Severe Weather Discussion 2010-4-1

Severe Weather Discussion for April 22-23, 2010.
Valid: 04/19/2010.

Highlights:

– Severe thunderstorms are possible across the eastern Texas Panhandle
and western Oklahoma on Thursday. Additionally, severe thunderstorms
are possible across southern Kansas and parts of west Texas.

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

Discussion:

18z NAM continues prior model trends of developing a large western
U.S. closed low by late Tuesday. Models then translate this system
slowly east for several days. There are still differences among the
models, with the ECMWF the slowest. The NAM/GFS are seemingly closer
to each other and for now appear appropriate. This forecast is based
on these two models, with consideration of the ECMWF's prog.

Models seem to be finally catching on to the amount of moisture
available as the upper system inches closer to the region. 18z NAM
shows 65F dewpoints across western Oklahoma near the dryline over
TX/OK border. NAM also shows a warm front over northern Oklahoma.
This is a potential fly in the ointment as this will formerly be a
cold front on Wednesday driven by a northern stream shortwave trof.
Initially, the southward push should weaken but as the LLJ increases
late Wednesday, storms may develop on this boundary. Said storms
could retard northward movement or even result in some southward
displacement. These mesoscale details are quite difficult to assess
this far in advanced and for now will accept model progs near the
KS/OK border.

NAM develops precipitation across parts of Texas from what looks to be
some support in the subtropical jet. This probably is a non-issue for
Oklahoma as development looks to be rather isolated. There does seems
to be a lot of rain along the warm front, but that is a given this
time of year. The dryline and warm front look to meet-up over
northern TX panhandle/Oklahoma panhandle by Thursday afternoon. NAM
progs CAPE values greater than 1500 J/Kg across much of western
Oklahoma, with isolated amounts above 2500 J/Kg. This appears
reasonable if temperatures warm to expected values and of course
moisture is present. Deep layer moisture appears likely given 850mb
dewpoints of 12-15C.

Wind fields may be a problem, but the NAM typically under-forecast
winds this far out. This is noted by an increase in the atmospheric
wind profile between the 12z and 18z runs. Surface-500mb crossover is
at least 45 degrees on the NAM/GFS. Helicity values are marginal on
the NAM, but this may change over the next few model runs.

Current indications are that thunderstorms will be ongoing Thursday
morning in northern Oklahoma and likely continue during the day. The
front is expected to end up near the KS/OK border by late Thursday but
this is subject to drastic changes depending on convective activity.
Thunderstorms will develop along the dryline in the Texas panhandle
Thursday afternoon with wind fields/thermodynamics possibly favoring
supercells.

Model solutions make forecasting for Friday quite difficult. Going
with the faster GFS, still yields a severe weather threat. Models are
also getting a handle on cap (EML) strength and this will be vital to
severe weather on Friday, along with convection on Thursday. Wind
fields certainly support an active severe weather day and marginal
thermodynamics may be enough.

Probabilities:

Risk area for any part of OK/TX (valid: 04/20 Day 3):

Slight: 100%
Moderate: 0%
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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