Severe Weather Discussion 2007-7-3

Severe Weather Discussion for April 13, 2007.
Valid: 04/12/2007.

*Significant Severe Weather Event is possible Tomorrow*

Highlights:

– Outbreak of severe thunderstorms and tornadoes is expected across the eastern half of Texas on Friday. This is a potentially dangerous situation with the likelihood of widespread severe weather. Primary threats are tornadoes (some long-track), destructive hail, and damaging winds.

– Severe thunderstorms are possible across the southern half of Oklahoma on Friday with hail to nickel size the primary threat.

– Excessive rainfall may result in flash flooding across parts of Oklahoma Friday into early Saturday.

– Snow accumulations are possible in northwest Oklahoma late Friday into Saturday, accumulations should be less than three inches.

Discussion:

No change to prior forecasts. Situation for Friday continues to look prototypical (classic) of a severe weather outbreak. This one will be localized to the eastern half of Texas, generally east of I-35. Southeast Oklahoma may get in on the action, but low confidence of that part of the forecast negates mentioning it. Also, I do not want to dilute the potential in Texas.

Robust deep layer moisture return will commence tonight as previous surface ridge moves east and the Gulf opens up in response to pressure falls across the region. Upper system responsible for this action will progress rapidly from the southwest U.S. into the central U.S. by late Friday. A wide variety of weather conditions are expected across the region during this system.

A warm front is expected across north Texas Friday morning with widespread convection across Oklahoma in response to 40-50kt low level jet. Warm front location Friday afternoon is difficult to pin down right now as convection to the north will govern the moment of this boundary. The boundary's location may range from I-20 northward to the Red River. Friday morning conditions will provide ample information on what this boundary should do.

Farther west a surface low should move into northwest Texas by late afternoon with a dryline west of I-35. East of the dryline dewpoints will be in the mid/upper 60s with 850mb dewpoints AOA 15C. Models generally agree that CAPE values will be AOA 3000 J/Kg with LIs below -6. There are some concerns regarding the thermodynamics due to widespread cloud cover and/or shower activity. Models seem to be struggling with the cap (EML) as they did a few weeks ago. Tough to assess this, but my belief is that warm sector shower activity may not be as robust as models prog. There will be lots of lift ahead of this system due to very strong wind fields aloft. However, with the warm front in north Texas and the upper system slower than originally progged, the cap should be able to contain convection until late afternoon.

If current indications hold, severe thunderstorms will erupt Friday afternoon along the dryline and warm front. These storms will move east in very favorable speed/directional shear along with favorable thermodynamics. East/west position of the warm front lends concern to long track tornadoes for any storm(s) that can anchor on this boundary. Dryline storms may initially struggle to produce tornadoes, but as storms increase and produce boundaries, tornadoes should become more common. Model progged soundings indicate 0-3km helicity values above 350 m^2/s^2 at Waco and 0-1km values around 220 m^2/s^2. This combined with any boundary interactions will lead to favorable conditions for long-track tornadoes.

SPC has a moderate risk up for generally College Station northward to Ardmore and east to Louisiana. A slight risk covers all of eastern Texas, including Houston/Galveston. If the area was a little bigger, I think SPC would do a high risk. for now, I'm putting 40% on the high risk since conditions are favorable. Morning convection in the warm-sector will solve this issue.

Probabilities:

Risk area for any part of OK/TX (valid: 04/13 Day 1):

Slight: 100%
Moderate: 100%
High: 40%

Chase Status:

Level 1 – Normal – a little far south for this group.

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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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