Severe Weather Discussion for April 21 – 27, 2014

Severe Weather Discussion for April 21 – 27, 2014.
Issued:  04/20/2014.
 
Highlights:
 
–  Showers are expected across the state on Monday.
 
–  A few severe thunderstorms are possible in western Oklahoma on Wednesday.
 
–  A few thunderstorms are possible in eastern Oklahoma on Friday.
 
–  Severe thunderstorms are possible across the state on Saturday/Sunday, one or both of these days may present a medium-end event.
 
–  Temperatures will be in the upper 70s Monday, near 80F Tuesday, low 80s Wednesday, mid 70s Thursday, low 80s Friday, mid 80s Saturday, and low 80s Sunday.
 
Probabilities:
 
–  Severe Thunderstorms:  50%
–  Storm Chase:  30%
 
Discussion:
 
Weakening southern stream system will continue to pass over the state tonight into Monday.  Ongoing showers and thunderstorms across western Oklahoma will continue to push east, although overall decrease in intensity should occur at sunset.  Moisture is marginal for severe thunderstorms, although a few have occurred. 
 
A cold front will push through the state late Monday and push moisture south again.  A larger system will  move into the western U.S. on Tuesday and start moisture return late in the day.  Models have a variety of solutions for moisture return by Wednesday afternoon.  The GFS/EC look the best as I don’t see the NAM’s far western Texas panhandle prog verifying.  The trof is seemingly well timed but ongoing short wavelength trofs has put a dent in meaningful Gulf moisture.  SPC does have a Day 4 up for western Oklahoma but I see this as a low-end event.  Hence, despite the hint at a moderate risk, I’ll forgo special discussions.  Seems to be the standard this year.  Either way, EC shows CAPE values in the 1500 J/Kg range with decent winds, as such a few supercells seem likely.  Large hail appears to be the primary threat at this juncture and probably the 30% area SPC is hinting at.
 
Upper trof swings through with moisture being pushed into north Texas and poised for rapid return.  EC goes a little nuts with precipitation on Friday and shockingly the GFS seems to hit the same area.  I really don’t want to put POPs in for Friday, but guess I have no choice.  Like the NWS, I went with low POPs. 
 
Attention then turns to Saturday/Sunday as another longwave trof moves into the lower 48.  Models greatly amplify the 500mb pattern across the CONUS and seemingly due to downstream blocking near Greenland.  The models handle this pattern change different with the GFS developing a strong northern U.S./Canada mid level ridge and the EC more elongated to the east.  WPC has also sided with the less amplified EC as opposed to the GFS.  There are important differences going into Saturday/Sunday as the GFS has a much stronger surface ridge nosing southward into the central U.S.  The model then struggles with moisture return into Oklahoma as the warm air gets held up by the 1032mb high.  The EC is much different and certainly leads to a more robust severe weather setup.  WPC’s D6/D7 drawings match up well with the EC and this lends credibility to the forecast.  Admittedly I have seen several good severe weather setups a few days out, only to fall apart.  Wednesday is one such example. 
 
12z EC progs strengthening 500mb flow atop a warm and moist surface airmass.  CAPE values Saturday/Sunday are in the 3000 J/Kg range with a dryline in Oklahoma.  Upper system will swing by on Sunday and appears to take on a negative tilt.  If the EC is correct and that’s a big if, special discussions will be needed starting Wednesday.
 
I went with storm chase probabilities since I’d certainly like to go if my commitments next weekend don’t go into the late afternoon hours.  I won’t know that schedule for a few more days.
 
Number wise, I lined up okay with the NWS.  The EC is rather warm for next weekend and I didn’t go that high.  I’ll say that if the dryline is faster than progged on Sunday, some 90s are likely in central OK.
 
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