Severe Weather Threat Sat May 11

Upper-Air Pattern Fcst To Amplify And Simplify By End Of Period.
Transition Will Be Completed From Current Split-Flow Regime To One Dominated By High-Amplitude Ern Synoptic Trough And Ridging From Ca Across Nrn Rockies And Wrn Canada. Intense Canadian Shortwave Trough Will Evolve 500-Mb Cyclone That Is Expected To Move Ewd Across Nrn Ont. Large Area Of Height Falls And Cyclonic Flow Aloft Will Spread Across N-Central Through Nern Conus...From Upper Midwest Across Great Lakes And Oh Valley To New England.

Meanwhile...Broad...Positively Tilted Area Of Cyclonic Flow -- With
Weak/Embedded 500-Mb Low -- Will Be Over Much Of Nrn/Wrn
Mex...Drifting Ewd Then Becoming Quasistationary During Latter Half
Of Period As Ern North American Troughing Moves Farther Away.

At Sfc... Leading Cold Front Should Extend From Low Over Nrn Ny Swwd Across Oh...Ky And S Tx Around 11/12z...With Second And Much
Stronger Cold Front From Ls Swwd Across Wi...Ia And Central Plains.
By 11/00z...Leading Front Should Extend From Hudson Valley Area Swwd Across Srn Appalachians...Becoming Ill-Defined Over Mid-South Region And Lower Ms Valley. Outflows From Prior Tstm Complexes Likely Will Obscure S Tx Segment Of This Front. By End Of Period...Both Fronts Will Merge Over New England And Mid-Atlc Regions...While Leading Boundary Remains Diffused By Convective Processes Over S Tx And/Or Nwrn Gulf.

Mid-Atlc And Vicinity...

Band Of Tstms May Develop Along/Ahead Of Sfc Cold Front During
Aftn...With Potential For At Least Isolated Gusts Near Svr Levels.
Flow Aloft Will Be Largely Parallel To Frontal Or Prefrontal Zones
Of Lift...Contributing To Likelihood Of Quasi-Linear Convective
Structures. Weak Lapse Rates Aloft And Probable Presence Of Cloud
Cover Also Indicate Limited Instability...Leading To Mlcape Under
1000 J/Kg In Most Areas. However...Modified Model Soundings Suggest Pockets Of 1000-1500 J/Kg Mlcape...Juxtaposed With 30-40 Kt Effective Shear Magnitudes...Are Possible. Farther S Across
Va/Carolinas...Deep Shear Should Be Somewhat Weaker...But With
Greater Buoyancy. Wherever Any Corridor Of Stronger Heating/ Destabilization Can Develop...Svr Threat May Be Enhanced... However There Is Too Much Mesoscale Uncertainty To Assign Specific 15%/Slgt Categorical Area Attm.

Deep S Tx...Rio Grande Valley...

Convection Should Be Ongoing At Start Of Period Over Parts Of S Tx.
Resulting And Prior Boundaries May Focus Subsequent/Aftn Development Over S Tx And Adjoining Portions Mex Near And S Of Remnants Of Front. Additional Activity Should Form Over Higher Terrain Of Nern Mex As Deep Layer Of Moist...Low-Level Ely/Nely Flow Moves Upslope...Though Sfc-Based Buoyancy May Be Limited By Low Cloud Cover. Presence Of Stg Mid-Upper Winds Nearly Opposing Boundary-Layer Flow Should Contribute To Effective Shear Magnitudes 50-65 Kt...Though Weak Low-Level Shear May Limit Hodograph Size. Where Sustained Diabatic Heating Occurs...And Air Mass Has Not Been Too Overturned By Prior Convection...Steep Midlevel Lapse Rates And Rich Low-Level Moisture May Support 2000-3000 J/Kg Mlcape. However...Svr Potential Appears Too Conditional And Uncertain Attm For More Than Mrgl Probabilities.

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