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Quincy

Severe Weather May 20-25, 27th, 2020

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The storms over the Yucatan peninsula attained overshooting tops of -90C by my estimation, based on the color scale (bottom of this image). Based on models, this tropical atmosphere had an equilibrium level of 15 kilometers, at -70C.

2duX4er.jpg

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I've taken a closer look at tomorrow's threat across Texas...

Area averaged sounding from the 00z hrrr at 20z/3 p.m. CDT for approximately a four county area just west of Waco shows that convective initiation (CI) is imminent. Convective temperatures are in the mid-80s and that is the same from other high resolution CAMs as well. 

Note that low-level shear is weak and even the 850mb jet is much weaker than you'll see under most ENH risk areas. With that said, deep layer shear is more than sufficient for supercells and moderate CAPE profiles thanks to cool temperatures aloft should result in several large to potentially very large hail producing storms. While not particularly large, 0-3km hodographs are enlarged, thanks to NW winds at 500mb backing to S/SE at 850mb. In addition to weak low-level shear, seasonably lackluster dew-points seem to imply that the tornado threat is very limited during the first few hours of storm development. (dew only around 60F in the initiation zone at this latitude in late May is unimpressive)

Most CAMs show an initially supercellular storm mode, but the NSSL and multiple WRF members show clustering/upscale growth within a couple of hours. The HRRR, HRRRv4 and 3km NAM hold onto a semi-discrete mode until at least 22-23z, before things get messier toward 00z. It's hard to predict MCS evolution since the CAMs are not in the best agreement, but I would think that the highest risk for a damaging MCS would be near/east of I-35 after 22z. An MCS seems very likely by 00-01z, moving toward the Texas coast during the night.

Basically, on the lower end of expectations, I'd think there are a couple of initially supercellular storms that produce large to possibly very large hail before storms grow upscale. If storm modes remain more discrete, as storms encounter an air-mass with richer low-level moisture to the southeast and the low-level jet, then the tornado threat may increase. If we dissect the tornado threat, I'd say it's very low, but non-zero through 6 p.m. and then increases toward sunset, assuming storms are still at least semi-discrete by that point.

 

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Considered making the drive from Kansas down to central Texas for tomorrow, in what will very likely be the only central/southern Plains severe threat for at least a week or week and a half. But decided to hold off in lieu of hope for better northern plains potential later on in June. As Quincy referenced, in spite of what will probably discrete/semi-discrete storm modes for a few hours after initiation, dew-points are very unimpressive tomorrow for central Texas, and low-level shear will be pretty mediocre until roughly 00z -- and at that point storms will probably be a sloppy mess. Ultimately there will likely be beautiful supercells tomorrow, but much like Friday of last week, these supercells will likely struggle to obtain strong enough low-level rotation to produce notable tornadoes. All the twitter talk about a 10% sig tor for tomorrow completely ignores low-level thermos/kinematics that are critical for tornadogenesis in favor of mixed CAM output showing potentially discrete storm modes.

Tomorrow probably is not worth a 7 to 9 hour drive down there for me... unfortunately this is how May 2020 severe chances will end on the plains. Unless tomorrow pulls a rabbit out of the hat, we will end up with the lowest number of May U.S. tornadoes since 1970. 

FWIW I'll go with a hypothetical starting point/target of Temple, TX.

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