Severe Weather Discussion – 2008-7-5

Severe Weather Discussion for April 3, 2008.
Valid: 04/02/2008.

Highlights:

– Isolated severe thunderstorms are possible tonight. Primary threat
is large hail.

– Active severe weather day is likely on Thursday for much of Texas
and the southeast 1/2 of Oklahoma. Primary threats are destructive
hail, tornadoes, and damaging winds.

Discussion:

12z models changed the pattern rather significantly and the 18z models
have continued this trend. While it does not ruin the event, the
threat has shifted towards large hail as opposed to tornadoes.

Intense WAA has resulted in widespread showers and thunderstorms
across the area this afternoon. This activity is expected to continue
and eventually shift east overnight. The primary threat will be large
hail. Excessive rain is also possible, which may cause flooding
issues in eastern Oklahoma.

Thursday continues to be a potent day with the potential for a
significant severe weather event. Models have caught onto a
complicating issue relative to how the southern stream shortwave trof
will phase with the northern stream system over the region on
Thursday. A lead shortwave trof will ejected eastward out of the mean
longwave position early Friday. This will assist a 1023mb surface
ridge in pushing into Oklahoma during the afternoon. The cold front
associated with this ridge should be near I-44 by mid-afternoon.
Looking back west the main system will approach resulting in a surface
low developing in the Texas panhandle. This low will have a dryline
attached to it somewhere in southwest Oklahoma southward to into the
Big Bend area.

Dewpoints will be above 60F across the area south of the cold front.
Temperatures should be in the low/mid 70s across much of the area.
This will yield CAPE values in the 2500-3000 J/Kg range. Complex
500mb pattern noted above will create an issue with speed shear as the
850/700mb flow appears weaker than earlier thought. Despite this
issue, there will be ample turning, especially with expected backed
flow in southwest OK/northwest Texas. This setup favors destructive
hail and it appears SPC and NWS offices are on the same page. Day 2
17:30z was a great read and certainly picked up on the model changes.

NAM really drives the moisture out of here at the surface and 850mb.
I am rather suspicious of this solution, especially with the GFS
farther west. Norman has continued the fire weather watch for three
counties in southwest Oklahoma. I can't argue with it and contrary to
my prior discussion the dryline may very well make it to Altus before
the surface low reforms.

I do not expected many changes on the 00z models, but will be looking
closely at the NAM for items of interest. The cap has yet to shutdown
convection, given ongoing activity. However, ongoing activity will
have very little instability to work with and will most likely not be
severe. Current radar shows the back edge at I-35, moving rapidly east.

Unfortunately I do not have any case studies to compare this event
too. So, if anything it will make a good case study for the future.

SPC's moderate risk looks good, I'd go with a 45%/10% hatched area
tomorrow for hail, pretty much same area as depicted. The tornado
threat seems more around 15%, but we'll see what they think in the
morning. Damaging winds will probably be around 30%.

Chase wise, I think a trip to Lawton is a good idea. If anything,
this will be a good hail hunt. Although, I'm willing to get into
southeast Oklahoma where conditions will be better for rotating
storms. Thinking that departure time around 14:00 will work. Surface
evolution will be key to what we do and I certainly think the chase
target will be shifted some direction.

GFS and ECMWF agree on the development of a longwave trof in the
western U.S. next week. ECMWF would hint at severe around the 9th,
while the GFS hits the 8th. This will need to be monitored as lack of
Gulf intrusions and persistent southwesterly mid-level flow will
provide favorable environment for severe thunderstorms.

Probabilities:

Risk area for any part of OK/TX (valid: 04/03 Day 1):

Slight: 100%
Moderate: 100%
High: 0%

Chase Status:

Level 3 – Planning on going to Lawton.

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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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