Having lived in the Southeast for most of my life, I find it a bit dishonest to say that there is a defined “severe weather season” for this region. We, after all, are the ones who rang in the new year with a tornado outbreak. We’ve had tornadoes a couple days before Christmas, in January, during hurricanes, and occasionally even during the summer months. We have certainly had powerful thunderstorms in the summer months that have brought extreme wind, hail, and lightning. However, it is fair to say that there are periods of the year when the most destructive types of severe weather are more common than others, and perhaps these are what should be defined as the “severe weather seasons” for the South. If this is the case, then we are entering the spring severe weather season. We’ve already had a taste of it when the powerful squall line came through last week on Thursday and Friday, generating several tornadoes and 100-mile-an-hour winds in Tennessee. That, however, was just a teaser, and an event is currently in the process of unfolding that promises to be more significant. Tonight’s event has a couple of parameters in place that last week’s did not, at least not to a great extent, plus all the parameters that last week’s event did have. I am using images from the RUC model, which is a short-range model containing live observations that is run every hour.
First of all, there is a strong jet over the region.
This jet will have uplift occurring on its northeast side. Since it is a 120-knot jet, this will provide significant uplift.
Secondly, there is a decent thermal gradient and fairly strong winds at the 850 mb level.
This will provide fuel to the strengthening low pressure center that is the source of the upcoming bad weather. The advection of warm air is also a source of uplift.
There is also an area of vorticity (essentially, rotational motion of wind). Vorticity is not the source of tornadoes, despite the suggestive name; in fact, uplift associated with vorticity is not directly under the strongest area, but is instead downstream of it. The vorticity is being advected in that direction by the motion of the air.
Incidentally, that accounts for three parameters providing favorable severe weather conditions over pretty much the same spot: Memphis/St. Louis/NE Arkansas.
Now, the parameters that last week’s event did not have in place relate specifically to supercell formation. Last week’s event had plenty of uplift and wind shear, just not a lot of atmospheric instability and helicity. Tonight’s expected outbreak is a different story.
Here’s atmospheric instability, as measured by CAPE (convective available potential energy):
There are values approaching 2000 J/kg expected over, yes, the same area where a lot of uplift is supposed to be occurring. This isn’t a stupendous value, but it’s pretty significant for an early-season outbreak. Some of the highest values of CAPE I’ve ever heard of were on the order of 6000 J/kg for events that produced F5 tornadoes. Where other favorable parameters exist, CAPE is directly correlated with the formation of thunderstorms.
The peak swath of high helicity is a little off-center compared to the rest of the favorable parameters, but a pretty significant area of it is nonetheless centered right over the area in the lower Midwest/upper South where the rest of the favorable parameters are converging.
I am expecting a tornado outbreak over this area tonight, pushing eastward into Tennessee in the early morning hours. This is not good, as nighttime tornadoes are easily the deadliest of all. (See my previous blog entry about the tornado outbreak of February 5, 2008 for more about Southern tornadoes.) I completely agree with the Storm Prediction Center’s decision to issue a moderate risk for this region tonight and tomorrow. I think it is possible that tornadoes may occur a bit farther south than currently indicated, mainly because of that high helicity index, but I am not expecting there to be a very great threat much south of Highway 82.