Day 91: Thin Squall Line

Posted by on 1 Apr 2010 | 3 comments

Day 91

Tonight I don’t have much time, so the post won’t be as long or as detailed as it has been the last few nights.

Last night’s post mentioned convergence along a dryline as being the most likely location for thunderstorms to develop this evening.  Well, that’s exactly what happened.  In the image above, notice how the thunderstorms are located in a thin, narrow line.  This squall line, as it is called, is developing along along a convergence zone very similar to what the model’s in last night’s post predicted.  Note, even though last night’s models did not predict thunderstorms, they did predict convergence.  This is where a skilled meteorologist can make improvements to a model forecast by having an up-to-date understanding of the situation.

Update: Please read the first comment from the Storm Prediction Center’s (SPC) Warning Coordination Meteorologist, Greg Carbin.  He points out that I was making extreme oversimplifications regarding the evolution last night, and is absolutely correct.  My response to his comment is below.

  • http://wxvu.net/20100401/WUNIDS_map.gif Greg

    Patrick,

    You should know that it’s never so simple as “convergence along a dryline”. In fact, what appeared to aid in the initiation of thunderstorms was a collision of what appears to be a retreating dryline and an advancing cold front over western Kansas. You can see the two boundaries heading toward each other, colliding, and initiating deep, moist convection in this long loop that I saved from wunderground.com last night:

    http://wxvu.net/20100401/WUNIDS_map.gif

    Most of the models depicted this scenario correctly but were about 3-6 hours too slow with storm initiation (actual initiation was about 03Z and most guidance indicated convection at 06Z-09Z). Additionally, most guidance suggested the convection would remain west of the OKC metro through early Friday. Obviously, the guidance was wrong there too!

    Overall, this case was not much of a surprise. We (SPC) expected storm initiation in an environment characterized by strong dynamic forcing for ascent where available CAPE would be near 1000 J/kg and shear would be sufficient for storm persistence/organization. The primary severe weather expected was marginally severe hail around 1 inch in diameter. With the possible exception of perhaps the last 1-2 hours of the 24h forecast when bowing structures in the squall line (farther east than expected) posed some threat of high winds, hail is what we got.

    http://w1.spc.woc.noaa.gov/climo/reports/100401_rpts.html

  • http://www.patricktmarsh.com pmarsh

    Hi Greg,

    You’re correct, the thunderstorms last night most likely developed along the converging dryline and cold front. It was lazy/sloppy writing on my part to leave the impression that the dryline was the sole factor responsible for the convergence. What I should have said was that the models forecast increasing convergence on the dryline during the evening that would eventually become strong enough to generate convection. As to what caused the increase in convergence along the dryline, well, that’s where the collision of the two boundaries (cold front / dryline) came in to play. I did not adequately (or not at all) explain this in the post. That’s what I get for trying to do it at the last minute when I was tired.

    Thanks for holding me accountable!

  • Greg Blumberg

    Thank you Greg for clarifying that. I noticed in the radar velocity data last night that the squall line had some very odd features to be purely dryline convergence. It did have a convergent signature initially in the convection just a bit north of DDC and mostly south of it, however after the line passed over the DDC radar site, the velocity pattern shifted to a look similar to when there is a cold front passage over a radar site. I could not understand why at that time. In retrospect, I probably should have spent time analyzing the surface data.

    I don’t have any exact data from the event or graphics, but maybe either of you two saw what I did in the radar data. If you did, was my interpretation of the radar data depicting the cold front passing over correct? I hope my question and description isn’t too vague.