Severe Weather Discussion for May 11-12, 201

Severe Weather Discussion for May 11-12, 2011.
Issued:  05/10/2011.

Highlights:

–  Severe thunderstorms are expected Wednesday afternoon and evening across western Oklahoma.  Primary threats are large hail and damaging winds, a few tornadoes are also possible.

–  Severe thunderstorms are possible Thursday afternoon and evening across central/eastern Oklahoma.

Discussion:

Main change to going forecast is to slow the closed low down, to account for expected cloud cover on Wednesday, and to add Thursday to the threat.

00z NAM continues prior trends of lowering surface temperatures across central Oklahoma due to cloud cover and some rainfall.  This was seen today, although CAPE values may not lower as much as seen on this model.  HRRR is a better tool for this situation but will not be within the time period until later tonight.  And, in all actuality, we’ll need to see how the thermodynamic environment evolves tomorrow.  Model wind fields are decent but once again not overly impressive.  Turning is good with decent turning noted.  There may be some weakness at 700mb which could lead to QLCS.  My hope here is the NAM is off and the GFS is more accurate.  The other issue here is the subtropical jet and the plethora of cirrus associated wiht such.  Another ill-timed shortwave trof moves through Texas and lights up QPF along I-35.  If this happens, the rain cooled air would be 50-60 east of the dryline and provide a decent warm sector.  This is a similar thermodynamic setup to May 7, 1995, when Ardmore got hit.  Either way, lots of short term forecasting on-tap for tomorrow and should be a challenge to nail things down.

Thursday has garnered additional attention the past few runs as models continue to slow down the closed low.  This trend is not a shocker and 00z NAM is believable with a dryline in central Oklahoma late Thursday afternoon.  Additionally, flow becomes much less meridional, such that the subtropical jet problem goes away.  So, for now I’ll address the threat for Thursday and look at the details later.

SPC Day 2 (today) was awesome and I don’t disagree at all.  Within that same thinking, I know they’ll keep the slight but the moderate continues to be iffy to me.  I know they have a 45% area and the areal placement is great, just not sure of the 45%.  We’ll see what SPC does in the morning, but looks like they’re on top of it.  Either way, I think it warrants a chase and we’ll certainly try.  Thursday warrants a slight risk and I think SPC will need to shift it back west for the new Day 2.  They may do a moderate but that would seemingly be farther east into eastern Oklahoma.

Probabilities:

Risk area for any part of OK (valid:  05/11, Day 1, Day 2):

Slight:  100%, 100%
Moderate:  75%, 50%
High:  0%, 0%

Chase Status:

Level 2 – looking to head out on Wednesday, target area is Weatherford, departure 2pm.

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