Thursday, April 21, 2016

April 26 Potentially Significant Severe Weather Event

A potentially significant severe weather event continues to show up in model guidance for Tuesday, April 26th over the Central and Southern Plains.

Storm Prediction Center
The Storm Prediction Center's long range outlook for Tuesday places a 30% chance of severe weather within 25 miles of a given point across northern Texas, much of central and eastern Oklahoma, and southern Kansas. A 15% chance of severe weather within 25 miles of a given point is situated from central Texas north into Oklahoma (outside of the panhandle), much of central and eastern Kansas, and extreme western portions of Missouri and Arkansas.

This event will begin taking shape on Monday, April 25th, when the storm system that will act to ignite this potential outbreak makes landfall on the West Coast. 500-millibar speed max near 100 knots barreling into southern California and western Arizona will be associated with a vorticity max rounding the base of the main trough, whose energy will be extended from northern California offshore Oregon and Washington by Monday morning. This speed max will shunt the aforementioned vorticity max east, where it will then round the lee side of the trough and act to pull the main trough northward by Tuesday morning.

The main energy associated with the trough will continue moving southeast into the Southwest U.S. and strengthen as another speed max rounds the base of the trough, this time making it negatively-tilted. Recall that a negatively-tilted trough means the highest wind speeds are 'pointing' the trough in a southeast direction. This indicates the trough is mature, and is optimal for severe weather.

As mentioned above, by Tuesday night at 7 PM Central Time, we see a speed max just breaching 70 knots traversing the Central Plains, indicating the trough is now negatively-tilted and at max strength. The trough here is in prime positioning for a severe weather event to unfold across the Central U.S. in the evening and overnight hours on Tuesday, and will set the stage for continued opportunities for severe weather as the week continues.

Strong moisture flow prior to the trough's entrance to the region will allow surface dewpoints to surge into the 70s by Tuesday evening, as the above graphic shows. As such, instability on the order of 4000 j/kg to 5000 j/kg is anticipated across Oklahoma into northern Texas, approximately in line with the 30% delineation in the Storm Prediction Center graphic at the beginning of this post. This ample moisture will allow for cloud bases to drop below 1000 meters- soundings from points across central and northern Oklahoma suggest cloud bases (also known as LCLs) will be around 600-800 meters off the ground, which can be conducive for tornado development. 800 meters off the ground is a little high for prime tornado formation conditions, but ample instability may act to offset a portion of this.

Lastly, supercell composite forecasts off the 18z GFS suggest the environment most conducive for supercell formation will be from southern Kansas across central Oklahoma, just into north Texas, again in line mainly with that 30% severe threat corridor. Although supercell composite values are high in central Kansas, the best axis of instability will be displaced in extreme southern Kansas and central Oklahoma, and it is this latter region where the best potential for severe weather lies. In this region, taking current model guidance at face value, all hazards of severe weather would be possible. This would include a threat for strong, possibly long-track tornadoes, very large hail, and damaging winds.

To Summarize:

- A severe weather event is being forecasted for April 26th.
- Substantial uncertainty exists regarding storm initiation, threat level, and placement of best environment, among other things.
- As such, confidence is low with this event.
- Going by current model guidance, a potentially dangerous severe weather situation could evolve across northern Texas, Oklahoma, and southern Kansas on April 26th.