Severe Weather Discussion for May 19 – 25, 2011

Severe Weather Discussion for May 19 – 25, 2011.
Issued: 05/18/2011.

Highlights:

– Active severe weather day is expected on Thursday across western and parts of central Oklahoma. Tornadoes, large hail, and damaging winds are all likely.

– Widespread thunderstorms, some possibly severe are likely on Friday. Eastern sections stand the highest threat for severe weather. Tough call on significance at this point since early morning convection will modulate the event.

– Severe thunderstorms are possible in eastern Oklahoma on Saturday.

– Storm chances remain Sunday-Wednesday but model disagreement makes forecasting this time period very difficult.

Discussion:

Little to no change to going forecast for Thursday. 00z NAM has upped CAPE values for tomorrow and seems this model better understands the convective setup. Storms attempted to develop this afternoon but were unsuccessful due to modest capping across the region. Prior model runs develop convection across the region early Thursday, limiting instability. This appears to be resolved now as the NAM is quiet through late afternoon. It does lower CIN values by mid-afternoon indicating a cap breach is likely by 4-5pm. The dryline will likely be in western Oklahoma with dewpoints in the mid/upper 60s to the east and 850mb dewpoints of 15-16C. NAM indicates better upper level support than previously thought across Oklahoma and this would support a more robust event. I have kept moderate probabilities the same, but I’d think SPC will upgrade much of the slight risk in OK/KS tomorrow. Storms will likely decrease later in the night as the atmosphere cools and capping increases.

Friday looks to be a widespread thunderstorm day with embedded severe thunderstorms. Given the event last week, cannot ignore this being a higher-end slight risk, even though it may not reach a moderate. If eastern Oklahoma can warm up just a little on Friday, then storms will likely become severe as they move into the region on Friday. Either way, a beneficial rain event looks likely for the region.

Saturday seems to have some decent potential, but is somewhat muddied by expected convection on Friday. GFS has copious amounts of convection in Texas and develops such in Oklahoma east of HWY 75. This appears reasonable given CAPE values 3000 J/Kg and decent winds. Directional shear will be marginal, but cannot ignore an organized severe threat. So, far now will go with slight risk for eastern OK on the new Day 3, but this look to be a low-end event right now.

Atmosphere looks to become progressive again by Sunday as the next southwest U.S. drops into California. Models don’t appears as favorable with mid/upper level winds as prior runs but CAPE values above 4000 J/Kg and CIN near 0 seem to warrant severe chances.

Monday-Wednesday will continue to be elusive as models struggle with the evolution of the western U.S. trof. GFS is more progressive than the other models but the ECMWF actually seems to lift the surface reflection east quicker on Tuesday. For now will have to continue to monitor but degree of moisture and expected instability seems to warrant medium-end chances as stronger flow aloft impinges on the region. Additionally, the GFS is now farther south with the low as it ejects northeast on Tuesday, taking the best winds into Texas. Either way, we’ll have to see what happens.

Probabilities:

Risk area for any part of OK (valid: 05/19 Day 1, Day 2, Day 3):

Slight: 100%, 100%, 100%
Moderate: 80%, 30%, 0%
High: 0%, 0%, N/A

Chase Status:

Level 2 – looking to head out on Thursday, target area is Watonga to Woodward, departure time is 2-3pm.

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