Severe Weather Discussion 2008-7-1

Severe Weather Discussion for March 30 – 31, 2008.
Valid: 03/29/2008.

Highlights:

– Scattered to numerous severe thunderstorms are possible late Sunday into Monday. Primary threats are large hail and damaging winds. However, isolated tornadoes are possible with discrete storms during the overnight hours. An excessive rain threat is likely for northern Oklahoma Sunday night.

– Severe thunderstorms are possible on Monday.

– Severe thunderstorms are possible late Wednesday into Thursday.

Discussion:

Models are in decent agreement regarding the evolution of the western U.S. longwave trof the next 48 hours and the resultant affects on the sensible weather.

Afternoon surface analysis reveals moisture remaining close to the Gulf as a result of the prior cold front on Thursday. As the upper system develops in the western U.S. and a lee trof intensifies in Southwest Kansas. Models indicate that the prior cold front will weaken as the surface ridge shifts east and return north as a warm front. Rapid surge of moisture is expected late tonight into Sunday across Texas with 60F dewpoints to the Red River by 12z Sunday. Moisture return will continue during the day with 65F dewpoints indicated into southern Kansas. A strong cap will develop as southwesterly 700mb flow draws warm air from the west Texas/New Mexico region where models prog temperatures in the low/mid 80s. Lack of dynamic lift should suppress convection much of the day.

By late afternoon the dryline is progged by the 12z NAM to be located in the eastern Texas panhandle extending northward to a 999mb surface low in the northern TX/OK panhandles. A warm front should be extending northeast through northwest Oklahoma into north central Missouri. There are two areas of concern for convection Sunday night into Monday. Area one is along the developing dryline in the eastern Texas panhandle and area 2) is along the warm front.

The most likely area for severe thunderstorms is along the warm front during the overnight hours. This will be in response to a 60kt low level jet and strong moisture transport from the Gulf. Presence of impressive directional and speed shear should aid severe thunderstorm development and maintain this threat during the overnight hours. Orientation of the warm front is of particular concern as any storm that can move along this boundary will have an enhanced tornado threat. The warm front location is the best location for tornadic activity. The other area long the dryline is expected to remain quiet through early Monday as the EML prevents deep convection from lacking upper support. Speed and directional shear will be equally impressive in this area with CAPE values around 2500 J/Kg. The threat is not completely zero as convergence late Sunday afternoon into the evening may be sufficient to breach the cap in isolated areas. LP supercells would seem the most likely storm type
given dewpoints in the low 60s. Primary threats look to be destructive hail and damaging winds. An isolated tornado cannot be ruled out, especially close to the triple point in the northern Texas panhandle.

Monday has the potential to be an active day, however overnight convection will play a huge role in the severe weather threat. Early on Monday, convection should be ongoing across far northern Oklahoma into Kansas and Missouri. 12z NAM/GFS keep the warm sector across Oklahoma quiet and this would seem correct given expected cap (EML) strength of 8C at 700mb. The NAM increased the 700mb temperature 2C across Oklahoma from the 00z to 12z run. This tells me that the models may be underdoing the cap. Irrespective of the cap, wind fields will be very impressive across Oklahoma during the day Monday with 60hr 12z NAM Bufkit helicity values of 417 m^2/s^2 in Oklahoma City. The concern continues to be the potential for overnight convection and associated debris clouds. So, two scenarios seems possible on Monday; 1) a squall line developing in western Oklahoma by early afternoon as the cap is breached across a wide area due to lowering heights as the upper system moves to the north of
Oklahoma. It also appears that a weak shortwave trof will side swipe the state during the late morning into early afternoon time period. This could also be enough to initiate convection. This is one area the NAM and GFS disagree. The 12z NAM is low on QPF through 84 hours with only 0.16 inches in Oklahoma per Bufkit data. The GFS is a little more robust. Whether we get a squall line or discrete storms on Monday seems dependent on cloud cover and when storms initiate. However, temperatures will start off at least in the mid 60s, so little warming is needed for storm initiation. Current indications point to a potentially significant severe weather event on Monday.

SPC has a slight risk up for much of Oklahoma on Sunday with a 10% hatched and 30% area. This looks reasonable given the expected coverage of storms. The dryline should keep a slight risk and I'd expect an upgrade to a moderate along the warm front on the 13z Day 1 issuance. The probabilities should remain the same since any storms that develop will have high-impact, whether along the warm front or dryline.

SPC Day 3 also looks good. The slight risk is very contingent on issues noted above. The probabilities are pretty good, although I think a 10% hatched area is needed. I'm going low on moderate potential right now to see what the models look like tomorrow morning.

Models point to a medium-end event for Thursday. Discussions appear likely at this point.

Probabilities:

Risk area for any part of OK/TX (valid: 03/30, Day 1, Day 2):

Slight: 100%, 100%
Moderate: 40%, 25%
High: 0%, 0%

Chase Status:

Level 2 – Monitoring Sunday and Monday

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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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