The Storm Prediction Center (SPC) has issued a Moderate Risk for eastern TX, LA, southern AR, southeastern OK, and western MS.
Associated with the Moderate Risk is a hatched 15% tornado threat ring, marked “SIG SEVERE”.
To summarize the SPC: A deep, fast-moving trough over NM/AZ will shift into the Great Plains by tomorrow morning. Embedded in the trough will be a jetstreak that will pass near this threat area. Storms will start out as high-based supercells in KS/OK, hailers with limited moisture and too near a polar front. Farther south, the low over Texas will continue to pull rich Gulf of Mexico moisture into the south. The upper level dynamics and strong southeasterly winds will create plenty of shear to support supercellular development. However, the convective mode will be messy: More than likely, it will form into a Quasi-Linear Convective System (QLCS) with embedded supercells, perhaps turning into a derecho in the early morning hours. Tornadoes, some of which will be long track, are possible.
The 12 Z upper air sounding from Lake Charles, LA, shows why the situation is messy today. There is plenty of moisture available, given the 65 F surface dewpoint and 1.46 inches of precipitable water. This environment shows 275 J/kg of skinny Convective Available Potential Energy (CAPE) and -152 J/kg of Convective Inhibition (CINH). The LCL is 184 m. The 0-3 km lapse rate is 4.0 C/km, and there is only a tiny capping inversion that will quickly mix out. The hodograph shows that the shear is strong, with 31 kts of low-level shear (due to directional changes), and 57 kts of deep-layer shear (due to speed changes). Because of all of this, the supercell parameter is 5.6.
The visible satellite imagery shows cloudy skies over the entire southeast, with pockets of convection along the western edge of the clouds, representing the approaching cold front.
The surface observations steady southerly to southeasterly winds from the Gulf of Mexico. The temperatures are mild and the dewpoints are high. The whole region is under cloudy skies.
The surface pressure chart shows that there is an elongated low ahead of the trough.
The HRRR simulated reflectivity is showing a powerful squall line moving through the southern plains late this evening and into the Mississippi River Valley just before sunrise.
The HRRR is showing a dryline ahead of the trough. It’s not as big of a player today, but it does demonstrate how much moisture is present in the south today.
The HRRR CAPE/CINH plot is showing only meager CAPE, with a maximum near Dallas this afternoon. After that, the CAPE will decrease, probably because of the cloud cover that exists already and the convection that is expected this morning.
The HRRR shows that the supercell parameter will be high, particularly near the western edge of the risk area.
The HRRR also shows that the SigTor parameter is high through northwestern Mississippi.
The HRRR shows helicity tracks are weak, but there are a lot of them.
The timing, terrain and storm morphology are awful. There’s too much forcing. Nothing about this day would go right if I was chasing this system. There is plenty of moisture, but the biggest source of lift is the fast-moving frontal boundary. There is also plenty of shear; the deep layer shear will support embedded supercells and the low-level shear will support tornadogenesis. Storms will go linear very quickly. In the bayou. At night. Nighttime tornadoes in the swamp.
This would still be a very difficult chase, and I wouldn’t attempt it. Well, if I did attempt it, I’d go for structure in northern Oklahoma or Kansas. Maybe I’d see the back end of a large squall line at sunset.