MODERATE risk for multiple significant tornadoes LA/MS


Day 1 Convective Outlook

NWS Storm Prediction Center Norman OK

1020 AM CST Mon Dec 16 2019

Valid 161630Z - 171200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS NORTHEAST LA TO CENTRAL MS... SUMMARY

Numerous to potentially widespread severe thunderstorms are anticipated across parts of the Lower Mississippi Valley and Deep South with tornadoes and damaging winds as the primary hazards. The peak tornado risk is expected through early evening, centered on northern and central Louisiana into central Mississippi. ...Lower MS Valley and Deep South...

Primary changes this outlook are to increase tornado and wind probabilities across parts of LA/MS. In addition, the SLGT risk has been expanded north for wind into middle TN near the expected surface cyclone track.

Probability of a tornado within 25 miles of a point. Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.

Gradual expansion of surface-based thunderstorms is expected across the Sabine and Lower MS Valleys, along/ahead of the surface cold front. Activity should grow in coverage and move across the Mid-South and MS/AL regions through this evening into tonight. Several tornadoes will be possible across the entire region, but with greatest potential in a corridor from northern/central LA across central MS where semi-discrete supercells are most probable, and should access the most favorable CAPE/shear parameter space. Damaging gusts should be the most common convective event area-wide. Severe hail is most probable over southwestern parts of the outlook area, essentially collocated with greatest tornado probabilities.

12Z soundings sampled the pronounced elevated mixed layer overlying the rich low-level moisture plume (characterized by upper 60s to low 70s surface dew points) emanating northeast from the western Gulf. Abundant cloud coverage will be a limiting factor, but moderate surface-based buoyancy characterized by MLCAPE of 1000-2000 J/kg will persist in the ambient environment ahead of the front across LA through parts of MS. Relatively backed surface winds, beneath a 40-60-kt southwesterly LLJ, will contribute to enlarged hodographs and 300+ effective SRH.

Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% or greater probability of wind gusts 65 knots or greater within 25 miles of a point.

Given potential for sustained warm-sector supercells, such as progged by the HRRR, HRRR-P and WRF-ARW, such an environment will support a risk for multiple significant tornadoes. Otherwise, regardless of warm-sector convective density, a band of thunderstorms should evolve close to the front and shift across the remainder of the outlook area through the period. This activity will offer a risk for numerous to widespread damaging winds and embedded QLCS tornadoes. It will eventually encounter gradually weaker low-level and mid-level lapse rates and less inflow-layer moisture content with northeast/eastern extent. This should result in a somewhat lesser threat for damaging winds and tornadoes tonight. ..Grams/Wendt.. 12/16/2019