Severe Weather Discussion for May 13, 2009.
– Active severe weather day is likely for much of Oklahoma. The
greatest impacts will occur east of a line from Alva to Hinton to
Duncan. In this area destructive hail to baseball size and
destructive winds to 80mph will be possible. An isolated tornado
cannot be ruled out, but the threat is limited.
– West and south of the greatest impact area (western and southern
Oklahoma) a threat for severe thunderstorms will exist. Large hail
and damaging winds will be possible.
– This is a potentially dangerous situation with widespread
significant severe weather expected late this afternoon into the
12z model data indicates that strong to extreme instability will
develop across Oklahoma by late afternoon given temperatures in the
upper 80s and dewpoints near 70F. Indeed 13z surface analysis
indicates CAPE values already 1500-2000 J/Kg across much of the state.
The main limiting factor for convection is a very strong cap. 12z
OUN sounding yields -636 J/KG CIN with a convective temperature of
92F. 12z NAM afternoon temperatures are very close to this and should
yield CIN values less than 30J/Kg by peak heating.
Main upper system and associated upward lift will pass well north of
Oklahoma. 500mb – 300mb winds will be unseasonably weak this
afternoon. However, degree of instability is expected to compensate
for the lack of speed shear aloft. Low level speed and directional
shear will be quite favorable for supercells across parts of Oklahoma.
By late afternoon, a cold front will be in northwest Oklahoma attached
to a surface low near I-40 in far western Oklahoma with a dryline
extending southward. The strong cap will likely keep storms from
forming through late afternoon. However, low level convergence,
heating, and microscale lift should be sufficient to breach the cap in
isolated locations along the dryline. The southward moving cold front
will provide more convergence and hence a more likely chance for
severe thunderstorm development.
Once the cap is breached, storms will rapidly become severe and
isolated storms will acquire supercell characteristics. High LCL's
due to wide T/Td spreads will initially limit tornado potential.
However, after 00z and as temperatures start to cool, LCL's will lower
and may enhance the tornado threat along with a strengthening/veering
low level jet. The primary threat throughout the day remains
destructive hail and destructive winds.
A severe squall line, possibly producing widespread destructive winds,
is possible by mid evening across parts of central and eastern
Oklahoma. The supercell and tornado threat should decrease as storms
merge into the larger storm complex developing southward along the
Level 4 – looking to head west to Clinton around 15:30 CDT.
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