Severe Weather Discussion for May 18, 2013.
*Active Severe Weather Day Expected*
– Severe thunderstorms will develop this afternoon across western Oklahoma. These storms will move northeast and east, and approach central sections by early evening. Primary threats are tornadoes, destructive hail greater than baseballs, and strong winds to 80mph. There is a strong tornado threat in northwest Oklahoma by early evening.
– The threats for Sunday and Monday continue, but this forecast will focus on today.
An area of elevated convection has evolved across southwest Oklahoma this morning. This area is likely rooted well above the cap layer and as such presents a strong wind and small hail threat. The 00z NAM caught onto this but the 06z lost it. The 10z HRRR seems to have it, although appeared a little too convective. This area should move north over the next few hours and eventually leave the threat area by early afternoon. Atmosphere modification is not expected.
All primary operational models develop convection along an eastward mixing dryline this afternoon. There are differences as to where storms are expected to develop, but several of them are clustered in west-central and northwest Oklahoma. Additionally, the convection-allowing HRRR develops 3-4 supercells in western Oklahoma. This model also shows extreme instability with CAPE values above 4000 J/Kg. Upper level winds will not be particularly strong but decent speed/directional shear in the low/mid levels along with extreme instability will compensate for this. Storm motion seems to be strongly northeast but a right mover would certainly travel east.
12z OUN sounding is interesting. At 6am with temp 71 and dewpoint 71, CAPE value is 3137 J/Kg and LI -10. Convective temp is 99F. Strong surface heating is expected today, which will likely yield the extreme instability noted above. The hodo is rather messy, but that was known going into this. 500mb winds will improve today, especially across western Oklahoma.
Models agree that a shortwave trof will move through the region late morning. There will be subsidence in its wake with a more substantial shortwave trof moving into the region by mid/late afternoon. As such, explosive severe thunderstorm development is likely by 4pm across western Oklahoma. Supercells will be favored, with attendant threats.
Latest SPC outlook has maintained the moderate risk and 10% hatched tornado area. This looks good and see little reason to change ATTM. They also note the weak wind fields and that these could be compensated for by the extreme instability.
A quick look at Sunday and Monday yield no major changes ATTM. NAM continues to be an outlier with cold front position late Sunday and subsequent warm front movement back north on Monday. Time will tell. Significant and extremely dangerous severe weather is expected Sunday.
Level 1 – heading west to Clinton @1:30pm. I see no reason to change the intended target area. There is agreement among the group of where to head.by