Severe Weather Discussion – 2008-8-5

Severe Weather Discussion for April 9 – 10, 2008.
Valid: 04/08/2008.

*Significant Severe Weather Event for Texas Tomorrow*

– Outbreak of severe thunderstorms and tornadoes is expected for parts
of Texas Wednesday afternoon through early Thursday. Primary threats
are strong tornadoes, destructive hail, and damaging winds.

– A continuation of the above outbreak is expected for parts of
Louisiana, Arkansas, and Missouri on Thursday.

– Excessive rainfall event is likely for much of Oklahoma Wednesday
afternoon through early Friday. Rainfall of 2-3 inches will be common
with localized amounts up to 4 inches.

Discussion:

Large longwave trof is in the process of developing across the western
U.S. this evening. Farther east in OK/TX, a cold front has pushed
into central Texas per evening surface analysis. An area of strong to
severe thunderstorms continues along the front from east Texas
northeastward.

As the incoming longwave trof undergoes significant amplification, the
surface will respond. A 995mb surface low is expected in the
panhandles by Wednesday evening with a dryline extending southward to
the Big Bend area. A warm front will be located just south of the Red
River Wednesday evening and should be moving north. The position of
these boundaries will provide for a large warm sector. Lack of
recently frontal intrusions into the Gulf will yield 70F dewpoints
moving northward behind the warm front. There may be a double warm
front structure tomorrow as the initial front shifts winds to
southeasterly and the secondary front represents true tropical moisture.

As the upper system moves towards the region on Wednesday a 30-40 kt
jet will develop, assisting in rapid transport of moisture. By
afternoon, deep layer moisture is expected across much of Texas. CAPE
values are progged from 1500-2500 J/Kg across the warm sector, with
LIs to -9. Showers and thunderstorms may be ongoing during the day as
lift develops above the cap. Assuming that clouds clear, discrete
storm development is likely. Given very impressive speed/directional
shear, supercells are likely. 200mb winds of 100kts will limit anvil
seeding.

The primary complication for a significant event tomorrow is expected
shower activity across the warm sector. However, with the upper
system west of the area, there may not be enough lift to overcome the
cap until the afternoon hours or sustain elevated convection above the
cap. Tough call at this point, but this will be monitored. SPC has
posted a moderate risk across much of northern Texas with a slight
elsewhere except southeast. This looks good and this situation does
not warrant a high, but I went ahead with low probabilities anyway.

Oklahoma still gets to play in this situation, but it looks to be on
the heavy rain side. With the warm front in Texas, the very moist air
will be lifted over this boundary, resulting in widespread
thunderstorms. Presence of elevated instability may yield severe
thunderstorms with hail to golfball size the primary threat. I
disagree with the models and others that the front will get very far
into Oklahoma. It really depends on cloud cover and rainfall
tomorrow. If we are clear, then the front should move into the state.
Otherwise, the cold sector will likely be reinforced by clouds and
rain. Despite where the warm front ends up, excessive rainfall is
likely for much of the state. Given frequent rainfall events in
eastern Oklahoma, flash flooding is a concern.

The threat should shift east on Thursday, but may remain in eastern
parts of the region. Depending on the ultimate setup, SPC may do a
high risk just east of the region on Thursday. A limited area of the
region will be impacted, but I went with 5% anyway.

Probabilities:

Risk area for any part of OK/TX (valid: 04/09 Day 1, Day 2):

Slight: 100%, 100%
Moderate: 100%, 100%
High: 5%, 5%

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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