Weekly Severe Weather Discussion

Severe Weather Discussion for April 13 – 19, 2009.
Valid: 04/12/2009.

Highlights:

– Showers and thunderstorms are possible Thursday – Saturday. Severe
thunderstorms are possible and even a medium-end event. However,
model agreement is poor making efforts to delineate an area difficult.

– Temperatures will be near 60F on Monday, warming to the low 70s on
Tuesday, then upper 70s Wednesday – Thursday. Friday will be in the
mid 70s, with a cold front by Saturday. Temperatures Saturday low 60s
and Sunday mid 60s.

Probabilities:

– Severe Thunderstorms in Oklahoma: 30%
– Storm Chase: 20%
– Winter Precip: < 5%

Discussion:

Very complex setup this week as all models indicate another closed low
will impact this region. The problem seems to be a pattern change,
which may feature a ridge in the west and trof in the east by next
weekend. The rather reliable ECMWF brings the closed low into the
western U.S. and slowly moves it east and even develops a rex block in
the western U.S. It has held to this solution for a few runs. The
GFS on the other hand is rather progressive with the closed low
initially and then slows it down late in the period. Although, it
does not develop a rex block, but a western U.S. longwave ridge. The
18z NAM favors the ECMWF at 84 hours. I believe the solution is
somewhere in the middle of all the models. Further, with the
disagreement, I'd hesitate to hang my hat on one model.

A cold front will move through the state tonight, although impacts
will be limited as we have already been cool today. The clouds should
clear, allowing temperatures to warm on Monday with a continued
warming trend through Thursday. Moisture will get pushed to the Gulf
with the current cold front. There are differing views as to how far
moisture will get pushed into the Gulf. Return flow should setup
moisture return by Wednesday with low 50s dewpoints moving northward
by Wednesday night.

Thursday – Saturday is where model problems greatly impact the
forecast. At this time I think a compromise of the models will be the
best solution, with a dose of persistence. I'm going with the ECMWF's
timing through Friday morning and the GFS's speed beyond that point.
This would seemingly point to severe weather chances Thursday for the
panhandles and statewide on Friday. Confidence is much lower than
usual. If moisture returns as expected by late Thursday as progged by
the GFS, then severe thunderstorms would be possible in the
Texas/Oklahoma panhandles. Presence of strong flow aloft and
directional shear would provide a favorable environment for organized
severe thunderstorms. Degree of instability remains a question this
far out, but if 12z GFS CAPE values materalize then a medium-end event
would be possible.

Seemingly, a similar setup would be possible farther east on Friday.
However, the ultimate evolution of this stage is quite complex.
Thursday convection and occlusion of the surface low will likely muddy
possible impacts. If a medium-end threat develops, special
discussions will be needed.

Saturday will likely be showers if anything, I went ahead and left
POPs in the forecast as the upper system should be exiting the region.
Sunday will be sunny.

12/18z NAM are followed through Wednesday, 12z GFS Thursday, with some
modifications Friday – Sunday.

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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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