Severe Weather Discussion 2009-5-1

Severe Weather Discussion for April 25-26, 2009.
Valid: 04/22/2009.

Highlights:

– Severe thunderstorms are possible Saturday afternoon and evening
across the eastern Texas panhandle/far western Oklahoma northward into
Kansas.

– Severe thunderstorms are possible across northwest Oklahoma on Sunday.

Discussion:

Model disagreement resolved somewhat with the 18z GFS showing a
pattern closer to the ECMWF than previous runs. The cold front
advertised by the GFS for the past few days appears to remain in
Kansas through Monday. This would make sense given southwest flow
aloft and nothing to push the front southward. Issue then turns to
severe weather potential. The NAM is now the slowest of the models
with the upper system as it closes the low off over southern
California. This solution is an outlier and is not used for this
forecast. I think the ultimate outcome will be somewhere between the
ECMWF and the GFS.

What appears likely at this point is moisture return under increasing
flow aloft. Temperatures in the upper 70s to low 80s along with
dewpoints in 60s should produce CAPE values around 2500 J/Kg,
primarily northwest Oklahoma per the 18z GFS valid Saturday evening.
LIs are in the -6 to -8 range. GFS sounding from Gage shows CAPE of
1900 J/Kg with CIN at -49 J/Kg around 7pm. Using a surface
temperature of 79F and dewpoint of 58F, both of which are probably
underdone. Wind fields look favorable for rotating storms given
south-southeast flow at the surface, veering to south at 850mb, then
southwest at 700mb, and west-southwest at 500mb. Due west winds at
200mb would reduce the chance of anvil seeding.

Overall model progs appear supportive of severe thunderstorms along
the dryline Saturday afternoon and evening. I think the GFS is too
far east with the dryline and prefer a location westward just inside
the Texas panhandle. The dryline will likely be arched to the
north-northeast. Now, the GFS does indicate a dryline bulge and this
is certainly possible. The biggest factor for storms will be enough
lift to breach the elevated mix layer. However, CIN values less than
-20 J/Kg are easily breached by microscale events. The GFS does
develop QPF along the dryline Saturday afternoon.

PRIND: A slight risk will be issued on the Day 3 due out Thursday
morning. The area will cover parts of northwest Texas northward
across eastern Texas panhandle/western Oklahoma into central Kansas.

The severe weather threat looks to continue into Sunday per the medium
range models. Sunday does become more difficult due to GFS progged
weakening of the mid-level winds and westward movement of the dryline.
I'm not convinced the GFS is accurate with the progged wind fields
anyway. I frequently see the models underestimating wind field this
far out. Conditions certainly appears favorable for severe
thunderstorms again across the Texas panhandle and northwest Oklahoma
on Sunday. Sunday could end up being a medium-end event depending on
the ultimate frontal behavior and wind fields. The primary item will
be a continued increase of moisture with the GFS showing dewpoints in
the low/mid 60s statewide. The GFS is likely under doing CAPE values
across the state due to lots of cloud cover. The new Day 4 should
incorporate the Texas panhandle dryline along with the cold front over
southern Kansas/Oklahoma.

The threat for severe thunderstorms may continue into Monday, but at
this point I'm not convinced and it is well beyond my forecast time
period.

I am cautiously optimistic that Saturday and/or Sunday will present
chasing opportunities for the group. We'll have to see how things
evolve and the 00z NAM will arrive in about 100 minutes, not that I'm
counting.

Probabilities:

Risk area for any part of OK/TX (valid: 04/23 Day 3):

Slight: 100%
Moderate: 0%
High: N/%

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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