Severe Weather Discussion 2009-5-2

Severe Weather Discussion for April 25-26, 2009.
Valid: 04/24/2009.

*Active weather expected through the weekend into next week*

– Severe thunderstorms are possible in far western Oklahoma and the
panhandle Saturday afternoon and evening. Primary threats look to be
hail to tennis ball size and isolated tornadoes.

– Severe thunderstorms are possible across much of western and
central Oklahoma on Sunday. Primary threats are large hail, excessive
rainfall, and isolated tornadoes.

– Severe thunderstorms may occur each day, however only a few days
are expected to present medium-end impacts. Concerns develop relative
to multiple medium-end events across the state due to the variety of
severe weather expected over several days.

– Chase Status is increased to Level 2.


All models finally agree on the development of a western U.S. longwave
trof this weekend. Last cold front intrusion into the Gulf was Sunday
and it was a significant intrusion. Increasing sun angel and a rapid
return of onshore flow will provide a favorable fetch of Gulf moisture
across the state. Model data indicates that impressive moisture
return will occur from the surface up to 850mb Saturday through next
week. The western U.S. longwave trof is expected to remain anchored
through at least Thursday 04/30. As such, the deep return flow of
Gulf moisture will continue across Texas with cold front intrusions
into the Gulf unlikely.

Morning model data indicates that periodic shortwave trofs will be
somewhat out of phase with peak heating. This time of year convection
is less dependent on these features with subtle atmospheric ripples
more meaningful. The location of the cold front to the north will be
instrumental in where the heavy rain threat develops. This part of
the forecast is still 4 days out and refinements will be needed the
next few days.

18z NAM hasn't offered much new to the overall forecast. It does show
a different scenario for Sunday with a shortwave trof ejecting
northeast during the afternoon hours. This would provide a very
favorable environment for severe thunderstorms. Either way, both
Saturday and Sunday will be monitored for chasing. Sunday's forecast
is complicated by expected convection Saturday evening and the NAM is
rather robust with convection Sunday morning. I have adjusted the
dryline west on Saturday as I don't think it will reach the OK/TX

SPC has a 10% hatched area and a 30% area on the Day 3. Kind of
unusual and indicates they're thinking of a moderate risk for
Saturday. However, I'm going to hold off a day. The threat area does
remain the same as noted yesterday and the SPC Day 3 reflects the same
thinking. Day 4 (Sunday) is essentially the same and I'd think we'll
see a similar forecast.


Risk area for any part of OK/TX (valid: 04/24 Day 2, Day 3):

Slight: 100%, 100%
Moderate: 30%, 0%
High: 0%, N/A

Chase Status:

Level 2 – Monitoring Saturday with interest in the Shamrock, Texas
area to Pampa.

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About Putnam Reiter

Putnam has been storm chasing since 1990 and is a co-founder of For his day job, Putnam works in emergency management for information technology.
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