Severe Weather Discussion 2008-8-2

Severe Weather Discussion for April 7 and 9-11, 2008.
Valid: 04/05/2008.

Highlights:

– Isolated severe thunderstorms are possible on Monday. Primary threat is large hail.

– Significant excessive rain event appears to be developing for Wednesday night through Friday. Rainfall totals of 3-5 inches are possible across a large part of central and eastern Oklahoma.

– A freeze is possible late Tuesday into Wednesday for northern Oklahoma.

Discussion:

The more I look at the models for next week, the less encouraged I am for chasing weather. Concern is rapidly turning towards an excessive rainfall event and likely flash flood situation. Of particular concern is the 4+ inch 60hr precip maximum on the 126hr GFS 12z panel in southeast KS. This is in a favorable location to adversely impact eastern Oklahoma reservoirs/rivers that have already receive an unusual amount of rainfall the past two months. The 00z/06z runs are similar in depicting an excessive rainfall event Wed – Fri for Oklahoma.

12z NAM continues to keep the 500mb jet stream unamplified on Monday. However, even though the 12z GFS has more amplitude to the northern stream system, the setup doesn’t seem favorable. This is due to the warm sector being removed eastward from the best directional/speed shear. The cold front that passed through the area late Thursday into Friday will make moisture return difficult into the area of best dynamics given a slower upper system. SPC’s Day 3 product looks good. I still think that robust convection may occur overnight as dynamics increase across the warm/moist sector over Oklahoma. The area will be at nocturnal min in temperature and hence instability. This would seem to limit the severity of the event. I made that mistake last Sunday night and will be careful again here. GFS CAPE values are near 1500 J/Kg and with a low level jet at least 30-40kts, storms will be able to organize into severe activity. GFS places a cold front along I-44, in a favorable SW-NE fashion for storms to anchor upon. I’m going low on the moderate chances, but I still think SPC will hold off.

GFS shows a Canadian connection to Monday’s system on the 00z/12z runs, in fact the 12z run was colder than the 00z run at 12z Wednesday. GFS surface temperature map valid 12z Wed has temperatures in the 30s across much of Oklahoma. Given the warm weather the past few weeks, a killing frost/freeze is certainly possible. It also takes the wind out of a potentially significant severe weather day as I doubt the Canadian air will be kind enough to pack up and leave by Wednesday afternoon. In fact the GFS shows the Canadian air holding tough across the state, even into the TX/OK panhandles. At this time, it appears the best severe weather threat will be in Texas where a medium-end event may occur.

In Oklahoma, isentropic lift will combine with ample moisture to produce an excessive rainfall event. As noted above, model QPF values are 4-5 inches in some areas. Eastern Oklahoma has been particularly hard hit by excessive rainfall the past two months. As the powerhouse longwave trof develops over the western U.S. late Tuesday into Wednesday, surface dewpoints of 65-70F and 850mb dewpoints of 14-16C yields unseasonably high atmospheric moisture content across the region. This will be carefully monitored the next few days and I anticipate continuing discussions for this threat. Tomorrow’s discussion will be combined into the normal weekly issuance.

Probabilities:

Risk area for any part of OK/TX (valid: 04/06 Day 2):

Slight: 100%
Moderate: 15%
High: 0%

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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