Severe Weather Threat thru May 16

Sun May 11 Convective Outlook
Storm Prediction Center Norman Ok
Mdt Risk Of Svr Tstms for Neb...Ia...Mo & Ks...
Slgt Risk Of Svr Tstms for Mid Mo Valley...Cntrl & S Plains...
Mid & Up Ms Valley...S Great Lakes & Oh Valley...

A Severe Weather Outbreak Including The Potential For Very Large
Hail...Damaging Winds And Tornadoes...Some Of Which Could Be Intense Will Develop This Afternoon Into Tonight From Central Kansas
Northeastward Through Eastern Nebraska Into Central Iowa.

A Couple Changes Have Been To The Outlook For This Issuance.

The First Change Is To Add A Narrow Corridor Of Enhanced Tornado Threat Across Se Neb Into Wrn Ia.

The Latest 1 Km Visible Satellite Imagery Suggests Storm Initiation Is Taking Place Near The Sfc Triple Point In Webster County Neb.

Moderate Instability And Strong Deep Layer Shear Across Se Neb Should Result In Rapid Supercell Development With Cells Tracking Newd Along The Warm Front Late This Afternoon.

Enhanced Storm Relative Helicity Along The Warm Front Should Be Very Favorable For Tornadoes. At This Point...Confidence Is Great Enough To Warrant Adding A 15 Percent Tornado Probability Contour From Fillmore County Neb Newd Across The Vicinity Of Omaha Neb To Just West Of Des Moines Ia.

A Long-Track Strong Tornado May Occur Along The Corridor Late This Afternoon Into Early This Evening.

Other Tornadoes May Occur Ewd Across Parts Of Ia And Swwd Across Parts Of Cntrl And Wrn Ks.

The Second Change Is To Expand The Moderate Risk Area Slightly To The Northwest Across Ecntrl Neb Where Tornado Watch 142 Is Located.

Prev Discussion...

12z Upper-Air Data Indicate Mid And Upper-Level Jet Streaks Progressing Through The Base Of The Parent Trough Over The Lower Co Valley. These Wind Maxima Will Continue Ewd/Newd Into The Cntrl/Srn High Plains By 12/12z...In Tandem With The Srn Extension Of Full-Latitude Trough.

A 40-50+ Kt Sly Llj Will Be Maintained Ahead Of The Trough Across The Cntrl/Srn Plains Today With This Feature Migrating Ewd/Newd Into The Upper Midwest And Ozark Plateau Region By Early Monday.

At The Surface...A Lee Cyclone Over Wrn Ks Will Develop Newd Into N-Cntrl Ia Or S-Cntrl/Sern Mn By The End Of The Day One Period.

An Associated Warm Front /Reinforced By Convective Outflow/ Will Attempt To Move Nwd Through Parts Of Ern Neb And Srn/Cntrl Ia While A Cold Front Surges Swd Through The Cntrl Into Srn High Plains.

The Cold Front Will Gradually Overtake A Dryline Which Will Become Established From N-Cntrl Ks Sswwd Into The Far Ern Ok/Tx Pnhdls Into W-Cntrl Tx By Mid/Late Afternoon.

Mid/Lower Mo Valley Into Cntrl Plains Today Through Tonight...

Morning Raob Data From Top And Sgf Show That Considerable Moistening Of The Warm Sector Has Occurred Overnight With Lowest 100-Mb Mean-Mixing Ratios Of Around 13 G/Kg And Boundary Layer Dewpoints In The Mid 60s.

This Moisture Will Combine With Daytime Heating And Steep Midlevel Lapse Rates To Yield Afternoon Mlcape Values Approaching 2500-3500+ J/Kg.

The Remnants Of A Nocturnal Mcs Ongoing Over Wrn/Cntrl Ia Have
Recently Shown Signs Of Re-Intensification Owing To The Gradual
Destabilization Of The Air Mass Along The Nwd-Moving Warm Front.

A Growing Risk For Damaging Winds...Large Hail And Perhaps A Tornado Or Two Will Exist With This System As It Continues Ewd Today.

In The Wake Of The Morning Storms...Rapid Tstm Development Is
Anticipated From The Vicinity Of The Surface Triple Point Over
S-Cntrl Neb Swwd Along The Cold Front And Dryline Into Wrn Ks.

Ample Vertical Shear Will Exist For Supercells As The Initial Storm Mode With A Risk For Very Large Hail And Tornadoes.

However...The Largely Parallel Orientation Of The Deep-Layer Shear Vector To The Cold Front As It Overtakes The Dryline Suggests A Fairly Quick Upscale Growth Of Storms Into A Linear Mcs.

The Exception May Be Across Parts Of Ern Neb Into Wrn And Cntrl Ia This Afternoon And Evening Where Strong Low-Level Shear /I.E. Effective Srh Values Of 300-400 M2 Per S2/ And A More Favorable Deep-Layer Shear Vector Orientation Suggest A Greater Risk For Tornadoes /Some Possibly Intense/ Given A More Discrete Storm Mode.

Expect One Or Multiple Bowing Mcs/S To Evolve Tonight With A Hail
And Damaging Wind Threat Spreading Ewd Into The Midwest/Upper
Midwest And Lower Mo Valley.

Srn Plains This Afternoon Into Tonight...

Strong Heating W Of The Dryline Coupled With Weak Height Falls Aloft
Will Likely Promote Isolated Storm Development By Late Afternoon / Early Evening.

Forecast Soundings Indicate That Considerable Boundary Layer Mixing Will Result In 30+ T-Td Spreads Just E Of The Dryline With Steep Low To Midlevel Lapse Rates Contributing To Mlcape Of 1500-3000 J/Kg.

Associated Hodographs Exhibit A Vertically Veering Wind Profile With 30-40 Kt Of Deep-Layer Shear Which Will Be Sufficient For Supercells Capable Of Very Large Hail.

The Majority Of These Diurnally Driven Storms Are Expected To
Diminish With Nightfall Owing To The De-Coupling Of The Boundary
Layer.

However...Some Tstm Activity --Including An Isolated Risk For Damaging Winds And Hail-- May Persist Overnight Along The Ewd / Sewd-Moving Cold Front.

Upper Midwest Into Mid Ms/Oh Valleys into Tonight...

Despite The Presence Of Midlevel Ridging Downstream From The
Evolving Wrn U.S. Trough...Increasing Low-Level Moisture...Daytime
Heating And Strengthening Midlevel Lapse Rates From The West Will
Promote Moderate Instability By Afternoon With Mlcape Of 1000-2000
J/Kg.

Specific Details Regarding Storm Development And Evolution Remain Unclear...But The Presence Of A Nwd-Moving Warm Front And
Existing Convective Outflows May Serve As Foci For Tstm Development Later Today.

Other Storms Ongoing Across Ia/Nrn Mo Will Likely Begin To Affect Parts Of The Region This Afternoon Into Evening.

While Vertical Shear Will Be Weaker Than Points To The W...The Degree Of Instability Will Support Organized Storm Structures With A Risk For Primarily Damaging Winds And Severe Hail.

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Mon May 12 Convective Outlook
Storm Prediction Center Norman Ok
Slgt Risk Of Svr Tstms for Low Mo Valley...Mid & Up Ms Valley & S Great Lakes...
Slgt Risk Of Svr Tstms for Sw And Cntrl Tx...

Clusters Of Severe Storms Are Possible From The Southern Great Lakes Southwestward Into The Mid To Upper Mississippi Valley Monday Afternoon And Evening.

Severe Thunderstorms Will Also Be Possible In The Southern Great Plains Monday Afternoon And Evening.

Damaging Winds And Large Hail Should Be The Primary Hazards Although An Isolated Tornado Threat Can Not Be Ruled Out.

Mid To Upper Ms Valley/Great Lakes...

An Upper-Level Trough Will Move Into The Cntrl And Nrn Plains On
Monday As A Cold Front Advances Sewd Across The Mid Mo Valley And
Cntrl Plains.

At The Start Of The Period...Scattered Thunderstorms Appear Likely To Be Ongoing Near The Exit Region Of The Low-Level Jet In Parts Of The Upper Ms Valley And Mid Mo Valley.

This Activity Should Move Ewd During The Day As Moderate Instability Develops Ahead Of The Front From Ia Swd Across Mo.

Thunderstorms Should Initiate Along This Corridor During The Afternoon Along And Just Ahead Of The Boundary.

Other Storms May Initiate Across The Warm Sector Newd Across Lower Oh Valley Into The Great Lakes Region.

Nam Forecast Soundings At 00z/Tue From Chicago Il Swwd Into Cntrl Mo Show Mlcape Around 1200 J/Kg With 30 To 35 Kt Of 0-6 Km Shear.

This Should Be Enough For Storm Organization As Cells Increase In
Coverage Late Monday Afternoon.

Steep Low-Level Lapse Rates And Unidirectional Wind Profiles Should Be Favorable For Strong Wind Gusts With Short-Line Segments And Multicell Clusters.

Isolated Large Hail May Also Occur In Areas That Destabilize The Most.

Further To The Northeast...Model Forecasts Develop A Pocket Of Enhanced Instability Across Lower Mi Beneath An Ewd Moving Upper-Level Ridge.

In Response...Thunderstorms May Initiate On The Nrn End Of The Stronger Instability As Low-Level Convergence Becomes Enhanced Monday Afternoon.

Forecast Soundings At Detroit At 21z/Mon Show Mlcape Around 1200 J/Kg With 35 Kt Of 0-6 Km Shear.

In Addition...Low-Level Lapse Rates Are Steep And There Is Some
Directional Shear Below 850 Mb.

This Should Be Enough For An Isolated Severe Threat With Wind Damage And Hail Possible.

Srn Plains/Ozarks...

The Srn Extension Of An Upper-Level Trough Will Move Across The Srn Rockies On Monday.

At The Sfc...A Cold Front Will Advance Sewd Across The Srn Plains.

A Cap Across The Warm Sector Should Weaken Quickly By Midday Allowing For Scattered Thunderstorm Initiation Just Ahead Of The Boundary From Nern Ok Swwd Into Nrn And Cntrl Tx.

Model Forecasts Expand Convective Coverage Markedly By Monday
Evening Where Linear Mcs Development Will Be Possible From The
Ozarks Swwd Into Ncntrl Tx.

Further To The South...Storms May Initiate Ahead Of A Dryline Extending Swd From The Tx Hill Country To Near The Rio Grand Where A Few Storm Clusters May Develop Monday Evening.

Nam Forecast Soundings At 21z Mon From Fort Smith Ar Swwd To Near Dallas Tx Show Mlcape From 1200 To 1500 J/Kg With 0-6 Km Shear Of 20 To 25 Kt.

This Should Support A Marginal Severe Threat As Cells Increase In Coverage During The Late Afternoon.

Isolated Wind Damage And Hail Would Be The Primary Threats.

Further South...The Nam Forecast Sounding For Laredo Tx At 00z/Tue Show Substantially More Instability With Mlcape In The 2000 To 3000 J/Kg Range.

In Addition...Substantial Directional Shear In The Low To Mid-Levels
May Result In Moderate Deep Layer Shear.

This May Be Enough To Support Supercell Development If Cell Initiation Can Occur To The East Of The Dryline.

Isolated Large Hail And Wind Damage Would Be The Primary Threats Although A Tornado Or Two Can Not Be Ruled Out.

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Tue May 13 Convective Outlook
Storm Prediction Center Norman Ok
No Svr Tstm Areas Forecast...

Localized Severe Storms May Occur On Tuesday Morning To Afternoon
Over Parts Of South Texas And The Central Great Lakes States.

An Amplified Upper-Level Trough Will Persist Over The Cntrl States.

One Mid-Level Impulse And Attendant Speed Max Should Shift Nwd From The Upper Ms Valley Early In The Period.

Models Remain Quite Divergent In The Evolution Of An Impulse Embedded Within The Basal Portion Of The Trough Invof Srn Plains.

At The Surface...Primary Cyclone Will Track Across Nwrn Ontario In Association With The Lead Impulse.

A Trailing Cold Front Should Progress E/Sewd Across The Great Lakes To Off The Wrn Gulf Coast By Early Wed.

Central Great Lakes...

Middle 60s Surface Dew Points Should Persist Ahead Of The Cold Front With A Probable Increase In Diurnally-Driven Tstm Development By Early Afternoon.

However...Instability Should Be Weak Owing To Extensive Convection On D1-2 And The Meridional Nature Of The Upper Trough.

But...Within A Trailing Belt Of Around 35-45 Kt Swlys At 700 Mb...Some Organization Into Line Segments With Locally Damaging Winds Will Be Possible.

S Tx...

Although Deep-Layer Flow Should Be Modest...Substantial Veering Of
The Wind Profile With Height Should Be Sufficient For Moderate
Effective Shear Ahead Of The Approaching Cold Front Amidst
Moderately Steep Mid-Level Lapse Rates.

Uncertainty Does Exist With Regard To The Evolution Of Convection Late D2...With Potential For An Ongoing Cluster At 12z/Tue.

This Activity May Develop Sewd And Move Across Most Of S Tx Through Midday...Unfavorably Timed For Robust Diurnal Destabilization.

This Warrants Only Marginal Severe Probabilities For Wind/Hail.

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May 14-18 Convective Outlook
Storm Prediction Center Norman Ok

Primary Concern For Severe Potential Remains Mid-Week In Association With A Shortwave Impulse Embedded Within The Basal Portion Of An Amplified Upper-Level Trough Over The Cntrl Conus.

Guidance Spread Increases By D3 With The Spatiotemporal Evolution Of This Feature Along With The Degree Of Downstream Amplification And Potential Cyclogenesis Along A Cold Front Anticipated To Extend From The Lower Great Lakes To Lower Ms Valley At 12z/Wed.

The Ecmwf/Ukmet Are More Bullish With Kinematic Fields From The Cntrl Oh Valley To Ms/Al On D4...Followed By The Cntrl Appalachians To S Atlantic Coast On D5.

Still...With Meridional Wind Profiles And Likely Weak To Modest
Instability At Best...Overall Setup Might Only Yield A Lower-End
Severe Risk.


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