Weekly Severe Weather Discussion

Severe Weather Discussion for October 26 – November 1, 2009.
Valid: 10/25/2009.

Highlights:

– Showers and isolated severe thunderstorms are likely Monday morning
in southeast Oklahoma with a continued chance of showers into the
afternoon.

– Isolated severe thunderstorms are possible Wednesday evening across
southern Oklahoma.

– Thunderstorms are likely on Thursday across much of Oklahoma, severe
thunderstorms are possible across central/eastern sections.

– Temperatures will be in the low 60s Monday, upper 60s Tuesday, low
70s Wednesday, upper 60s Thursday, mid 60s Friday, upper 60s Saturday
– Sunday.

– Time change on Sunday 11/01/2009 – turn clocks back one hour.

Probabilities:

– Severe Thunderstorms in Oklahoma: 30%
– Storm Chase: 5%
– Winter Precip: < 5%
– Tropical Development: 10%

Discussion:

Wild week ahead as the 7 day pattern continues for the central U.S.
12z NAM/GFS/ECMWF all in reasonable agreement on the evolution of a
southern stream longwave trof this week. Models do differ on the
progression of this system Thursday and beyond, so will make some
modifications to model outputs.

Current southern stream shortwave trof is located over KS/CO per RUC
output. Energy on the upstream side of this trof is expected to dig
into Texas on Tuesday and leave the region late in the day. Models
indicate near 60F dewopints returning from the Gulf but is of
sufficient quality to produce 1-3 inch rainfall amounts, mainly in
southeast Oklahoma. OUN has issued a flash flood watch for this area.
Central and eastern Oklahoma will also get some rainfall but the
frontal boundary will push the big stuff into southeast OK. Rainfall
should taper off on Tuesday as the upper system moves east of the
region.

By Wednesday morning a powerful southwest U.S. southern stream
longwave trof takes shape. Models are in amazing agreement given the
GFS's issues the past few runs. Cold front associated with the
current weak southern stream shortwave trof will move into the Gulf
and push moisture offshore. Initially, moisture return will be
limited on Wednesday, but by late in the day models indicate decent
moisture return. Since moisture return should be ongoing at 925/850mb
I've put some pops in for storms Wednesday night in what should be an
isentropic lift environment. Models agree on lifting the closed low
of the system into eastern CO Thursday morning and then diverge after
this point. ECMWF rotates the base of the trof into the Great Lakes
by Saturday morning while the GFS holds it back over NM. There also
seems to be some differences on how the models handle the upstream
kicker. I am tempted to slow the eastward progression of this system
down, but got burned on it a few weeks ago. I do think the timing
will ultimately be a little slower, but not enough to change my
forecast around all that much. Severe potential looks decent for
Thursday, especially in eastern Oklahoma. Maybe an early morning
forced squall line or if the system slows, something a little more
robust.

ECMWF shows the pattern to be amplified into next weekend but we'll be
on the west side of the trof and in rather dry air. As such, we
should clear out with mild temperatures. I'm not going that cold next
weekend since the incoming high is Pacific origin.

I've gone above MOS Monday – Thursday. Thursday was the toughest day
to deal with as early morning storms may give way to sunny skies that
afternoon. This being the case and westerly winds behind the dryline
may result in a decent warmup. MOS guidance (GFS, ensembles) look
good for the end of the week. I left POPs out on Sunday.

Both the GFS and ECMWF hint at a tropical system this week. I think
it is BS but put 10% in there out of boredom.

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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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