Severe Weather Discussion for May 18-21, 2013


Severe Weather Discussion for May 18-21, 2013.
Issued:  05/15/2013.
–  Severe thunderstorms are expected Saturday late afternoon into the evening hours for western Oklahoma.
–  Severe thunderstorms are possible Sunday and Monday across a large part of Oklahoma.
–  Multiple severe weather episodes appear possible for Oklahoma during the forecast period, although Sunday and Monday are technically beyond the scope of this discussion.
Models agree that a medium-wavelength trof will enter the western U.S. on Friday and translate eastward.  This trof may take on a slight negative tilt as it impinges on the high plains Saturday afternoon and evening.  Ahead of this system, deep layer moisture will return northward.  Said moisture will be characterized by mainly upper 60s surface dewpoints with potentially some low 70s.  850mb moisture will be 12.5C range, potentially greater.  Models are in good agreement regarding these parameters.  Winds aloft will increase across the panhandle late Saturday and progress into Oklahoma during the evening hours.
18z GFS/NAM/HWRF and 12z ECMWF all agree that a dryline will be located in the eastern Texas panhandle.  As always, there are a variety of viewpoints among the models as to the ultimate location of the dryline.  The ECMWF surges the dryline east into western Oklahoma during the day and then retreats it during the afternoon.  It develops thunderstorms in western Oklahoma as this occurs during the late afternoon hours.  Extreme CAPE develops on the EC just east of this feature.  The GFS and NAM dryline positions are located farther west in the Texas panhandle.  The GFS develops low 850mb dewpoints and subsequently mixes out surface dewpoints Saturday afternoon.  The most recent run finally mixes the dryline eastward as a result of this.  NAM somewhat does the same thing.  Despite this, all models develop QPF somewhere in the Texas panhandle and/or far western Oklahoma on Saturday.  Winds are decent but not overly impressive.  If robust instability can develop, then storms may have better ability to rotate.  Either way, large hail and damaging winds are expected with any mature thunderstorm.
By Sunday a large trof is expected across the western U.S. and somewhat into the central region.  Models are in decent agreement regarding the evolution of this trof.  The ECMWF lifts out a small shortwave trof early Sunday and veers the surface flow across Oklahoma.  This is a departure from the 00z run and is different than the other models.  Current trend is for a dryline to setup in western Oklahoma with CAPE values AOA 2500 J/Kg to the east.  Wind shear should be much better than Saturday.  PRIND severe thunderstorms are possible during the afternoon and evening hours across central/eastern Oklahoma. 
Monday’s threat is well beyond the scope of this forecast.  But, to trend the models there is decent agreement that a closed low will develop over the north-central U.S.  This would keep a dryline somewhere in Oklahoma along with decent winds aloft.  Models have the dryline near Oklahoma City Monday afternoon with CAPE values of 3000 J/Kg.  Of course this is way far out, but certainly something to watch.
Risk area for any part of Oklahoma (valid:  05/16 – Day 3):
Slight:  100%
Moderate:  25%
High:  N/A
Chase Status:
Level 4 – Watching Saturday.
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