Severe Weather Discussion 2007-8-4

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

Highlights:

– Severe thunderstorms are possible later today, primary threats are large hail and damaging winds. Isolated tornadoes are possible with discrete storms.

– Active severe weather day is expected on Tuesday with an outbreak of severe thunderstorms and tornadoes. Primary threats are tornadoes, destructive hail, and damaging winds.

– Chase Condition remains at Level 3.

Discussion:

12z NAM/GFS continued the 00z trend of moving the eastern most position of the dryline to the west….confused? In other words, instead of the dryline ending up just past Oklahoma City, it is now progged near Weatherford, per the GFS. I'm still ignoring the NAM's dryline location. This is very similar to what we saw on March 28th when the dryline was 100 miles farther west. I wouldn't be surprised if a more westward correction occurs tonight. However, I'm satisfied at the current location. It is very tough for a dryline to mix out 150mb of 10C plus moisture. Not saying it can't happen, but not an easy thing to do. Also, late April is in an iffy climatology area where drylines can punch to I-35, reference April 19, 2003 and April 24, 1993. For this situation the upper low and surface low will be northwest of Oklahoma and the mid-level jet max will be north-northeast to south-southwest. There will be decent 700mb winds pushing into the dryline, but that's about it.

Another complicating factor regarding dryline movement and the overall severe weather threat is overnight convection or even early morning showers. Models have lots of QPF but nothing much more than 0.10 or so. This tells me they're struggling with the deep layer moisture across the region and "convince" themselves there has to be precipitation. The models seem to have a good handle on the cap, especially the GFS. The 12z GFS is my pick BTW. The NAM still over drives the dryline to I-35 by 7pm. The GFS holds it back west and all things considered is the better solution. There is no reason to pick showers or no showers tonight/tomorrow. The panhandles are heating up this afternoon and a dryline is in-place. Lubbock had some good points in their AFD, however SPC just issued an MD for the panhandles and part of western OK. A tornado watch appears likely. What these storms do tonight has huge implications on Tuesday.

Another issue to consider is early morning shower activity. Situations like this can end real fast if the atmosphere turns over. Tuesday morning will be such a case and we'll just have to wait and see. The cap looks strong enough to hold down activity and may even keep things isolated late tomorrow afternoon.

All other parameters appear in place for a significant severe weather event on Tuesday, including strong tornadoes. 12z GFS sounding for Oklahoma City at 7pm Tuesday yields CAPE values around 2400 J/Kg and LIs to -6, EHI 3.2 with about 30 J/Kg CIN at 7pm. Although, the GFS does increase the cap during the day. Either way, appears the cap should hold. Also, lapse rates are 7.1 C/Km 850-500mb late tomorrow afternoon.

PRIND: Storms may develop along the dryline today and push towards Oklahoma. Storms should diminish tonight as they move east into a strengthening cap. On Tuesday the dryline will mix into western Oklahoma and stall near Weatherford. Storms should erupt along this feature by late afternoon as upper system moves into Kansas. Discrete supercells are expected with attendant tornado threat. I think this is a high risk situation, but SPC will probably hold off until 16:30z and I'm with them there.

Probabilities:

Risk area for any part of OK/TX (valid: 04/24 Day 1):

Slight: 100%
Moderate: 100%
High: 50% – conditional on morning activity.

Chase Status:

Level 3 – Monitoring Tuesday, current target area is El Reno, leave time 14:00 CDT.

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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