Severe Weather Threat Day 1 Sat Apr 27

In Mid-Upper Levels...Split-Flow Pattern Will Persist Through Period
Over Conus. Shortwave Trough -- Now Evident In Vwp/Profiler Data
And Moisture Channel Imagery From Ern Ks Swwd Across Tx Panhandle -- Is Fcst To Move Generally Ewd Through Period. By 28/00z...Weak 500-Mb Low Should Evolve Around Primary Vorticity Max Over Nern Mo...With Main Shortwave Trough Remaining Positively Tilted Swwd Across Ok. By End Of Period...Trough Should Extend From Central Il Swwd Across Portions Nern/N-Central Tx.

At Sfc...Weak Low Should Be Along Cold Front At Start Of Period... Over Sern Ok Or Adjoining Red River Valley. This Low Should Migrate/Redevelop Newd To Sern Mo/Nern Ar While Cold Front
Reaches Portions Central/Sw Tx. Kinematic Col Is Expected Along
Cold Front Over Central/E-Central Tx During Afternoon...Amidst Weak
Sfc Warm-Sector Winds...Substantially Limiting Low-Level
Convergence. However...Svr Tstm Potential Is Greater On Either Side
Of That Col...In Each Area Discussed Below.

Portions Sw Tx...Rio Grande Valley...

Initiation Of Widely Scattered Tstms Is Possible This Aftn Into Evening On Two Primary Foci -- Frontal Zone...And High Terrain From Davis Mtns Sewd Through Big Bend Region Into Mex. This Regime Of Flow Aloft...Favorable Low-Level Moisture...Anticipated Stg Sfc Heating S Of Front...And Substantial Ely/Upslope Component In Boundary Layer Supports Potential For Orographically Forced Tstms. Although Only 25-35 Kt Of Flow Is Progged Over This Region...Near-Sfc Ely Component Contributes To Favorable Deep Shear -- E.G. Effective Shear Magnitudes Around 40-50 Kt.

Common Algorithmic Fcsts Of Tstm Motion Indicate Any Convection That Forms Over Mex Highlands And Becomes Discretely Supercellular Before Crossing Rio Grande May Remain Over Mex...Moving Slowly Ssewd. However...Even If Orographically Initiated Right-Moving Supercells Do Not Cross Rio Grande...Smoothed Low-Level Hodographs Do Appear Small Enough To Support Left-Movers With Risk For Large Hail And Stg/Damaging Gusts As Well. Fcst Mlcape Increases From Around 1500 J/Kg Over Big Bend Region To 2000-2500 J/Kg Range Between Lrd-Drt. Cold-Pool Development Also Is Possible With Any Clustered Convection... Leading To Resultant Forward Propagation Down Rio Grande Valley.

Portions Lower Ms Valley/Mid-South Regions...

Areas Of Precip Including Tstms Should Be Ongoing Through Much Of
Morning...Along And E Of Cold Front...Over Portions Sern Ok...Ar And
Ewd Across Mid-South Region. This Activity Should Contribute To
Differential-Heating/Outflow Boundary Over Portions Ern Ar/Wrn Tn.
More Specific Location Of Any Such Mesoscale Feature Remains
Uncertain Attm...However.

Meanwhile...Expect Cold Front Or Prefrontal Convective Line To Move
Into Progressively Moistening And Diabatically Destabilizing Warm
Sector Throughout Afternoon. Resulting Lift Will Lead To Increasing
Coverage Of Tstms Near Sfc Low...Trailing Swwd On Sfc Front.
Damaging Gusts And Ocnl Hail Are Possible. Coverage Is Fcst To
Lessen With Swwd Extent Across Arklatex Region And Nrn La In Closer Proximity To Appreciable Capping At Base Of Eml.

Given That Factor...And Weaker/More Veered Low-Level Flow...Svr Risk Is Smaller And More Conditional Swwd Toward Col. Sfc Dew Points 60s F Should Contribute To 1000-2000 J/Kg Mlcape Amidst 35-45 Kt Effective Shear Magnitudes Over Much Of Warm Sector In This Region. However...Small Area Ne Of Sfc Low And Along And S Of Convective/Cloud Boundary May Contain Sufficiently Backed Winds To Enlarge Low-Level Hodographs And Boost Deep Shear Magnitudes Above 50 Kt. Enhancement To Helicity...Along With Relatively Maximized Vorticity On Any Such Boundary...May Yield Some Tornado Threat. However...This Potential Appears Too Mesoscale/ Conditional In Nature To Assign More Than Mrgl Probability Attm.


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