Severe Weather Discussion 2007-12-1

Severe Weather Discussion for October 13 – 14, 2007.
Valid: 10/10/2007.

Highlights:

– Isolated severe thunderstorms are possible Saturday evening and overnight across the TX/OK Panhandles into far western Okahoma.

– Severe thunderstorms are possible across western and central Oklahoma on Sunday.

Discussion:

I'm going to take a bath on the forecast this week, but hey that's how this forecasting thing goes. Situation for Sunday rapidly turning into a moderate-end event.

Recent runs of the GFS are in very good agreement and 00z ECMWF is in close agreement, albeit a little slower. Excellent model agreement makes the overall forecast somewhat less complicated. The 12z NAM-WRF/GFS and the 00z ECMWF agree nicely at 84 hours regarding placement of the upper system and associate jet streaks. A somewhat disturbing occurrence. However, it does lead some credibility to the GFS's forecast parameters behind 84 hours.

A southern stream shortwave trof will move into the southwest U.S. on Friday and progress rapidly east into the longwave ridge position over the central U.S. This system is progged to slow some as it move through the southwest U.S. Model placement has the system on the Utah/Nevada state line Saturday evening with a jet streak extending through the base of the trof over southern Arizona into central New Mexico. There is some disagreement on the strength of the 500mb jet max, but that really is nothing to get into right now. Upper system is then progged to move into the region by early Sunday afternoon. The system should completely clear the region by Monday afternoon, however there is potential for timing changes the next few days. I doubt much of a slow down will occur as a longwave trof is progged to approach the western U.S.

Previous cold front and the backdoor front expected tonight will effectively close the Gulf for a few days with dewpoints commonly in the mid 40s across the region. Moisture return should begin late Thursday as the upper ridge starts to move east, but will initially be shallow. Previous frontal surges pushed 65F dewpoints into the Gulf and there isn't much time for this moisture to recover. With the upper system to the west and little upper support, only isolated severe thunderstorms are expected. The atmosphere will be strongly sheared and there should be decent heating. Moisture values in the upper 50s may be enough to set off a few LP supercells. The NAM-WRF may be underdoing moisture return, we'll see.

Attention then turns to Sunday as the upper system is progged to lift out across the region. Favorable timing hints at this being a moderate-end event for parts of the region. Wind speeds aloft increase during the overnight hours into Sunday afternoon as shear also increases. Moisture will deepen across the region with surface values in the low/mid 60s along with 850mb dewpoints near 14C. The surface low will be in southwest Kansas early Sunday morning and is progged to progress slowly east during the day with a dryline along the TX/OK border. Storms should initiate during the early afternoon as temperatures warm into the mid/upper 70s. Initial instability figures places LI to -8 and CAPE values around 2000 J/Kg.

The instability will likely change since I'm using the GFS and the NAM-WRF ending at 84 hours is a little lower on moisture return. Additionally, models indicate deep layer UVV's in advance of the upper system, which may result in widespread shower activity. A big different between October and April is that we have the ground heat and it doesn't take much sun to warm things up. Also, the models tend to indicate that the early morning lift will be separated to the north from the best jet structure during the afternoon. This may place the best severe threat over southwest Oklahoma into central and south-central sections. Details will hopefully become a little clearer in a few days.

SPC has things well covered on their Day 4-8 outlook. I'm not sure we'll get a slight on Day 3 for the panhandle or western Oklahoma. It really depends on what 00z models do. It wouldn't surprise me, which is the main reason for the 75%.

Probabilities:

Risk area for any part of OK/TX (valid: 10/11 Day 3):

Slight: 75%
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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