Weekly Severe Weather Discussion

Severe Weather Discussion for April 28 – May 4, 2008.
Valid: 04/27/2008.

Highlights:

– Isolated severe thunderstorms are possible on Wednesday across
western Oklahoma/Texas panhandle.

– Severe thunderstorms are possible across much of
Oklahoma/Kansas/Nebraska on Thursday.

– Temperatures will be near normal much of the week in the mid 70s,
with Thursday the warmest day.

Probabilities:

– Severe Thunderstorms in Oklahoma: 40%
– Storm Chase: 25%
– Winter Precip: < 5%

Discussion:

12z models are in decent agreement regarding the 500mb pattern this
week and the evolution of a medium wavelength trof/potentially closed
low out west on Wednesday. This week is expected to be unseasonably
quiet with one potentially active day. Thursday is the main interest
this week from a severe weather stand point and the probabilities are
set for an expected medium-end event.

Longwave trof is taking shape over the region today and will progress
east and northeast through Tuseday. The unusually strong southern end
of this trof will help drag a Canadian high pressure system into the
Gulf. Said cold front has already passed through much of the region,
resulting in temperatures below normal this afternoon. This front
will push deep layer/quality moisture well off shore through Tuesday.
As such, days will be mild and nights a little chilly.

By late Tuesday the next upper system moves onshore in the NW and a
lee trof develops, assisting in return flow on the west side of the
prior Canadian high pressure. Moisture return will initially be weak
as surface through 850mb ridging remains along the coast with the
1016mb line west of Houston. As the upper system digs into the
western U.S. the surface low will intensify. Moisture will attempt to
surge northward, but will remain limited on Wednesday due to continued
ridging and overall lack of moisture depth. Additionally, moisture
return will likely have to battle dirunal mixing due to expected
shallow nature of return. Despite this, 55F dewpoints are possible
immediately east of a developing drying in Texas/Oklahoma, potentially
into Kansas. 12z NAM shows shortwave ridging at 500mb and this
combined with limited moisture should keep storms chances very low.
Southwest flow aloft will help temperatures warm into the upper 70s.

The upper system is progged to approach the region on Thursday. 12z
GFS has continued a slowing trend noted in the 06z run. However, I'm
not going to bite onto this right now. Given strong jet streaks at
500mb – 200mb, I would expect the models to slow the system some in
future runs. Dryline is progged near I-35 late Thursday with
impressive jet structure across Oklahoma. Issue really becomes how
much moisture can return to the region prior to convective initiation.
Progs would indicate that 60-65F moisture is likely with 850mb
dewpoints of 12-14C. I would really like to see better moisture
return, but in a year with unusually strong eastern U.S. longwave
trofs I'll take what I can get. Thursday should be a warm day,
especially west of a the dryline. BTW – I'm not convinced the dryline
will reach I-35, but we'll see.

Friday is a tough call, I'm wondering if it will be warm with strong
westerly winds as the parent system warps up into a closed low. OUN
has 68F right now for OKC, but will not be surprised if that changes
some.

The weekend should be in the 70s as the closed low ejects northeast
across the Great Lakes. The resulting cold front should not impact
the Gulf as the amplitude and direction of the system is not favorable
for a Gulf penetration. This should allow for rapid moisture return
next week and a potentially active period if the 00z/06z GFS runs are
to be believed. The 12z GFS changed the song and dance, but not a
shocker. The 00z better resembled reality.

Special discussions do look necessary for Thursday, starting tomorrow.

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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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