The storm didn't actually "move" sothwest...it propagated. As with any supercell, you get continuous new updrafts to keep it going. Quite often, epsecially in May and June, these cells can appear to remain stationary when in fact the updrafts are getting moved at 30 knots. The area of new updraft generation remains stationary or will drift in one direction deviant from the mid and upper flow. Just take a look at what happened on June 5th of this year in SW OK. Even with what is considered "weak" mid level winds of only 30 knots, if a storm remains stationary, the "storm relative" shear is at least 30 knots...ideal.
So, after all of that long-windedness :wink:, the one thing common in all of these types of setups are very strong to extreme instability and a stationary boundary with some cap (need an isolated sotrm...not a cluster). If the boundary is parallel to the mid level flow, all the better!
The Jarrell event had everything...stationary boundary parallel to mid flow of 30 knots, extreme CAPE of 6000-7000 (I read in a report that the Texas A&M intercept team luanched a near-storm environment sounding and found like 7300 CAPE). That's a ton-o-energy ready to explode! Once the first cell exploded on the NE aprt of the stationary boundary, new updrafts would be generated to the SW of the cell....likely enhanced by outflow traveling SW from the mature cells. This created a "zipper" like effect southwestward along the stationary boundary largely influenced by greater instabilities to the SW. I also contend that some localized moisture pooling (convergence) right along the boundary locally increased instabilities in a narrow axis to keep the updrafts focused in one spot. Overall, it was the perfect balance and timing of many parameters in my opinion. It was a rare event.
One very important factor involved was a pronounced gravity wave propagating SW aong the boundary from early morning storms over Lousiana and NE TX. It seemed that this helped to initiate rapid updraft development and from what I read...updrafts only formed in the wake of the gravity wave. Al Moller mentioned one time that Texas A&M did a computer simulation of the entire event and removed the gravity wave factor. At least in computer simulation, no tornadoes formed.
Since that day, I've paid very close attention to similar setups. Since gravity waves are such a peculiar and barely understood phenomena as well as impossible to predict, I don't bother with that aspect. But, I do look for strong to extreme CAPE and a stationary boundary relatively parallel to the mean flow. I also prefer the boundary to be strongly heated on both sides with strong/full insolation. I also look for stationary drylines with perpendicular mid level flow (N/S dryline with westerly mid level winds). This is a bit more tricky as you have to get the storm to get firmly rooted along the dryline and not move off it.
I had this happen to me on June 2 this year near Liberal, KS as a storm exploded, went nuts, got a TOR on it, and then promptly fizzled as it moved off the dryline...too strong of a cap was a factor I think. The other situation was June 12th of this year where storms kept erupting along the dryline in Crosby county. They would look pretty impressive and start getting their act together and quickly fizzle as the moved off from the dryline. As the dry punch arrived and sharpened up the dryline and convergence as well as reaching max heating/instability, we had quite a helluva show.
June 8th in eastern KS was a similar setup, but very weak to no capping created a big cluster of storms erupting almost simultaneously...but still produced tornadoes. I think CAPE values that day were 5000+. I can't help but think if the capping was stronger and we got one big supercell that day (I was playing with an isolated cell in extreme SE NE).
One particular situation that was "Jarrell-like" was the impressive F3 Lake Whitney tornado May 12, 2000. Stationary boundary, CAPE 5000+, parallel to 25-30 knot mid level flow. It did the same thing and propagated SSW along the boundary before producing a strong tornado (that I missed by about 10 minutes..argh!) around Lake Whitney. That was really the only tornado of the day, but it was impressive by the photos I saw. The radar images were quite impressive too and the cells were DITOR warned for awhile afterwards. Check out Lon Curtis' site:
http://www.vvm.com/~curtis/May_12.html
That's my $0.02 worth...check out theses sites for the experts' analysis:
An OUTSTANDING study by Al Moller and Lon Curtis:
http://www.vvm.com/~curtis/Jarrell/Jarrell.htm
A great assessment from NWS:
http://www.nws.noaa.gov/om/assessments/pdf...dfs/jarrell.pdf
A good study by Jon Davies regarding weak shear strong tornadoes:
http://members.cox.net/jondavies1/wkshrtor.PDF
Texas A&M Intercept Report (check out the awesome soundings!):
http://www.met.tamu.edu/research/texacal/outbreak.html
Great White Paper from Stephen Corfidi:
http://www.spc.noaa.gov/publications/corfi...idi/jarrell.htm
Other great links:
http://cimss.ssec.wisc.edu/goes/misc/970527.html
http://www.spc.noaa.gov/coolimg/jarrell/
http://rsd.gsfc.nasa.gov/goes/pub/goes/970...70527.texas.mpg (very cool timelapse visible satellite)
I hope this helps.
