Severe Weather Discussion for May 8 – 11, 2011 – Updated 5/7

Severe Weather Discussion for May 8 – 11, 2011.
Valid:  05/07/2011.

Highlights:

–  One or two severe thunderstorms are possible on Sunday along and near HWY 81.  Primary threats are large hail and damaging winds.  Of course, a brief tornado cannot be ruled out.

–  Severe thunderstorms are possible on Tuesday in western Oklahoma.

–  Active severe weather day is anticipated Wednesday across parts of Oklahoma.

Discussion:

18z NAM/GFS continue prior trends of developing a large trof across the western U.S. by Sunday.  In general, there are no changes to prior forecast at this point.  For now, I’m going to update some of the specific items seen in the models.

For Sunday, SPC has risked a small area along HWY 81 in southwest Oklahoma and this appears good.  Very hot afternoon temperatures may be sufficient to breach the cap in a location or two.  Primary threat will be large hail and damage winds with the tornado threat rather low.  NAM develops QPF while the GFS is dry.  CIN values on the NAM are near -30 J/Kg with the GFS not showing much lower than -100 J/Kg.  Tough call on which model is correct, but I certainly think a storm or two will be possible tomorrow.

Both models agree on no precipitation for Monday and this appears reasonable given CIN values near or below -100 J/Kg.

Tuesday continues to offer some potential as models lower CIN values, potentially aided by a shortwave trof.  Both models develop QPF, although in different areas.  NAM/GFS greatly reduces 850mb moisture with the GFS recovering better by late in the day.  This may allow the dryline to surge farther east than would otherwise be expected.  850mb winds back by late in the day and may allow increasing moisture to move west.  Either way, CAPE values should be at least 2000 J/Kg by late afternoon with the GFS in the 3000 J/Kg range.  Given improving winds aloft, supercells will be likely with any sustained updrafts.

Next up is Wednesday, which holds the best potential for active severe weather.  18z GFS continues to support CAPE values AOA 3000 J/Kg with deep layer moisture across the area.  NAM is slightly out of range, ending 1am Wednesday, but even this panel starts to bring better 850mb into the region.  Trof is forecast by ECMWF/GFS to eject eastward into the state by mid-afternoon along with very good wind fields aloft.  CIN values finally erode to AOA -30 J/Kg, indicating the cap can be breached in several spots.  The main question is, location of the dryline at this time.  GFS places this feature near HWY 183, which isn’t bad but may still be too far east given 850mb moisture values near 16C.  All this yields CAPE AOA 3000 J/Kg and LIs near -11.  Suffice it to say that Wednesday continues to hold our attention.

SPC discussions are great and I see little reason to change anything.  I’d expect current thinking to continue for Days 1-4.  I doubt they’ll risk Monday, with Sunday and Tuesday being conditional.  Wednesday continues to be the higher impact day.

Probabilities:

Risk area for any part of OK (valid: 04/08 Day 1, Day 2, Day 3):

Slight:  100%, 25%, 75%
Moderate:  0%, 0%, 0%
High:  0%, 0%, N/A

Chase Status:

Level 4 – maybe a local chase tomorrow, but not really organized.  Still expecting chase potential Tuesday and especially Wednesday.

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