Severe Weather Discussion 2010-5-1

Severe Weather Discussion for April 28 – May 1, 2010.
Valid: 04/26/2010.

Highlights:

– Severe thunderstorms are possible on Thursday across western/central
Oklahoma.

– Severe thunderstorms are possible on Friday across central/eastern Oklahoma.

Discussion:

Model disagreement continues regarding the evolution of a significant
longwave trof across the western U.S. during the forecast period. GFS
remains quite robust in blasting the trof into the central U.S. but
has recently grasped onto the ECMWF's idea of hanging back trofing
over the southwest U.S. Friday and Saturday. The NAM is slightly
slower than the GFS at 72 hours and the ECMWF is slower still than the
NAM. Ensemble plots rapidly diverge at 96 hours, so there is little
help beyond Thursday on the ultimate evolution of the western U.S.
trof. The hemispheric pattern looks to be a 5-wave and this is likely
causing a significant amount of problems for the models. The ECMWF
has been pretty reliable the past several days but admittedly may be a
little to slow with the shortwave trof on Thursday as opposed to the
NAM's 4-corners region. This forecast is based on the NAM with much
consideration of the ECWMF and a slight nod to the GFS. Given time of
year and expected winds aloft, severe weather is possible with
sufficient moisture and this will be big player through the forecast
period.

All of that being said, models are close enough in agreement for
Thursday to warrant special discussions. SPC Day 4-8 product
highlights much of Oklahoma for 30% combined severe threat and this
looks good. 12z NAM has a dryline near Elk City with 60-65F dewpoints
just east of said feature. Recent frontal intrusion and current one
have/will push tropical moisture to the southern Gulf. However,
modest moisture will remain near the Gulf coast and poised to return
northward on a 40/50kt LLJ. All three models show at least 60F
dewpoints east of the dryline by Thursday afternoon with CAPE values
at least 1500 J/Kg. Some models get near 2000 J/Kg and 850mb
dewpoints are around 13-14C. 18z NAM just rolling in and continues
prior trends. It now shows 65F dewpoints making to southern OK by
evening, so this will need to be monitored. Along with CAPE values a
little above 2000 J/Kg. Sfc-500mb crossovers are greater than 45
degrees and in-general winds are impressive. Given degree of
instability, directional/speed shear, and timing of the event I will
bump up the moderate probs just a little. I don't think SPC will do
it on the Day 3, but who knows. Moisture will likely remain an issue
until we know for sure what happens along the Gulf coast. Either way,
it appears that a medium-end event is setting up for Thursday.

Model disagreement beyond Thursday makes any attempt at forecasting
futile. As such, I'll keep storms in the forecast for Friday and
Saturday preferring to use future discussions to determine weather
threats. HPC/local forecasters prefer the ECMWF and I see no reason
to differ.

Probabilities:

Risk area for any part of OK/TX (valid: 04/27 Day 3):

Slight: 100%
Moderate: 20%
High: N/A

Chase Readiness:

Level 1 – Normal, watching Thursday

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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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