Weekly Severe Weather Discussion

Severe Weather Discussion for September 18 – 24, 2006.
Valid: 09/18/2006.

Highlights:

– Temperatures near or below normal all week. Chance of showers and thunderstorms on Thursday.

– Fall starts this week.

Probabilities:

– Severe Thunderstorms in Oklahoma: 25%
– Storm Chase: 15%
– Winter Precip: < 5%

Discussion:

Oh what a pleasant pre-fall morning. Of course in a few days, we can call it a fall morning. Anyway, lots of rainfall across Oklahoma yesterday with the best stuff falling near I-44. Texas got 1-2 inches in areas that really needed it. Oklahoma City is about 3.91 inches below normal after Sunday’s rainfall, current normal high is 83F.

Longwave trof will remain over the central U.S. as Sunday’s system lifts northeast into Canada. 12z GFS is similar to the 00z GFS/ECMWF runs digging the next system into Colorado by late Wednesday. GFS has a closed low as it moves eastward across Nebraska/Kansas on Thursday. Models then prog the low to stall as energy moves into the west side of the longwave trof. The entire thing lifts northeast early Saturday morning as the longwave pattern deamplifies.

The main issue for this forecast deals with sensible weather as the initial low moves into the region on Thursday. The 12z NAM-WRF is much faster with the upper system as it doesn’t dig it as far south as the other models. The big difference here is the NAM-WRF keeps the 35-40kt jet max on the east side of the low, causing it to lift out quicker while the ECMWF/GFS keep the jet energy on the west side of the low. I’m going with the two global models for now, especially since the NAM-WRF was so far off last week.

Issue then becomes what to make of storm chances with this system. Surface ridge from the prior cold front remains over the area and will slowly move east the next few days. The Gulf will be closed during this time period. By late Wednesday the high should move far enough east to allow the lee surface low to inititate moisture return. However, the Gulf may not fully open until early Thursday. GFS shows 60-65F dewpoints in the state mid-day Thursday. 850mb moisture is around 14C. Upper system is progged to move rapidly through the state as it wraps up into a large system late Thursday. GFS develops QPF across the region and rain chances certainly seem good. The question is severe weather chances. Currently, I’m going on the low-end of the scale for severe weather. However, if the models slow the system much more the resulting severe weather threat will increase.

NHC tracked Gordon last week, which was a major hurricane, the first of 2006. While Gordon has recurved, Helene is out there and may also become a major hurricane. Neither of them are a threat to the U.S. at this point.

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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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