Severe Weather Discussion 2007-5-1

Severe Weather Discussion for March 20 – 23, 2007.
Valid: 03/20/2007.

Highlights:

– Isolated severe thunderstorms are possible in west Texas northeast into south-central Oklahoma on Tuesday. Primary threat is large hail.

– Severe thunderstorms are possible across western Texas into western Oklahoma on Wednesday.

– Severe thunderstorms are possible across eastern Texas on Thursday and Friday.

Discussion:

Eastern U.S. longwave trof is progressing away from the lower 48 today. This resulting surface pattern has pushed a Canadian cold front through the region and into the northern Gulf of Mexico. This front has pushed 50F dewpoints well south to near Cuba, however the western Gulf maintains 50-55F dewpoints.

Zonal flow will dominate for about a day or so as models prog a western U.S. longwave trof to develop by Tuesday. Models have been at odds regarding the overall development of this feature, but have recently come into agreement on a slow eastward movement of the southern part of this trof. In fact, 00z ECMWF/GFS and 12z GFS indicate split flow developing with this system. The southern stream portion will close off and become cutoff from the main flow by Wednesday. This system is then progged to hang around the southwest U.S. through Friday. Models do disagree on when this system will lift out, but would not expect it to be around much longer than Friday.

As a whole, closed lows are not a great severe weather producers. However, the evolution of this system and slow movement opens the door for severe weather across parts of the region. On Tuesday the mid-level flow will be subtle as a transition to southwest flow aloft occurs. Dewpoints should be in the mid/upper 50s across parts of the region. Wind fields will be weak, but there is the chance for sufficient lift to generate isolated severe thunderstorms. 12z GFS has CAPE values around 1500 J/Kg with the 12z NAM-WRF around 2500 J/Kg. I'm inclined to go with the NAM-WRF as it seems to be handling moisture better. This best location for activity appears to be west Texas into north Texas during the afternoon with additional activity in Oklahoma as the low level jet develops Tuesday evening. Moisture return may be a little delayed as the 1016mb line is west of Corpus, this will need to be watched.

Deep layer moisture return will continue overnight into Wednesday morning on 45-50kt LLJ. Upper system has closed off by this time and taken up residence in the SW U.S. Models differ on surface low location but an average is fine for this point, which would place the surface low in eastern Colorado. Next question is the eastward progression of the dryline. Wind speeds will increase at all levels and this may help the dryline mix eastward during the day. GFS places this boundary near the Cap Rock of the eastern Texas panhandle. This solution looks reasonable as there isn't any significant push on the dryline. Dewpoints should be at least 60F to the east in Oklahoma and Texas, potentially near 65F. I believe the GFS is too bearish on dewpoints and prefer to use an extension of the NAM-WRF for dewpoints Wednesday afternoon. GFS does develop mid-level lift and well-timed jet streaks Wednesday afternoon from central/west Texas northward into southern Oklahoma. Primary concern
for Wednesday is cirrus due to the southern latitude of the closed system and overnight convection. March is a fine line between severe weather and heavy rain events. Initial impression is that severe weather is possible for western parts of the region Wednesday afternoon. Wind patterns and instability would favor supercells.

Late Wednesday into Thursday appears to be the heavy rain event of this system. I think that rainfall is likely for much of the region as the upper system remains closed, cutoff, and west of the region. Severe weather threat appears to be lowest on this day.

Friday may see severe weather in Texas with isolated stuff in Oklahoma. By this time cloud cover should have really shutdown daytime heating, but these systems have a way of causing severe weather as they lift out.

Probabilities:

Risk area for any part of OK/TX (valid: 03/18, Day 3):

Slight: 50%
Moderate: 0%
High: N/A

Chase Status:

Level 1 – Normal

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About Putnam Reiter

Putnam has been storm chasing since 1990 and is a co-founder of Hook-Echo.com. For his day job, Putnam works in emergency management for information technology.
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