Severe Weather Discussion 2009-3-1

Severe Weather Discussion for March 23-24, 2009.
Valid: 03/20/2009.

Highlights:

– Severe thunderstorms are possible Monday afternoon and evening
generally west of I-35 in Oklahoma, much of Kansas/ Nebraska, and
northwest Texas.
– Severe thunderstorms are possible Monday overnight into Tuesday for
the remainder of Oklahoma, Kansas, Nebraska, and north Texas.

Discussion:

12z models have not shed much more light on what is expected to happen
this Monday. While a complicating factor, certainly not the end of
the world. Models agree that a western U.S. trof will develop on
Sunday and remain progressive into the central U.S. by Monday
afternoon. There are a variety of different speeds and dynamic
evolutions offered by the models. At this point I have no reason to
pick one model over the other. I have been expecting a slow down of
the upper system in the model progs the past few runs. This slow down
has not occurred, but I'm not ready to abandon my thinking. The
models seem awfully progressive with the system given progs of 125kt
jet max on the upstream side. If the current runs are to be believed,
we may not have a good answer until Sunday night when the upper system
is sampled by the CONUS raob network. The NAM/ECMWF would seemingly
be the most favorable for severe weather in Oklahoma. The 12z GFS,
while not unfavorable, adds a few more questions such as an early
start to the event or general rainfall.

NAM/GFS moisture fields show mid-50s dewpoints making the northward
track into Oklahoma. In fact, mid-50s may surge to southern Nebraska.
This past Wednesday a Canadian cold front moved into Oklahoma with
the frontal boundary stalling across north Texas. The center of the
high went through the Great Lakes area. While the Gulf has not been
whacked in a few weeks, the high will settle over the southeast U.S.
and cause ridging over the eastern Gulf. Models maintain said ridging
through Sunday evening and then rapidly lessen it. There are some
differences between the models on the amount of ridging Sunday night,
but general agreement shows the 1016mb line at Houston or east by 12z
Monday. By Monday night the 1016mb line is way east and the Gulf is
considered fully open. I'm not convinced the models are handling the
expected moisture return well. If a cold front had moved into the
Texas Gulf Coast or even into the Gulf, I'd be more inclined to follow
the model moisture progs. However, I have a tough time with it given
ample moisture in the Gulf today and expected moisture surge as the
ridging weakens early Monday. The 12z NAM strongly hints at this, but
seems to mix out the moisture during the afternoon hours. Despite
this issue, NAM BUFKIT data from Hobart Monday afternoon shows deep
layer moisture of 10C and greater from the surface to 850mb. The
moisture quality and depth does improve throughout the day. Despite
the 55F or so dewpoints the NAM does develop widespread 750-1000 J/Kg
CAPE in western Oklahoma Monday afternoon with a few pockets of
1000-1500J/Kg. I think the ultimate evolution will show low 60s
dewpoints throughout much of western Oklahoma Monday afternoon. The
dryline is the next issue and I think the 12z NAM is perfect with the
positioning. I will not modify at this point. The eastern Texas
panhandle fits very well with the 500mb evolution of the upper system
and low-level mass response.

Thermodynamics aside, this system will be very dynamic. Of course
this is typical of late March and the resulting speed and directional
shear fits climatology. 12z NAM BUFKIT data has helicity values above
400 m^2/s^2 across much of western Oklahoma. This is quite impressive
and favorable for supercells. The current setup does favor high
shear, low CAPE; athough I think we'll see the CAPE improve in future
model runs. The NAM is likely not handling the CIN well as it erodes
the cap (EML) by 7pm. I doubt this is the case given low 80s
temperatures in west Texas the next few days. I was not able to find
much of a cap on the BUFKIT data, so think the models have yet to
catch on to it.

In general I'm not using the GFS for this discussion and will continue
to monitor the solutions that it offers. However, it is wrong on the
temperature today and I'm not going with it. Day 3 will likely sport
a large slight risk area from northwest Texas into Nebraska, they may
pull it south to Abilene, which I think they should. I'm not sure if
they'll do a 10% hatched area, most likely west-central Oklahoma
through central Kansas. There seems to be plenty of time to review
all of this and no need to get carried away this early in the game and
with many questions remaining.

PRIND: A dryline will be located in the eastern Texas panhandle
Monday afternoon and storms will develop around 5-6pm along this
feature. Presence of deep layer shear and CAPE values around 1000
J/Kg should yield supercells as the activity is expected to remain
discrete. Tornadoes and large hail are possible at this point.

Probabilities:

Risk area for any part of OK/TX (valid: 03/21, Day 3):

Slight: 100%
Moderate: 25%
High: N/A

Chase Status:

Level 1 – Normal – we are watching this one very closely.

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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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