Weekly Severe Weather Discussion

Severe Weather Discussion for May 11 – 17, 2009.
Valid: 05/10/2009.

Highlights:

– Isolated severe thunderstorms are possible all week along with
widespread showers, except for Sunday. It will not rain everyday in
the same spot, but will continue to oscillate north to south and vice
versus as we have seen the past 8-10 days.

– Temperatures will be in the low 60s on Monday, near 80F Tuesday, low
80s Wednesday, upper 60s Thursday, low 80s Friday-Saturday, upper 60s
Sunday.

Probabilities:

Severe Thunderstorms in Oklahoma: 20%
Storm Chase: 15%
Winter Precip: < 5%

Discussion:

Unseasonably quiet severe weather period continues across Oklahoma
this week. While low-end severe weather events appear likely,
confidence in a medium-end event is low. Wednesday would be the most
likely time for a medium-end event given presence of a surface
boundary, strong instability, and directional shear. A big problem
this week is the fast zonal flow with weak shortwave trofs. Models
cannot accurately resolve these small features, which present weather
impacts due to residual boundaries and tropical moisture.

Models are in reasonable agreement through the period with the GFS
going whacky at 168 hours, which is also the end of the ECMWF run.
Persistence seems the best solution, which is a continuation of the
pattern experienced last week. It will also mean unusual variations
in temperatures this week, as noted by the 62F today. Last week I
went low 80s yesterday and today, so I'll take a 20F hit today. We'll
see how bad it is next week.

Fast zonal flow will continue across the CONUS this week with two
perturbations of interest. The first one is due in on Wednesday and
the second late Saturday. Both of these will result in isentropic
lift over old frontal boundaries and usher in new frontal boundaries.
There is some concern that Wednesday may end up as a medium-end event,
but I'm going to hold off on such. I disagree with OUN on direction
shear as the 12z NAM BUFKIT data shows helicity above 300m^2/s^2. The
primary limiting factors appear to be 1) cap strength as afternoon CIN
is greater than 200 J/Kg and 2) weak 500mb flow. The latter would
seem to favor merging updrafts, as we have seen a lot this year.
However, if storms form, severe thunderstorms are likely given
expected strong instability.

Saturday would be the next chance for organized thunderstorms as
another upper system moves through the northern U.S. As with the past
several systems, the cold front races ahead of the stronger flow
aloft. This reduces the overall severe threat in the warm sector.
The GFS is much faster with the cold front and in a normal year I'd go
against it, but not sure right now.

Precipitation is possible pretty much everyday next week. Friday and
Sunday appear dry right now, although that's a dangerous thing to say
in this pattern. Isentropic lift is likely on Monday with a cool,
wet, cloudy day expected. Front moves north of the region on Tuesday
with possibly a continuation of Monday's conditions across the
northern parts of the state. Wednesday will see warm temperatures
statewide ahead of the next cold front. Precipitation is expected
late Wednesday into Thursday along this boundary. Next upper system
moves in late Friday into Saturday with a good chance of precipitation
along the cold front. Sunday has the best chance of being dry as the
front will be well south in Texas.

NAM is used through Wednesday, with only minor modifications to high
temperatures. I've used a little of the GFS Thursday-Sunday with
extensive modification to Saturday as I think the frontal movement is
too fast.

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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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