Weekly Severe Weather Discussion

Severe Weather Discussion for September 21 – 27, 2009.
Valid: 09/20/2009.

Highlights:

– Severe thunderstorms are expected Monday afternoon and evening
across central/eastern Oklahoma. Primary threats are large hail and
damaging winds. Isolated early afternoon thunderstorms will have the
potential to produce golf ball or larger hail. Showers and
thunderstorms are in the forecast Monday – Friday, chance of severe
weather decreases after Monday.

– Temperatures will be in the mid-80s on Monday, near 70 Tuesday –
Wednesday, mid 70s Thursday – Friday, upper 70s Saturday – Sunday.

– Fall starts on Tuesday!

– Tropics remain unseasonably quiet with little to no activity
ongoing. We're still in the peak season for tropical activity, so the
probabilities remain up.

Probabilities:

– Severe Thunderstorms in Oklahoma: 35%
– Storm Chase: < 5%
– Tropical Development: 20%

Discussion:

ECMWF/NAM//GFS Ensembles continue to be the models of choice for this
week. These models agree on a medium wavelength trough closing off
into a cutoff low by Monday afternoon across Kansas. This sets up a
Rex Block across the lower 48 during much of the week. Models show
the northern stream ridge progressing eastward by the weekend and the
cutoff low opening up and ejecting northeast next weekend. GFS is
much different as it retrogrades the low into the four corners region
during the week and then ejects it eastward on Saturday. This
forecast will follow the NWS and the former suite of models.

Sensible weather impacts will be highest on Monday/Tuesday of this
week as the closed low forms over the central US. On Monday, a
Pacific cold front will move into the state. Areas downstream from
the front will be warm and moist during the mid-afternoon hours. An
EML will be present, keeping convection at a minimum until peak
heating. Upper system and frontal convergence will be sufficient to
breach the cap, resulting in widespread thunderstorms. Some of these
storms will likely be severe given CAPE values around 2000 J/Kg and
moderate shear. Linear forcing should keep discrete activity at a
minimum and westward displacement of stronger flow aloft will keep the
tornado threat limited.

Cold front moves through the region late Monday and ushers in cooler
weather. In a normal year the cold front would likely be very
welcomed as it would end upper 80s temperatures. However, this
September has been on the cool side and the front will pretty much
continue what we've felt the past few weeks.

The upper system will remain nearby and as such, cannot rule out rain
chance through Friday. After Friday the upper system should be east
of here, setting up a pleasant weekend for the region.

I've gone a little lower on temperatures this week than others given
persistence and no real sign of a strong warm-up. The biggest change
this week will be morning temperatures, as they drop into the low 50s.
I'll have to remember to put weather appropriate clothing on the
kids! Normal high is 83F and low 61F.

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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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