Weekly Severe Weather Discussion

Severe Weather Discussion for September 11 – 17, 2006.
Valid: 09/11/2006.

Highlights;

– Near normal temperatures through Wednesday, increasing to slightly above normal through Sunday.

– Severe thunderstorms are possible Friday – Sunday.

Probabilities:

– Severe Thunderstorms in Oklahoma: 60%
– Storm Chase: 25%
– Winter Precip: < 5%

Discussion:

Very complex setup for the latter part of this week, have I mentioned how much I love these Day 5-7 setups. Forecast confidence is below normal.

For Mon – Thu, conditions will be normal September weather. Temperatures should be about normal, maybe slightly below on Tuesday. Current normal high is 86F. This is a result of a small southern stream shortwave trof moving east across the region today. Said trof is progged to intensify the next few days as it moves into the eastern U.S. This trof will help drag a surface ridge into the region, with the associated cold front moving to the Gulf Coast. By Thursday, a shortwave ridge will dominate the region as a northern stream longwave trof develops out west.

It is the progged northern stream longwave trof that makes this forecast complex. 12z GFS remains consistent with the overall placement of this feature and the 00z ECMWF is similar. This agreement among the global models gives decent confidence on the development of said feature during the expected time period. As the longwvave trof sets up over the western U.S, southwesterly flow should develop across the region. This will warm temperatures to above normal Thursday, where they should remain through Sunday.

The GFS has offered differing speeds of this system with the ECMWF a little more consistent. However, the 12z GFS slowed the system and is in better agreement with the ECMWF. There is some placement differences among the models, but for now I’m not going to get caught up in those issues.

First item of interest is Friday as weak southern stream low gets ejected east by the longwave trof. Said low moves into the central U.S. as a shortwave trof around Friday afternoon. Model progs are consistent with the timing and placement of this feature, with unusual agreement between the GFS/ECMWF and run to run consistency. The downer is potential lack of moisture due to incoming surface ridge. We’ve been hosed by this pattern a lot in 2005 and 2006. However, it is September and 70F dewpoints are easy to come-by. So, I’m not as pessimistc as SPC and OUN. Given increasing wind speeds aloft, deep layer lift, and dryline in the Texas panhandle, severe thunderstorms are possible. GFS is very bearish with CAPE values, only going 900 J/Kg. I’m not sure I buy into the low instability at this point.

Setup becomes much more interesting on Saturday and especially Sunday as moisture continues to increase across the region. The issue with Saturday appears to be neutral lift and Sunday timing. These are issues that will be tracked during the week. If the 12z GFS is correct a medium-end event is possible on Sunday for eastern parts of the region. I certainly expect some timing issues as models typically move these things too fast. Probabilities are set as a first and best guess. Since this is issued once a week, I have to make it count.

If current trends continue, special discussions will be needed starting tomorrow.

The tropics remain seasonbly active with Hurricane Florence moving near Bermuda and a tropical depression behind it. To date none of the tropical systems have lived up to NHC’s forecasts and no major hurricanes have occurred. September 10th was the climatological max in tropical storm season, with a slow ramp down through early October.


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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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