Tuesday, September 29, 2009
Thursday's Severe Weather
We are still looking at a threat of severe weather developing Thursday across the Central U.S. but depending on which forecast model you go by, there is a very different result. The image above shows the forecast for Thursday evening at 7PM CDT. Notice the cold front is much farther east than the forecast for the NAM (see below). The scenario forecast by the NAM would be much more conducive for supercell and tornado development Thursday afternoon and evening so this will be the model of choice to discuss, since the GFS shows a fast moving squall line with mainly a gusty winds small hail event.
Looking at the forecast map for the NAM for 0Z Friday ( 7PM CDT Thursday Evening) you see an area of low pressure with a warm front extending east through central Illinois and a cold front draped across central Kansas and northwestern Oklahoma. Out ahead of the front a dryline sets up and would be the focusing mechanism for thunderstorm development if this scenario unfolds. There are two areas that seem favorable for supercell development which could lead to isolated tornadoes late Thursday.
The first area that seems likely a few supercells could develop is along and east of I-35 in central Oklahoma south into North Texas north of Dallas. A large area of low level helicity will combine with moderate instability and moderate CAPE across these areas. 0-1km storm relative helicity are between 100 and 250 from the Dallas Forth Worth area through the Eastern third of Oklahoma (Fig 1). This should provide adequate low level turning of height to help produce low level meso cyclones as thunderstorms develop. In the same area surface based CAPE is on the order of 1500 to 2000 J/kg (Fig 2), not extreme but enough to have robust updraft strength in this area of low level helicity. Finally the surface based lifted index is around -4 in this same area providing moderate levels of instability (Fig 3).
The second area of supercell development and in my opinion may be the big winner when it comes to tornado development is out ahead of the surface low across southeast Nebraska, southwestern Iowa, eastern Kansas, and northwestern Missouri. An area of higher instability, CAPE and strong low level helicity will combine for the development of scattered supercells along the dryline/cold front. The images below show the surface based lifted index and CAPE for 4PM CDT Thursday Afternoon. LIs approach -5 (Fig 4)with CAPE approaching 2000 J/kg (Fig 5) in southeastern Nebraska. This area will also be experiencing very strong low level relative helicity, approaching 250 (Fig 6). For chasing Thursday I really like the northern option out ahead of the surface low for tornado development.
Again the following forecast is based on the NAM forecast model. I feel the GFS may be bringing this system through too fast as the water vapor imagery shows the 500mb vort max is moving into the Pacific Northwest slower than the GSF advertised on the 0Z run. I will issue another update later this afternoon after analyzing the 12Z model runs.