4/24/2006 DISC: OK/KS/TX

[Kenny Drake]

I'm surprised a discussion hasnt been created for this yet. I was in a meteorology lab today and had to create a sattelite animation with GEMPAK, and decided on April 24 hoping to find something interesting. I did.

Here is the link to the animation:

http://weather.ou.edu/~kdrake/metr2024/gempak6/animation.gif

Watch the cloud pattern in western/NW Texas. As the day progresses it almost seems as if the chances for dryline development are done with (the Cu field starts to dissipate). But, if you watch over TX there is an advancing (probably upper level, it was behind the dryline) clouds moving NE. As soon as they hit the dryline, BOOM the cells go up within the time frame of 1/2 hour. I don't have access to the upper level data right now but I'd say that was the edge of the jet streak associated with the 500mb shortwave approaching the area. This may be nothing new, and is definitely not uncommon. But I think the thing to take out of this is, if you are a storm chaser... pay attention to these kind of things! I've been kicking myself the last few days for not fully evaluating the conditions on Monday. I think if I did, 1) I would not have been sucked into the cells in N.C. OK that ended up moving right into a very well defined cold pool, and 2) I would have been ready about 5-6pm for that approaching upper level "disturbance".
I think this is a good example of how important upper level dynamics comes into play. It may be nothing surprising to most of you, and its not really to me, but new chasers (like me) can definitely learn how to better forecast real time on the chase day.

 

[Jeff Snyder]

Similar thing happened on 4-1... Storms intiatied right along the leading edge of cirrus, which likely indicated the leading area of enhanced upward ascent ahead of the elongated vorticity max and an area of upper-level divergence.

The other interesting part is that the only two storms that produced tornadoes were the only two whose forward motion almost stopped. Looking at the 0z OUN hodograph, it's little wonder why... Given an E motion at 12 m/s (I think that was the Bunker's right-moving supercell motion) there was strong streamwise vorticity in the 0-1km layer, but the 1-3km layer was characterized by nearly full crosswise vorticity. However, for a storm whose motion was nearly stationary or only a few m/s to the E or ESE (which both the El Reno supercell and the supercell down near the Red River had), there was much more streamwise vorticity in the entire 0-2.5km layer. From Davies-Jones (1984), the ingestion of streamwise vorticity leads to increased updraft rotation...

The question of WHY those two storms slowed to near stationary is left unanswered, though I surmise it probably had to do with locally enhanced convergence along the boundary that lead to upstream development, making it look like the storms were nearly stationary. In addition, there is a feedback mechanism invovled -- stronger rotation in a strongerly veering wind profile would likely lead to enhanced vert pert pressure gradients that would lead to more development to (in this case) the southwest of the updraft, again aiding in keeping the storm nearly stationary (or only moving very slowly).