Severe Weather Discussion 2007-8-1

Severe Weather Discussion for April 21st and April 23 – 25, 2007.
Valid: 04/20/2007.

Highlights:

– Active severe weather day expected on Saturday across the Texas/Oklahoma Panhandles. Destructive hail and strong winds are likely. The tornado threat is very conditional, but SPC indicates potential for isolated strong tornadoes.

– Severe thunderstorms are possible Monday – Wednesday across the region. Tuesday appears to be the most significant threat.

Discussion:

Models continue prior trends of depicting a negative tilt shortwave trof lifting northeast into the panhandles on Saturday. All things considered this is a very favorable setup for severe thunderstorms. However, the limiting factor from a high risk and/or outbreak of tornadoes is lack of deep layer moisture. This isn't to mean that there won't be decent moisture in-place. 12z NAM/GFS runs indicate that surface dewpoints will be around 55F with 850mb dewpoints around 11C, with PW values near 2.5 – 3cm. The moisture values here are in contrast to the event on March 28th when dewpoints where near 60F and 850mb dewpoints near 14C. Another issue is directional shear across the threat region, although speed shear should be quite nice.

Current indications are that strong daytime heating will occur near the dryline resulting in it mixing to the east during the late moring/early afternoon hours. There is some hint of mid-level clouds, but model RH's aren't that high. The dryline should arc from near Guymon south-southwest to west of Lubbock by early afternoon. Combination of deep layer lift due to height falls and well-timed speed maxes should be sufficient to breach the cap by mid-afternoon. Severe thunderstorms should rapidly develop given CAPE values around 1500-2500 J/Kg and LIs to -6. Storms may evolve very similar to that of March 28th, but too close to call given reduced moisture and less directional shear. If the cap can hold, discrete supercells are likely with low-precipitation supercells the primary type. This would point to a destructive hail issue for the threat region.

Storms should then congeal into an MCS and track east during the night as the negative tilt shortwave trof continues to lift northeast. The severe weather threat will continue into western Oklahoma and central Kansas by early Sunday morning. Storms should decrease in intensity as they move into central Oklahoma/eastern Kansas around 3-6am Sunday.

Attention then turns to next week, which looks active for the region. 12z GFS has come around to the ECMWF's more progressive solution with our next system. As such, the potential for another significant severe weather event will exist. Moisture should be better with southeast trajectories off the Gulf. 12z NAM places the dryline in the western Texas/Oklahoma panhandles with 60-65F dewpoints to the east of this feature. 850mb dewpoints will be 14-16C, resulting in CAPE values of 2000-3000 J/Kg and LIs to -8 across this region. Another issue to watch is the warm front the NAM shows in northern Oklahoma during the afternoon. These two regions may be very active given deep layer near-tropical moisture across the region. There seems to be some potential for significant severe weather on Monday if current progs hold true. NAM shows deep layer shear near the warm front in northern Oklahoma. Kind of early to get into so much detail, but I'm seeing some strong severe weather
signals for this day.

Tuesday may end up being an issue of what happens on Monday. Neutral tilt closed low is progged to move east into the region during the day Tuesday. Models move the dryline into central Oklahoma by late afternoon. Given a strong PVA gradient with this system, not sure if the cap can hold. The GFS develops lots of 0.05 QPF across the region, but this is typically moisture return. And, we saw a similar situation last Friday for Texas when the models developed lots of QPF during the overnight and early morning hours, with nothing happening. Placement of the system would be favorable for an outbreak of severe weather if clouds and rainfall do not reduce daytime heating. This has some similarities to April 19, 2003 and possibly April 24, 1994. It should be noted that Tuesday is April 24th.

PRIND: It certainly appears that Saturday, Monday, and Tuesday all favor severe weather outbreaks across the region. At this time I do not think a chase is necessary for Saturday, but Jay had better go. Monday and Tuesday will be closely monitored given proximity to Oklahoma City.

Probabilities:

Risk area for any part of OK/TX (valid: 04/21, Day 1 and Day 3):

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

Chase Status:

Level 1 – Normal

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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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