Weekly Severe Weather Discussion

Severe Weather Discussion for March 29 – April 4, 2010.
Valid: 03/28/2010.

Highlights:

– Isolated severe thunderstorms are possible Thursday afternoon in far
western Oklahoma.

– Severe thunderstorms are possible Friday across much of Oklahoma.

– Showers are possible on Saturday across the southeast 1/2 of Oklahoma.

– Temperatures will be near low 70F on Monday, near 80s
Tuesday-Wednesday, upper 70s Thursday – Friday, low 70s Saturday,
upper 60s Sunday.

Probabilities:

– Severe Thunderstorms in Oklahoma: 30%
– Storm Chase: 15%
– Winter Precip: < 5%

Discussion:

12z models agree that a western U.S. longwave trof will develop this
week and move slowly across the southern U.S. Agreement stops at this
point with the ECMWF being the preferred model. The GFS solutions do
not look realistic with a meandering closed/cutoff low over Texas
through Sunday. Sensible weather impacts appear to increase closer to
climatology than what we saw during much of March. The ECMWF is used
throughout much of the forecast, with GFS convective parameters used
where appropriate.

Monday-Wednesday will be characterized by mid-level ridging across the
central U.S. By late Wednesday the upper ridge shifts east to the
eastern U.S. as the western U.S. longwave trof develops. About the
same time a large closed low develops over the western Atlantic
forming an Omega Block just east of the region. This is the primary
issue for differences among the models. I believe the ECMWF has a
better handle on the maturing closed low as it ejects into the central
U.S. and moves northeast into the western side of the longwave ridge.
As noted above the GFS wants to meander the closed low southward from
Texas and eventually drops the circulation altogether. While the
atmosphere will be in a blocking pattern, there isn't much support for
the GFS's solution.

On Thursday, upper system progresses into the southwest U.S. as a lead
shortwave trof ejects northeast across CO/NE. This will allow a
surface low to spin up and draw moisture back into the state. Initial
return will be marginal as the upper system from yesterday will have
cleared out the Gulf. Despite this, the GFS has consistently
indicated upper 50s dewpoints into Oklahoma and it has support from
the 18z NAM. Initial return will be shallow as 850mb depwoints remain
AOB 5C. By late Thursday, GFS indicates dewpoints 10-12C with PW
values 2cm across western Oklahoma. CAPE values are unimpressive at
this juncture 500-750J/Kg with 750-1000 J/Kg in NW TX. This may
improve as Thursday draw's closer and models get a better handle on
the general setup. Wind fields at all levels will be quite impressive
and if the EML can be breach supercell storms will be likely. The
threat is very conditional and as such, I have gone with isolated
severe chances for this threat. Primary threat looks to be large
hail, with potentially a tornado threat.

Friday looks a lot more interesting as the upper system moves closer
to the region. This is where the ECMWF is used and seemingly more
potent than the GFS. The GFS solution would result in more
unidirectional flow across the region and hence QLCS. The GFS is used
for dewpoint and temperature fields. Deep layer moisture continues to
increase into Friday with surface dewpoints near 60F maybe into the
low 60s. 850mb moisture increases to 12-13C with 15C in south Texas.
GFS shows impressive CAPE values in Texas with a min in Oklahoma due
to precipitation, hence lower surface temperatures. This is most
likely in error due to the speed of the upper system per the GFS. As
such, I'd think that CAPE values will be much higher in Oklahoma as a
dryline takes shape in southwest sections. Storms are likely along
boundaries Friday afternoon as the EML is breached with stronger flow
aloft and any perturbations in said flow. Severe thunderstorms are
certainly possible with supercells favored due to speed/directional
shear. How this comes together remains to be seen but the threat will
be monitored.

Upper system lumbers by on Saturday per the ECMWF and a cold front
moves through the state on Saturday. As such, there may be a chance
for showers in southeast Oklahoma, but at this time an organized
severe threat does not appear to exist. Sunday will see the state
in-between systems, with a relatively pleasant day expected.

Another upper system is expected around the 5th/6th and appears to be
rather interesting right now. Moisture should not get pushed too far
south as the prior upper system ejects northeast across the central
U.S. The next upper system may have more moisture to work with as the
western Atlantic Oemga block appears to deamplify. That issue remains
to be seen and for now will not bet the farm on it going away.

For temperatures I used the NAM through Wednesday and mainly patch
work from the ECMWF through Monday. Largely the GFS is not used.
Special discussions may be needed starting tomorrow for the potential
later in the week.

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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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2 Responses to Weekly Severe Weather Discussion

  1. Anonymous says:

    Update PLEASE! You wrote about this on Sunday, but it is Wednesday! Big super cells if the wind comes down?

  2. Anonymous says:

    Are you there? I post a number of comments, but you don't post them! I am pulling out of this nonsense! Later Dude, Andrew Woods

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