Weekly Severe Weather Discussion

Severe Weather Discussion for March 30 – April 5, 2009.
Valid: 03/29/2009.


– Thunderstorms are likely across the eastern half of Oklahoma Monday
evening. Severe thunderstorms are not expected, but isolated small
hail is possible. Showers and a few isolated thunderstorms are
possible Wednesday evening.

– Severe thunderstorms are possible Saturday afternoon and evening
across much of Oklahoma.

– Temperatures will be in the low 70s on Monday, with a cold front
Monday night. Tuesday will be cooler with temperatures near 60F, with
a warmup on Wednesday to near 70F. Low 60s on Thursday following a
another cold front, with a warming trend on Friday into the upper 60s,
Saturday mid 70's. Another cold front is expected late Saturday with
Sunday temperatures in the upper 50s.


– Severe Thunderstorms in Oklahoma: 25%
– Storm Chase: 10%
– Winter Precip: 10%


Active weather pattern will continue this week, but a high-end event
is not expected. Prior system, which brought 25 inches of snow to
northwest Oklahoma, is moving into the Great Lakes this afternoon.
Longwave trof will remain over the central U.S. and three systems are
expected to move across the region this week.

12z models are in general agreement through Friday with Saturday and
Sunday a little iffy given the GFS bouncing around. Closed low will
progress through the central U.S. on Monday with a dryline moving
eastward across Oklahoma. Moisture will attempt to return north in
advance of this system. However, prior cold front has confined the
moisture to the Texas Gulf coast and decent return will be difficult.
Despite this, 12z NAM shows 55F dewpoints in the eastern half of
Oklahoma by Monday evening. Previously said closed low will become
vertically stacked by late Monday, resulting in decreased directional
shear. Despite this, warm surface temperatures and CAPE values around
500J/Kg should be sufficient for showers and thunderstorms across
eastern Oklahoma. This is in best agreement with SWODY2. Severe
thunderstorm chances remain minimal with this system.

Low lifts northward into central Canada as a trof move through Texas
on Tuesday. This feature will have little to no impact on Oklahoma as
moisture will have been pushed to the Gulf coast. Another shortwave
trof dives southward long the upstream side of the central U.S.
longwave trof late Wednesday and starts to move eastward into
Oklahoma. Moisture will once again attempt to move northward, but do
not expect much more than 40s for dewpoints. GFS is slightly faster
than the NAM with this system and is likely to be too fast. I'll side
with the NAM, although any impacts are slightly beyond the 84 hour
panel. Given lack of moisture, I'd expect impacts to be limited to
isolated showers in the eastern 1/3rd of Oklahoma. 18z NAM indicates
wrap around moisture in the panhandle and with the upper system nearby
I'm adding in snow chances. Although, just a trace this time. I
really don't think the warm front will make it out of Texas, so any
activity will be elevated across eastern sections.

Switching to the ECMWF, things are quiet Thursday and Friday as the
Wednesday system moves into the eastern U.S. and the longwave trof
reloads on the upstream side. GFS 12z run flipped to a solution,
which isn't that believable and given recent tendency to move systems
too fast isn't my pick. Even though it does have some support from
the ensemble members. Using the 00z run mass fields and the ECMWF
mid-level pattern would yield a severe weather chance for Saturday.
At this point, it is way too tough to determine given model
consistency issues and no dewpoint data from the ECMWF. I went with
climatology on the severe and chase probability. The rapid longwave
pattern is difficult to get moisture into Oklahoma this time of year
and hence better severe chances.

Temperature wise I've gone with the 18z NAM through Thursday and then
modified the 12z GFS beyond that time.

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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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