Severe Weather Discussion 2008-6-1

Severe Weather Discussion for March 17 – 18, 2008.
Valid: 03/13/2008.

Highlights:

– Severe thunderstorms are possible across much of Oklahoma and north Texas on Monday.

– Severe thunderstorms are possible across much of Texas on Tuesday. Excessive rain is possible on Tuesday across Oklahoma.

Discussion:

12z GFS completed the transformation to the ECMWF solution. While the GFS is a little stronger, actually closing off the low. The models are in decent agreement on spatial and timing issues.

A rather broad zonal flow pattern will continue for a few more days before significant amplification occurs over the weekend. Models continue to advertise a phased shortwave trof moving into the western U.S. with the southern system slowing down as it strengthens over the southwest U.S. The GFS now shows a closed low with trofing back southwest into Old Mexico while the ECMWF has an open system. The overall impacts initially aren't that great and may not matter too much through the forecast. The slower movement will place Oklahoma in a severe weather threat on Monday as modified moisture returns from the Gulf.

Canadian cold front expected to pass through Oklahoma late Friday into Saturday. As noted this past Sunday this feature will push to the Gulf, temporarily preventing moisture return. By late Sunday the ridge moves far enough east for moisture return across western parts of the region. However, as noted above the moisture will be of the modified variety due to recent frontal intrusion. Additionally, at 00z Monday the models both show the 1016mb line near Houston. This indicates that while moisture return will occur, we'll probably see only 50-55F at this point. The 12z NAM has picked up on this and is very bearish with dewpoints showing only 50-55F along the coast. The 12z GFS is much more robust with moisture return at this time period and this continues through its forecast. There really is no reason to pick one over the other, but the current pattern and forecast would seem to favor the NAM.

By midday Monday the upper system is closed off over AZ/NM/Old Mexico. A surface low is located in the Texas panhandle with a dryline in western Oklahoma southward into Texas. GFS dewpoints have 60-65F values northward into Oklahoma and I suspect that the NAM may be a little lower. We'll have to wait until 12z tomorrow for that run. Wind fields across the state will be increasing with some veering noted in the lower levels. Currently the GFS is rather veered with the 850mb flow, but I've seen that before. And, it is a detail not worthy of discussion right now. The 300mb/200mb wind fields are impressive with a 125-150kt jet max expected around peak heating. Despite expected modified moisture, this should be sufficient for isolated severe thunderstorms. I'm not sure if this will rise to a medium impact event, but the potential certainly exists. GFS develops a lot of precip along the warm front in northern Oklahoma and additional activity by midday along the dryline. There
could be early storms, but I'm not so sure given the slow movement of the upper system and EML. We saw today just how strong the EML can be and that shouldn't change as the cold front will primarily affect central and eastern areas. Moisture return should be in full swing by Monday evening, even though it may not reach Oklahoma by the best storm time.

Upper system is progged to migrate slowly across Texas on Tuesday with the GFS maintaining a close low. These type of systems have a habit of producing severe weather in Texas. By this time, much better moisture should be in-place with 60-65 dewpoints in Texas. While not great, it is seasonable and more than sufficient given proximity of the upper system. Additionally, I think there is a decent chance of overnight severe storms in Texas as the low level jet increases beneath strong flow aloft. It would only take a minor shortwave trof to develop storms in the warm sector into Tuesday. It does mean that Tuesday is kind of muddied across Texas, but there seems to be some potential for a continuation of severe.

In Oklahoma an excessive rain threat exists as the system moves south of the region and the surface low moves into north Texas. Strong isentropic lift should occur for a period of time before midday Tuesday. Models indicate in excess of 1-2 inches across central and southern Oklahoma. This appears reasonable and for now will watch and wait.

SPC Day 5 looks good and expect this to continue for Day 4.

Probabilities:

N/A

Chase Status:

Level 1 – Normal

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For more information, weather news, and chase summaries go to http://www.hook-echo.com.

Hook-echo Blog http://hook-echo.blogspot.com and for RSS Feed – http://hook-echo.blogspot.com/feeds/posts/default

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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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