Severe Weather Discussion for May 13 – 19, 2013

Severe Weather Discussion for May 13 – 19, 2013.
Issued:  05/12/2013.
Highlights:
–  Showers and thunderstorms are likely Wednesday – Friday, some severe thunderstorms are possible on Wednesday and late Friday.  It won’t rain all the time, as with last week, hit and miss.
–  Severe thunderstorms are possible Saturday and/or Sunday.  Multi-day severe weather events may be in the offering.  A medium-end or greater event appears possible.
–  Temperatures will be in the upper 80s Monday – Tuesday, low 80s Wednesday-Thursday, near 80F Friday, mid 80s Saturday, low 80s Sunday.
Probabilities:
–  Severe Thunderstorms in Oklahoma:  65%
–  Storm Chase:  60%
Discussion:
Deterministic and operational model trends yield the development of a western U.S. trof by 05/17.  This is also supported by the latest MJO phase change from two weeks ago.  WPC teleconnections analysis from the Alaskan low and eastern Pacific ridge would also yield said development.  Model agreement the past few days has been very good regarding the overall trends, although as usual a few bigger details remain.  The GFS is about 24 hours faster than the EC.  However, the 00z EC would seemingly support severe thunderstorms in western Oklahoma  on Saturday as it breaks down the shortwave ridge in that area.  The GFS 06z run for the past several days has minored out the trof and today was no exception.  However, it is not to the degree as noted 05/11 06z run. Seasonable moisture appears available as Gulf ridging is minimal at this time.  GFS forecasts low 70s dewpoints in Oklahoma but it has run too high recently on dewpoints as noted by SPC and WPC.  As such, I would expect upper 60s and this is in best agreement with the EC.  CAPE values should be AOA 3000 J/Kg, although not sure the 4500 J/Kg by the GFS as the surface 70F dewpoints are likely in-play.  12z EC/GFS and 18z GFS have not changed the prior thinking above.  The EC is still 24 hours slower overall than the GFS.  However, it still develops rather high CAPE values on Saturday and storms along the dryline.  Sunday would be the main player per this model, as the GFS shifts the trof eastward.  Although even the GFS doesn’t totally end the threat on Sunday as modest CAPE develops again and deep layer flow remains.  I should also note that the EC develops a western U.S. longwave trof by the 20th, so a rather active period may be afoot.  Despite selected model, a medium-end event appears possible Saturday and/or Sunday.  Special discussions could be needed as early as Wednesday, once uncertainty is removed regarding which day has the greater threat.
Before this weekend, a ridge will be in-place across the region Monday and Tuesday.  As such, a rapid warm-up is expected area wide.  By Wednesday, a weakening mid-level low will open up into a trof and shift east across Texas.  This system will provide rain changes for Oklahoma Wednesday and Thursday.  It will exit stage right on Thursday as the region comes under shortwave ridging aloft.  There maybe a chance of storms Friday night, even though we should be under shortwave ridging aloft.  The EC creates CAPE values @2500 J/Kg and develops QPF later in the afternoon.  Convection doesn’t get too far east as nocturnal cooling and ridging influence the area.  Tough call, so low POP’s are added.
Going model and NWS numbers look good and only made a few changes.  I dropped temps a little on Sunday given expected storms.
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