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Severe Weather Threat Day 1 Sat Sep 7

Upper-Air Pattern Over Much Of Conus Will Continue To Be Dominated By Large Anticyclone Centered Over Central Plains...With Ridging Nnwwd Across Sk. To Its E...Heights Are Fcst To Fall Across Sern Canada And Adjoining Areas Of Lower Great Lakes And Nern Conus...As Series Of Shortwaves Contributes To Synoptic Trough Amplification.

That Will Reinforce Seasonally Cool Low-Level Air Mass Across Much
Of Mid-Atlc...New England And Central/Nrn Appalachians Regions.

Meanwhile...W Of Ridge...Compact Mid-Upper Level Cyclone Remains Well-Defined In Moisture Channel Imagery Over Nrn Wa And Srn Bc. This Feature Will Move Little Through First Half Of Period. Low Then Will Move Slowly Ewd...As Height Falls Occur To Its N...Ahead Of Progressive/Stg Shortwave Crossing Gulf Of Ak.

At Sfc...Cold Front Now Moving Swd Across Portions Nd/Mt Is Expected To Progress By 8/00z To Lh...Lower Mi...And Srn Ia. Then... Frontal Segment Continuing Wwd Over S-Central Neb And Sern Wy
Will Have Stalled...Perhaps Moving Back Nwd As Warm Front Over Wy.
High-Plains Warm Front Then Should Move Nwd To Near Black Hills By 8/12z...While Remainder Of Front Reaches Nrn Mo...Srn Oh...Nern Pa And Maine.

Nrn Plains...

Widely Scattered To Scattered Tstms Should Evolve Over Portions Ern
Mt...Nern Wy...And Perhaps Wrn Sd And Swrn Nd By Late Aftn -- Either
As Temporal Extension Of Morning Convection Or Development Anew
Along Associated Outflow/Differential Heating Zones. Additional
Activity May Form And Move Off Higher Terrain Of Central/Srn Mt And
Extreme Nrn Wy...Wherever Ely To Nely Flow Can Contribute Upslope
Component. Isolated Threat May Extend As Far Se As Black Hills
Region...Though Deep Shear Will Be Weaker There Compared To Mt.
Main Concerns Will Be Damaging Gusts And Large Hail...With A Few
Supercells Expected.

With Salient Upper-Air Features Changing Little From Previous Day... Primary Contribution To Deep Shear And Buoyancy Will Arise From Low-Level Mass Response In Wake Of Fropa That Is Currently
Underway. Prind Post-Frontal Flow Will Veer Gradually Throughout
Morning Into Mid-Aftn...By Which Time Elys And Selys Should Converge Invof Boundary-Layer Moist Axis From Nwrn Sd Wnwwd Across Sern/Central Mt.

Resulting Corridor Of Moisture Advection/Transport...With Dew Points 60s F...Should Support Mlcape 1500-2500 J/Kg Where Sustained Sfc Heating Takes Place. Pockets Of Weak Flow Above Sfc Are Evident In Fcst Soundings...However...Stg Directional Shear Will Contribute To Favorable Bulk Values -- E.G. Effective Shear Magnitudes 40-50 Kt.

Sfc Thetae Should Decrease With Nwd Extent Over Nern Mt And Wrn Nd During Most Of Period. However...Elevated Low-Level Waa/Moist Advection With 35-45 Kt Sely Llj May Support Continuation/Upscale
Growth Of Some Activity During Evening Over Ern Mt/Wrn Nd. By
Around 8/06z...Declining Buoyancy Indicates Any Remaining Svr Threat Should Be Mrgl...And Diminishing With Time.

Mid-Upper Ms Valley Region...

Scattered Tstms In Clusters May Form Near Front...With Risk For
Isolated Damaging Gusts From Precip-Loaded Downdrafts. Isolated
Large Hail Also May Occur During Aftn And Early Evening. There Is
Some Suggestion Of Weak Frontal-Wave Low Development Over
Central/Swrn Ia...Especially In Those Progs Showing Subtle Vorticity
Max/Mcv Apchg Area From Prior/Morning Activity Over Nrn Plains. Any
Such Feature Would Enhance Mesoscale/Frontal Convergence To Its Ene. Flow Aloft Will Be Weak Relative To Other Areas With Svr
Probabilities...But Still Characterized By Stg Veering With Height.
Low-Middle Level Lapse Rates Will Be Sufficiently Steep To Support
2000-3000 J/Kg Mlcape In Presence Of Upper 60s/Low 70s F Prefrontal Dew Points And Around 1.75 Inch Pw.

Conditional Potential Exists For Cluster Of Convection To Expand During Evening And Move Swd Or Even Swwd Over Mid Ms Valley Region...In Which Case Damaging Wind Threat Could Get Better Organized On Mesoscale. For Now...Will Maintain Mrgl Unconditional Probabilities In Deference To Those Uncertainties.

Lower Mi...

Widely Scattered To Scattered Tstms May Develop Invof Sfc Cold Front
This Aftn...With Baroclinic Zone Potentially Reinforced By Outflow
From Antecedent Convection Farther N. A Few Gusts Apchg Svr Limits May Offer Risk Of Minor Damage. Warm Sector Should Experience Sufficient Diabatic Heating By Mid-Aftn To Offset Weak Midlevel Lapse Rates And Support Around 1000 J/Kg Mlcape In Some
Areas...Amidst Mid 50s To Low 60s F Sfc Dew Points. Though Some
Directional Shear Will Exist Beneath Mid-Upper Level Nwlys...Weak
Speeds And Wly Component Of Near-Sfc Winds Will Limit Both
Convergence And Low-Level Shear. Tstms Should Weaken During
Evening.


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