Severe Weather Discussion 2009-5-3

Severe Weather Discussion for April 25-27, 2009.
Valid: 04/24/2009.

*Significant and prolonged period of severe weather*

– Isolated severe thunderstorms are likely late Saturday afternoon
into the evening hours across the western one-quarter of Oklahoma.
Primary threats are hail to tennis ball size and isolated tornadoes.

– Active severe weather day is possible for the western one-half of
Oklahoma on Sunday. Primary threats are tornadoes and large hail.

– Severe thunderstorms are possible on Monday across the southern
two-thirds of Oklahoma. Primary threats are excessive rainfall and
large hail.

– A prolonged period of severe weather conditions will persist across
Oklahoma through Friday May 1st. It currently appears that Tuesday
and Wednesday will see lulls in activity, however this is a guess at
this point.

– Chase Status is increased to Level 3.

Discussion:

Afternoon surface analysis indicates low 60s dewpoints in
central/eastern Oklahoma with upper 40s to low 50s in western
sections. Southerly flow is occurring from the tropics allowing rich
Gulf moisture to spread north into Oklahoma. Morning sounding data
from Brownsville in Corpus Christi show deep layer moisture around 10C
and above up to 850mb with PW values of 3.5. A surface low was
located in western KS/eastern CO and a dryline extended southward
through the central TX/OK panhandles. The general conditions noted
here are expected to persist across Texas and parts of Oklahoma for
the next seven days. Gulf moisture will increase further as the
surface ridge over the Atlantic shifts far enough east to allow
additional moisture to stream northward. Current pressure readings
along the Gulf coast are in the 1017 to 1020mb range. Model progs
consistently lower pressures along much of the Texas coastline by
Saturday morning. Houston eastward remains at 1016mb or above.
However, this is not expected to impede moisture return, as noted by
the widespread 70F dewpoints in the Gulf and upper 60s along the
coastline.

Attention then turns to the mid-level features which are expected to
interact with the Gulf moisture over the next several days. All
models agree that a western U.S. longwave trof will become established
this weekend along with a southeast U.S. longwave ridge. This will
place Oklahoma in southwest flow aloft with the shortwave trof track
near or just north of the state. As is typical in late April across
Oklahoma, storm chances will be greatly governed by the general
atmospheric flow coinciding with warm moist air at the surface. The
presence of multiple convective events will start to muddy the
forecast beyond Sunday. However, given the general location of the
synoptic scale systems and convective interactions, I tend to accept
model placement of surface features through the next eight days.

On Saturday the western U.S. longwave trof will be developing and
shortwave trof will be in the base of what will become mean longwave
position. Atmospheric wind speeds will increase across the
Texas/Oklahoma panhandles and far western Oklahoma during the late
afternoon and evening hours. Warm daytime temperatures in the 80s are
expected to combine with dewpoints in the mid 60s to produce CAPE
values 2500-3000J/Kg. A dryline will initially be located in the
Texas panhandle along with a surface low in north-central sections. A
cold front is expected to move south into Oklahoma as a northern
stream shortwave trof moves into the Great Lakes region. By late in
the day, the cold front is expected to stall from near Sayre to Ponca
City. The surface low and dryline will intersect the cold front near
Shamrock Texas or maybe just a little farther east. A very strong cap
will be in-place much of the day, acting as a lid on the atmosphere.
By late afternoon a combination of strong daytime heating, dryline
circulation, and collocated UVV's should initiate a few severe
thunderstorms near I-40 and the Texas border, southward along the
dryline and northeast along the stalled cold front. Storms should
decrease during the late evening hours as temperatures cool and
mid-level support lessens.

Sunday is strongly predicated on convection Saturday afternoon through
Sunday morning. There are two equally possible scenarios for Sunday.
Scenario one: Storms decay late Saturday/early Sunday and have little
impact on the atmosphere across Oklahoma. A shortwave trof ejects
northeastward Sunday morning across Colorado into Nebraska. As
daytime heating destabilizes the atmosphere over Oklahoma, severe
thunderstorms will initiate along the dryline during the mid-afternoon
hours. The cold front should have returned northward as a warm front
and be located in central Kansas. This scenario would produce at
least a medium-end event with significant severe weather impacts
possible. Scenario two: Storms continue through the night or even
during the morning hours on Sunday. The cold front is reinforced and
not able to move north. Additionally, rain cooled air will be in
place across much of Oklahoma, preventing atmospheric destabilization.
This would result in isolated severe thunderstorms, with large hail
the primary threat. Which scenario verifies will be quite difficult
to discern until Sunday morning.

It is emphasized that there is the potential for prolong and
significant severe weather over the next eight days. Persons are
urged to remain weather aware throughout the weekend. Discussions
will be issued Saturday and Sunday mornings.

I don't think a moderate risk is warranted tomorrow. SPC has done a
great job on the forecast products and I just don't think coverage
will warrant it. The storms that do develop will produce significant
impacts, such as large hail and isolated tornadoes. The greater
threat appears to be Sunday if the atmosphere isn't worked over by
Saturday convection. I do think SPC will get to a moderate risk by
the 17:30z Day 2 issuance.

Probabilities:

Risk area for any part of OK/TX (valid: 04/25 Day 1, Day 2, Day 3):

Slight: 100%, 100%, 100%
Moderate: 30%, 75%, 0%
High: 0%, 5%, N/A

Chase Status:

Level 3 – Looking to go Saturday, target area is Sayre. Also,
monitoring 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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