Severe Weather Discussion 2007-8-3

Severe Weather Discussion for April 23 – 24, 2007.
Valid: 04/22/2007.

Highlights:

– Severe thunderstorms are likely across western Texas into Nebraska, including western Oklahoma on Monday. Tornadoes, destructive hail, and damaging winds are all possible.

– Active severe weather day is expected for parts of Texas northward into Kansas on Tuesday.

– Chase Condition Raised to Level 3.

Discussion:

Models did a near 180 on me with the 12z runs. Determining the significance of the upcoming events has become very complex since last night. Both the 12z GFS and NAM are about six hours faster with the upper system than they were on the 00z runs. Unfortunately a few more runs will have to pass before it is possible to determine if this is a trend or anomaly. To add a little more fire to the developing chaos, SPC has gone with an enhanced moderate for Monday afternoon in western Oklahoma. The use of the 45% area is one step below a high risk. I'm not convinced that Monday is a high risk day, but definitely an enhanced moderate day. In fact, I hinted at this on Friday.

The general forecast remains the same as a strong southern stream shortwave trof is still expected to dig into the southwest U.S. on Monday. Across the region a surface low will form in southeast Colorado with deep layer moisture return to the east of a developing dryline. Said dryline should be in the central Texas panhandle Monday morning and mix eastward to at least the edge of the cap rock by late afternoon. Aloft, wind speed and directional shear will dramatically increase during the day with NAM progged helicity values quite high across southwest Oklahoma.

Surface dewpoints should be near 65F in the threat region with 850mb dewpoints around 13.5C. Afternoon temperatures should reach the upper 70s resulting in CAPE values around 3000 J/Kg and LIs to -8. The cap will be an issue and I think all the models are under-forecasting it. The NAM does show 500mb vorticity moving across the region by 1pm, but this appears to pass with little fanfare. More robust activity should occur later in the afternoon as the cap weakens due to daytime heating and well-timed jet maxes. GFS and NAM are in good agreement regarding the particulars for Monday. Even confidence exists that the chase condition is raised.

I'm not going to waste my time on Tuesday at this point. Another model run is necessary to see if the 12z runs become a trend. I'd also like to take a look at the more reliable 12z ECMWF.

Probabilities:

Risk area for any part of OK/TX (valid: 04/23 Day 1, Day 2):

Slight: 100%, 100%
Moderate: 100%, 75%
High: 40%, 15%

Chase Status:

Level 3 – Monitoring Monday and Tuesday.
Monday is expected to be a southwest Oklahoma chase with a target near Altus.

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

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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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