Severe Weather Discussion 2009-7-1

Severe Weather Discussion for October 1, 2009.
Valid: 09/28/2009.

Highlights:

– Severe thunderstorms are possible Thursday afternoon through the
overnight hours. Models continue to hint at a medium-even event
during this time period. Most likely area to be impacted will be
central and eastern Oklahoma.

Discussion:

No change to prior reasoning from yesterday's weekly discussion. GFS
continues to do really weird things e.x. take a cold front from SW KS
at 7am to NE TX by 7pm. That's quite a journey for an early fall cold
front. I understand this is related to it's handling of the upper
system, but still. The 12z NAM and 00z ECMWF remain my preferred
models. The GFS has yet to hold on to a single solution the past two
days. This forecast is based on the prior two models and leaves out
the GFS. I'm not saying it can't happen, just saying I'm not
forecasting based on the GFS right now.

12z NAM did speed up the upper system just a tad and would expect
similar changes during the next few days. Current agreement among the
ECMWF/NAM has a dryline near Oklahoma City at 7pm Thursday with
convection ongoing in eastern Oklahoma. Taking the models at face
value would yield convection developing between 2-3pm, maybe a little
earlier, north/south of Weatherford and racing east. 12z NAM did
crank out some decent QPF across much of the area early Thursday
morning. This is kind of typical for this model at 72 hours and would
like to see if it remains the next few runs. If convection does
indeed develop early in the day, it may reduce the instability/threat.
I'm just not convinced it will work over all of Oklahoma that early.
Many forecasters, this one included, keep expecting some type of
slow down. Any reduction in speed of the upper system (whether an
hour or two at this point) may increase the threat for central Oklahoma.

Moisture appears to be a given and the NAM has expanded the moisture
field, showing 70F dewpoints moving into Oklahoma. Issues I'm seeing
from the 12z NAM/00z ECMWF revolve around directional shear at
convective onset and daytime heating. The former may or may not be an
issue but the models want to veer the flow at the surface up to 850mb.
This will be a case where if the surface flow can remain backed,
impressive helicity is likely to develop. The latter will remain an
issue until Thursday. There is no way to forecast 72 hours out
whether stratus will be around or even precipitation. Given that
morning lows will be in the upper 60s and early fall favors quick warm
ups, only a little bit of sun is needed. This is something of a
lesson learned from 10/09/2001 where western OK warmed up to near 80F
and several tornadoes occurred despite early morning convection.

PRIND: A medium-end event appears in the offering for central/eastern
Oklahoma northward into KS/MO on Thursday. A plethora of severe
weather types will be likely and the highest impact area has yet to be
determined. I don't see SPC doing a moderate on the Day 3, but would
expect a 30% prob area.

Probabilities:

Risk area for any part of OK/TX (valid: 09/29 Day 3):

Slight: 100%
Moderate: 40%
High: N/A

Chase Status:

Level 1 – Normal

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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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