Severe Weather Discussion – 2008-8-1

Severe Weather Discussion for April 7, 2008.
Valid: 04/04/2008.

Highlights:

– Severe thunderstorms are possible on Monday across central/western
Oklahoma into southwest KS/northwest TX and the panhandles.

Discussion

SPC Day 4 was a surprise and I'm still not sure discussions are
necessary. However, with two of the three models showing a favorable
setup for a near medium-end event, discussions are warranted. I agree
with Greg's review this morning and really think the focus is on next
Thursday. Of course, that is out of my four day forecast range for
special discussions. In light of what happened last Sunday, weak
events can still end up producing high sensible impacts.

I'll take just a little time to review the models and some basic
thinking for Monday. The NAM is on the lower end of a severe weather
event, in fact there would be very little little activity. And, any
storms that did develop would suffer from anvil seeding due to weak
flow above 300mb. The GFS's severe weather signal is somewhat
stronger and is border line between an upper low impact event to a
medium impact. Then, enter the ECMWF, which seems to strongly point
towards a medium end event.

Numerous problems present themselves in such a forecasting environment
as there is no conclusive reason to pick one model over the other.
The fact that the GFS and ECMWF are in the same ballpark is why I'm
initiating discussions for this event.

As SPC noted in the Day 4 product, along with Greg, and I certainly
agree; moisture will be an issue for Monday. At least that is the
current thinking. 1017mb surface ridge is currently pushing into the
Gulf as the weakening southern stream shortwave trof moves east of the
region. 12z models are in agreement pushing 55F + dewpoints off the
coast, leaving 50F at the coast. Surface high rapidly leaves the
region, however surface flow remains weak and disorganized through
Saturday. By Sunday a surface low develops in southwest Kansas as a
northern stream shortwave trof approaches in the northern U.S. This
wave passes by well to the north of the region by late Sunday,
dragging a cold front to the Red River. This front will be very weak
and should do little more than shift winds to the north.

The NAM fails to initialize the wave after this one and hence the
source of the model disagreement. The GFS brings in another northern
stream shortwave trof, but it is a little stronger and farther south
than its predecessor. In response to this wave a surface low develops
in the panhandles. The warm front is progged by the GFS to be along
the KS/OK border with a dryline in western OK. I agree on the dryline
location. The warm front is a different story and it is a tough call.
This boundary will be an item to watch the next few model runs.

NAM/GFS both return 60-65F moisture into Oklahoma and 850mb dewpoints
of 12C. If this does happen, then there will be sufficient
instability for severe thunderstorms. GFS wind profiles would support
supercells while the NAM's would support multicell clusters. I do not
have enough data from the ECMWF to make a call on what it is
forecasting. Looking at the 18z runs, the GFS and NAM are still at
odds. We'll see what happens. But the GFS's solution would favor a
chase. I think SPC will do a slight tomorrow on Day 3, don't really
see a moderate right now.

GFS/ECMWF hint at a longwave trof in the western U.S. this week. One
particularly intense shortwave trof is expected to move into the
region around Thursday. Lack of frontal intrusions very far into
Texas and southwest flow aloft may provide a favorable setup for a
severe weather event.

Probabilities:

– Risk area for any part of OK/TX (valid: 04/05 Day 3):

Slight: 100%
Moderate: 15%
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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