Severe Weather Discussion for March 28 – 29, 2007.
– Active severe weather day is likely on Wednesday with the potential for significant activity across western sections of TX/OK. Primary threats are tornadoes, large hail, and damaging winds.
– Severe thunderstorms are possible on Thursday across central/eastern parts of the region.
– Chase condition increased to Level 3, looking to go tomorrow.
No change to prior forecasts or reasoning. The ECMWF will likely verify with this system as the 12z runs have drifted south and a tad slower. However, all the models have done varying levels of good/bad during this event. There is better confidence on the evolution of this system, although the exact processes expected on Wednesday can not yet be determined.
Upper system is still progged to race eastward into Colorado where it will stall and close off. At the surface a dryline is progged to extend from an occluded surface low in northeast Colorado. This feature has been progged in a variety of location, but I prefer the GFS's farther west location about two row of counties west into the Texas panhandle. The 12z NAM-WRF has the dryline on the OK/TX border but think this may be too far east. Models agree that CAPE values should be at least 1500 J/Kg with LIs to -6. The NAM-WRF is much more aggressive with 3300 J/Kg CAPE at 1pm, decreasing it to 2000 J/Kg at 7pm. This seems mainly due to the continued odd dewpoint sink during the afternoon. The GFS does not hint at this and deep layer moisture would negate such. Some of it may be due to the shallow air the NAM has across central Texas, but once again I'm suspicious of this solution.
Speed shear looks good across the threat area with directional shear remaining a concern. Model soundings indicate that supercell potential will be AOA 70% in southwest Oklahoma and if the EML can hold, then storms should remain discrete. All models trigger convection along the dryline during the afternoon, most likely between 3-5 pm. This is why I'm putting 1pm as the departure time. There is something about southwest Oklahoma that the models keep developing QPF in this area first. The NAM/GFS/HIRESW all do this. It will be interesting to see what actually occurs tomorrow. The afternoon issuance of the Day 2 is good and I like the 10% hatched area across western OK. I'm not high on NW OK since the flow will be unidirectional until the warm front in Nebraska where better turning will be found. Other than that the better turning will be from southwest Oklahoma southward into Texas.
PRIND: The dryline will be in the eastern Texas panhandle Wednesday afternoon with convective initiation expected by 5pm. Storms may try to line out, but the EML/cap may keep them discrete in the early stages. This situation warrants a moderate risk and I'm continuing with 100%.
Little time to talk about Thursday but with the upper system hanging back west there is a severe weather threat for central/eastern parts of OK/TX on Thursday. Much of that will depend on atmospheric recovery, a rather difficult thing in March if there is a lot of convection on Wednesday. Potential certainly exists for another active severe weather day on Thursday. Heavy rain may become a problem, but not going to make the same mistake I did last week.
Risk area for any part of OK/TX (valid: 03/28 Day 1, Day 2, Day 3):
Slight: 100%, 100%, 70%
Moderate: 100%, 50%, 0%
High: 0%, 0%, N/A
Level 3 – Wednesday is the current target day. Looking to leave around 13:00 for Altus to Sayre. I'm not able to isolate it beyond those two places. Will also have to monitor Thursday for a local chase.
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