The SPC says a Tornado Watch may be needed for portions of Mississippi and Alabama shortly. This would be a “PDS” Tornado Watch. A “PDS” Tornado Watch is a “Particularly Dangerous Situation” Tornado Watch indicating the potential for strong and/or devastating, long-track tornadoes.
Here is the Discussion from the SPC:
Mesoscale Discussion 0200
NWS Storm Prediction Center Norman OK
1056 AM CDT Wed Mar 17 2021
Areas affected…Central MS…Central AL Concerning…
Severe potential…Tornado Watch likely Valid 171556Z – 171800Z
Probability of Watch Issuance…95 percentSUMMARY…
The environment is becoming increasingly favorable for tornadic supercells and a PDS tornado watch will likely be needed within the next hour or two.
DISCUSSION…
Regional radar and satellite imagery continues to show deepening convection within the broad and robust warm-air advection regime across much of MS and AL. Some modest clearing occurred briefly ahead of the leading showers over MS and temperatures across much of central and southern MS are now in the mid 70s. Slightly cooler surface temperatures exist across central/southern AL.
Dewpoints exhibit a similar trend, with upper 60s/low 70s across much of central/southern MS and mid to upper 60s across central/southern AL. These thermodynamic conditions have eroded much of the convective inhibition across the region, although forecast soundings do suggests some minimal (i.e. MLCIN of -25 J/kg or more) likely remains in place at the top of the boundary layer.
Continued moistening of the low-level profile coupled with slight cooling aloft should result in a removal of all convective inhibition. In addition to improving thermodynamics, the low-level wind fields continue to increase. Recent VAD profiles from LIX and DGX show 50 kts within the 1-2 km layer.
Latest VAD from DGX also sampled 0-3 km storm-relative helicity over 350 m2/s2. These strengthening low-level winds are expected to persist while gradually spreading northward/northeastward into more of northern AL. Deep-layer vertical shear is already in place over the region, with 0-6 km bulk shear currently 50 kt over central MS/AL, increasing to 70 kt over northern MS/AL.
In all, the overall environment is expected to becoming increasingly favorable for discrete supercells capable of all severe hazards, including intense tornadoes. These initial storms are forecast to develop within the 17-19Z time frame.
A prolonged threat for tornadoes is anticipated across the region, with conditions remaining favorable well into the evening
. ..Mosier/Grams.. 03/17/2021
CourTESY: spc.noaa.gov
