Weekly Severe Weather Discussion

Severe Weather Discussion for May 21 – 27. 2007.
Valid: 05/21/2007.

Highlights:

– Showers and thunderstorms are possible every day this week across the region. Tuesday and Wednesday hold the best potential for organized severe weather. Excessive rainfall may become a problem on Wednesday.

– Temperatures will be near or slight below normal this week.

Probabilities:

– Severe Thunderstorms in Oklahoma: 30%
– Storm Chase: 10%
– Winter Precip: < 5%

Discussion:

Models are in decent agreement regarding the evolution of the longwave pattern this week. The primary feature will be the development of a longwave trof in the western U.S. today, progressing east during the week. After the trof passes late Wednesday the atmosphere enters a zonal flow pattern with some perturbations noted through the weekend.

Moisture values remain below seasonal norms this morning across much of the region. However, deep layer tropical moisture is on the Gulf Coast as of 13z. This moisture should move north today and tomorrow as a surface low develops in Kansas. There will be another surface low in South Dakota. A dryline will form in the western Texas panhandle this afternoon. Isolated severe thunderstorms are possible given presence of upper 50s/low 60s dewpoints in this region. Wind fields aloft are not impressive so activity may tend to be mutlicellular. SPC has a slight risk up for this region and that looks good.

Tuesday and Wednesday look more active across the region with both days showing potential for a moderate-end event. SPC Day 2 highlights Kansas for the best activity and this is certainly true given positioning of the mid-level winds. However, it is not a stretch to bring northwest Oklahoma into the higher-end threat given 30kts at 500mb Tuesday afternoon. Instability is lower than would be expected this time of year, but 3000 J/Kg is still sufficient for organized severe. Concerns for this forecast are overnight activity and potential for several clusters to develop. Winds above 300mb are quite weak and could lead to anvil seeding. Given that the storms should move east this may not be a huge problem.

Mid/upper level flow increases dramatically on Wednesday from Oklahoma northward. However, the upper system is just enough out of phase that the main threat will be associated with a cold front across this region. Given time of year and that moisture values should be quite high Wednesday afternoon, excessive rainfall appears likely. The ground remains saturated across the region and would expect flash flooding if current indications verify.

Thursday will be quiet in Oklahoma with a chance of showers and thunderstorms in Texas along the old frontal boundary. Friday – Sunday will see a chance for thunderstorms each day, but very difficult to locate the frontal boundary at this point. Lack of a 588dm or greater ridge means that the atmosphere will be relatively uncapped. Oklahoma will be near the 30-40kt zonal flow and any perturbation in this flow will set off thunderstorms.

I'm not that high on chasing for Tuesday, but it certainly holds some potential. SPC Day 2 at 17z will be interesting, especially if they pull the 10% hatched area south. I'm not expecting such and I'm not convinced it is a really good setup. The weakness in the upper wind fields would seem to preclude a widespread tornado threat. However, presence of decent CAPE values and moderately strong flow aloft may be enough for a few supercells in Oklahoma on Tuesday.

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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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