Severe Weather Discussion for September 15 – 17, 2006.
– Isolated severe thunderstorms are possible Friday evening across the Texas/Oklahoma Panhandles into western Oklahoma.
– Active severe weather day is expected on Saturday from northern Oklahoma into South Dakota.
– Severe Thunderstorms are possible in eastern Oklahoma eastward to TN and northward to Ohio.
12z models continue in general agreement that weak southern stream low will be rapidly ejected eastward on Friday. Said system should move into the region as a shortwave trof, albeit rather weak. Gulf opens up early Friday as the current surface ridge moves east and allows for moisture return. It appears that the combination of the weak system and surface ridging will keep the severe weather threat limited on Friday. For now, only isolated severe thunderstorms are expected, with the main threat hail and wind.
Attention then turns to Saturday, which is appears the be the focus at this point. 12z GFS and 00z ECMWF agree on several items, one of which is timing. However, the 12z GFS has a trof through Saturday night, forming a closed low by Sunday morning. The ECMWF brings the system through the northern U.S. as a closed low. The 12z GFS increased the speed of the system, but the difference is it keeping an open wave. This would make sense as the formation of a closed low would tend to slow it down. For now I’m going with the agreed upon timing, but I’m not touching whether it will be an open wave or low. Current models clearly show the best area for severe weather to be Kansas and Nebraska. However, it doesn’t take a great leap of faith to place northern Oklahoma into the threat area. I have included it since a change of 50 miles will make a difference.
Rapid moisture return should be underway late Friday with 65F dewpoints in northern Oklahoma by early Saturday. These dewpoints should continue rapidly north into northern Kansas/southern Nebraska by Saturday afternoon. Power house trof is progged to impinge upon the region during the mid-afternoon hours on Saturday, near peak heating. Across Kansas, wind fields at all levels will dramatically increase as a 50-60kt 500mb jet rotates northeast out of the base of the trof. The best lift from PVA will be in northern Kansas and Nebraska. The rest of the region will need dryline convergence and other mesoscale lifting features to initiate convection. 12z GFS has decent instability around 2000 J/Kg and LIs -3 to -4 across the region. Progged 500mb temperatures are a little warm, but given that surface temperatures around 90F are expected concern is limited. Wind fields decrease rapidly in central and southern Oklahoma. However, speed shear may be enough, when combined with
favorable directional shear to result in organized storms. There is not a clear signal that convection will develop in Oklahoma along the dryline and in fact the GFS holds off convection until the cold front hits the dryline, resulting in a squall line. I think there is a window of opportunity for supercells in northern Oklahoma Saturday afternoon. At this time the best chance is Kansas, northward. SPC Day 4-8 has this well covered and current model progs provide increased confidence that a significant severe weather event will occur on Saturday.
Sunday is kind of a waste to talk about right now, given the current speed of the upper system. ECMWF and GFS have a closed low by Sunday morning with a rapidly moving cold front. Both models place the cold front south and east of Oklahoma by Sunday evening. There may be a window for severe thunderstorms early Sunday.
Risk area for any part of OK/TX (valid: 09/13 Day 3):
Level 1 – Normal
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