Severe Weather Discussion 2007-8-2

Severe Weather Discussion for April 21st and April 23-24, 2007.
Valid: 04/21/2007.

Highlights:

– Active severe weather day to begin shortly across the Texas panhandle northward to Nebraska. Primary threats are destructive hail and isolated tornadoes. Storms should progress east and after dark, the primary threat will be large hail and damaging winds.

– Severe thunderstorms are possible Monday and Tuesday across the region.

Discussion:

No change to prior forecast or reasoning regarding the setup for today. SPC has a moderate risk up for the panhandles into Nebraska. If dewpoints were a little higher, this would be a high risk. Clouds have held tough near the cap rock and expect them to erode during the next few hours. This has caused a temperature sink in this area, but right now that is not an issue as the dryline is in eastern New Mexico. Little in the way of CU is noted along the boundary right now, but expect that to change by 4pm. Visible satellite shows deep layer lift moving through New Mexico and this should overspread the dryline by late afternoon. Rapid thunderstorms development is expected by 5pm with storms moving northeast. Current indications are that storms will remain discrete for a period of time and then congeal into a squall line by evening. Said squall line should move east across the region as negative tilt shortwave trof lifts northeast. SPC has a 10% hatched area on the tornado
graphic and wouldn't be surprised to see one or two strong tornadoes today. However, lack of moisture should greatly limit this threat. Also, in the panhandles temperature/dewpoint spreads will approach 25F, resulting in high LCL's.

System moves east of the area on Sunday with moisture getting a modest push into Texas. Next longwave trof moves into the western U.S. on Monday and draws moisture back north. A dryline should once again develop in the panhandles, actually New Mexico during the day. Temperatures should warm into the 70s with dewpoints in the upper 50s. Speed and directional shear will improve dramatically during the day. Yesterday I noted the warm front in Oklahoma, but the models disagree on this issue. SPC Day 3 has sided with the GFS, which is much farther north than the NAM. This fight continued at 12z today and there is no reason to pick one over the other. The GFS is faster and farther north and the NAM is slower and farther south. We'll probably have to wait until Monday before we know. However, this setup look prototypical (classic), like many of the others I've talked about recently. In fact Monday/Tuesday look like a 1991 Day 1/Day 2 event.

Attention then turns to Tuesday, which is rather muddied by the models right now. Especially given timing issues noted above. However, despite the timing issues, the dryline should advance east into at least the eastern Texas panhandle by late in the day with CAPE values around or above 3000 J/Kg and LIs to -8. Impressive speed and directional shear will be in place with the upper system approaching late in the day. All of this should be sufficient for severe thunderstorm development during the afternoon. Big caveat on Tuesday is debris clouds from Monday, shower activity, or just clouds. No way to know, but the GFS has backed off on the QPF. As noted yesterday the widespread QPF on the GFS is usually a sign of intense moisture return. The NAM QPF progs look out to lunch, but won't ignore them at this point.

Monday and Tuesday both look like medium to high-end events. I think Monday is a little far west to raise the chase level, especially if the warm front moves north of the state. I'd think the warm front won't have much trouble moving north given strong heating across the region the past few days. Not like we're gonna have to get rid of cool air to move the thing north.

SPC Day 3 looks good and you rarely see a 10% hatched area. I'm thinking a moderate risk will be sported on the new Day 2 and frankly see no reason not too. Tuesday will probably get another hatched area but a slight risk given Monday evening convectiion issues.

Probabilities:

Risk area for any part of OK/TX (valid: 04/22 Day 2, Day 3):

Slight: 100%, 100%
Moderate: 100%, 30%
High: 10%, N/A

Chase Status:

Level 1 – Normal – will likely increase it tomorrow.

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About Putnam Reiter

Putnam has been storm chasing since 1990 and is a co-founder of Hook-Echo.com. For his day job, Putnam works in emergency management for information technology.
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