Weekly Severe Weather Discussion

Severe Weather Discussion for September 28 – October 4, 2009.
Valid: 09/27/2009.


– Severe thunderstorms are possible Wednesday evening through early
Friday. Current trends indicate the potential for at least a
medium-end event on Thursday. The rest of the week will be dry.

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

– TD 8 did not last long and NHC isn't noting any potential. We're
outside of peak season now and I have dropped the probabilities a


– Severe Thunderstorms in Oklahoma: 60%
– Storm Chase: 50%
– Tropical Development: 10%


12z models continue prior trends of amplifying the mid-level flow this
week and subsequently increasing sensible weather impacts. GFS/ECMWF
are in general agreement through the period with a few difference,
especially relative to Thursday. The general trend noted in the
models is for the current shortwave trof over Minnesota to continue
southeast and develop into a closed low by Monday. This will push a
cold front into the region that may have a little Canadian air mixed
in with the Pacific airmass. This front move through Oklahoma tonight
(10pm OKC per 12 NAM-WRF) and to the coastline by Tuesday afternoon.
500mb pattern remains progressive and the developing closed low moves
into far eastern Canada on Wednesday.

Right behind this system is a western U.S. longwave trof which the GFS
handles differently than the ECMWF/NAM. The NAM is actually the
slowest model with the ECMWF somewhat close and the GFS way faster.
The GFS pushes a cold front from southern KS at 12z Thursday to north
Texas by 00z Friday. I find this rather suspicious and of course the
quick frontal movement relates to the GFS's handling of the upper
system. While the GFS has had a good track record with this system,
the other models are similar and their progression appears more
realistic, especially the ECMWF.

Given these considerations have gone with medium-end probabilities for
a severe weather episode on Thursday. Moisture appears to be a given
with even the 12z 84 hr NAM BUFKIT data showing 58F dewpoints in OKC
Wednesday evening. Daytime temperatures on Thursday, any lead
shortwave trofs, and the ultimate speed of surface features will be
the determining factors for a significant severe weather event. This
has some similarities to 10/09/2001 although that system was a bit
slower. Despite the upstream kicker, models are typically too fast
with these systems and think in the end we'll see a slight slow down.
I am not buying the GFS's dryline position in east-central Oklahoma
late Thursday.

Either way, special discussions appear necessary starting tomorrow
with a focus on Thursday.

Between now and Wednesday a shortwave ridge will follow the system in
the northeast today. After Thursday's system leaves, ridging will
also dominate Friday – Sunday as a blocking pattern evolves over the
eastern Pacific. The kicker wave should settle over the southwest
U.S. with ridging over the central U.S. Given the expected cold front
on Friday and removing of moisture to the Gulf coast, I've excluded
POPs from the forecast after Friday morning. Just read AFDOUN and
seems to fit the overall thinking. Good notes about the NAM-WRF and
less energy diving into the southwest U.S. on Wednesday. Probably
another reason to stick with the ECMWF.

Followed NAM numbers through Wednesday and then seriously modified GFS
numbers through the weekend. I brought Thursday temps down a little
in OKC due to expected moisture and Gulf stratus. I do admit I blew
the temperature by about 16 degrees for today, but hey I think we all
did from last Sunday.

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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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