from the radar, respectively. (Note: the green triangle is where the radar algorithm detected the tornado, but I suspect it was slightly south of that based on the radar presentation; hence where I drew in the "x").
The tornado-producing storm also produced what's called a "hook echo" on radar, which is the classic indicator of a rotating supercell thunderstorm. On the right, you can see the radar image (this time displaying precipitation, as opposed to wind velocity in the above example) of the thunderstorm and its hook-like signature. I've noted where the tornado was likely located with an 'x' at the tip or point of the hook.
In addition to its definitive representation on radar, the Suffolk tornado was notable for its size. Initial indications from video suggest it was at least several hundred yards wide. It may have been wide enough to be considered a "wedge" tornado -- or one which appears wider than it is tall. NWS's Jackson suspects there may have been multiple vortices (columns of whirling air) within this large tornado:
Tornadoes this large typically have vortices an order of magnitude smaller that rotate about the main funnel. The nature of these multi-vortices as they revolve and move with the storm leads to the sporadic nature of tornado damage. I cannot confirm for sure that there were multi-vortices, but it did appear [there were]...
The intensity of the Suffolk tornado has not been confirmed. But judging from some of the video of the tornado and its damage, I'd guess it was an EF-3 or EF-4. EF stands for "Enhanced Fujita Scale," and the scale spans from 0 (least intense, 65-85 mph) to 5 (most intense, over 200 mph). Tornadoes of this intensity (EF-3 or EF-4) are pretty rare in Virginia.
The Suffolk tornado and the others that impacted southeast Virginia occurred on the six-year anniversary of the devastating La Plata, Md. tornado. Like the La Plata tornado, the Suffolk tornado will be remembered for years to come.