Severe Weather Threat thru Jul 27

Sun Jul 20 Convective Outlook
Storm Prediction Center Norman Ok
Slgt Risk Svr Tstms for Eastern Dakotas thru Cntrl & N Minnesota...

Severe Thunderstorms Will Be Possible Late This Afternoon Into Tonight Over Parts Of Minnesota And Perhaps The Eastern Dakotas.

Isolated Severe Storms Are Possible This Afternoon And Evening Across Northern Portions Of The Great Basin...From Northern Utah Through Northern And Central Nevada To In Vicinity Of...And To The North And South Of...Lake Tahoe...California.

Ern Dakotas To Nrn/Central Mn...

Trends In Visible Satellite Imagery Suggested The Cu Formation Across Srn Manitoba Has Greater Vertical Development...Than The Cu In Sern Nd Into Nwrn Mn. The Ascent Attendant To The Manitoba Convection Should Reach Nwrn Mn Around Or After 00z. As This Occurs...Tstm Development Will Be Possible Where The Air Mass Has Become Strongly Unstable /Mlcape 2000-3000 J Per Kg/.

Nrn Ut Across Nv To Interior Ca...

Lightning Data And Mosaic Radar Imagery Showed Tstms Have Developed As Expected Invof The Nwd Moving Great Basin Shortwave Trough. Given Instability Is Generally Weak...Low Severe Wind Probabilities Will Be Maintained.

Prev Discussion...

A Large Scale Pattern Change Will Commence Today And Eventually Support A More Active Severe Weather Regime Across The Northern Plains/Upper Ms Valley Areas Through Monday.

For Today...Modest Height Rises Will Spread Across The Great Plains And Upper Ms Valley And Great Lakes As A Broad Mid/Upper Level Anticyclone Takes Form. The Building Central U.S. Anticyclone Will Be Flanked To The East By A Stalled But Weakening Trough Covering Much Of The Area East Of The Ms River. A More Compact Disturbance Of Subtropical Origin Will Track North From Ca/Nv Across The Great Basin...On The Western Flank Of The Building Downstream Ridge. A Belt Of More Pronounced Mid-Level Flow Containing A Number Of Shortwave Disturbances Will Remain Situated Over The Northern Border States...From The Pacific Northwest Eastward To Mn.

Eastern Dakotas Across Central And Northern Mn...

The Combination Of Evapotranspiration In The Boundary Layer And Steepening Mid-Level Lapse Rates Associated With Eastward Advection Of Eml Plume From The High Plains Will Contribute To A Corridor Of Strong To Potentially Extreme Instability From The Eastern Dakotas Into Central And Northern Mn This Afternoon. Mlcape Of 3000-5000 J Per Kg Is Indicated In Forecast Soundings From The Nam And Wrf-Arw Given Surface Dewpoints In The Upper 60s To Lower 70s F And Temperatures In The Upper 80s F.

This Developing Volatile Warm Sector Is Forecast To Remain Capped With Only Weak Large Scale Ascent Likely Through The Peak Of The Diurnal Heating Cycle Based On Latest Guidance. However...A Shortwave Disturbance And Associated Frontal Wave Over Southern Manitoba Will Translate East Through The Late Afternoon As A Pre-Frontal Trough Strengthens From The Dakotas Into Mn. As The Cold Front Settles East-Southeast Across Nd And Into Northern / Western Mn And Merges With The Surface Trough Through The Early Evening...Most Guidance Suggests The Possibility Of At Least Isolated Storm Initiation.

Given The Presence Of 30-40kt Deep-Layer Shear Expect A Few Semi-Discrete Supercells Could Evolve And Pose A Threat For Both Large Hail...Possibly To Around 2 Inches In Diameter...And Damaging Winds. Tornado Potential Also Appears Possible Given Magnitude Of Cape Compensating For Weak But Non-Negligible Sr-Helicity In The Storm Updraft Layer. Activity May Evolve Into Clusters Or Perhaps Line Segments Near Advancing Trough/Front With Some Possibility Of Forward-Propagating Segments Maintaining A Wind/Hail Risk Toward Central Mn Later Tonight.

Great Basin Interior Ca Across Nv To Northern Ut...

Scattered To Widespread Relatively High-Based Convection Across The Great Basin Will Be Sustained By Large Scale Ascent Accompanying The Disturbance Migrating North On The Western Flank Of Building Upper Ridge. Mid-Level Flow Of 25-30kt May Further Contribute To Updraft Tilting And Storm Persistence With Evaporational Cooling Beneath Cloud Bases Supporting Gusty To Locally Damaging Winds.

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Mon Jul 21 Convective Outlook
Storm Prediction Center Norman Ok
Mdt Risk Svr Tstms for Eastern Sd...Cntrl & S Mn...& W Cntrl/Nw Wi...
Slgt Risk Svr Tstms surrounding the Mdt Risk for Dakotas E thru Upper Midwest...

Severe Storms Appear Increasingly Likely Across Parts Of The Upper Midwest From Monday Late Afternoon Into Tuesday Morning. Damaging Winds...Some With Speeds To Hurricane-Force...Should Be The Primary Hazard. A Tornado Threat And Large Hail Cannot Be Ruled Out.

A Strong Mid-Upper Level Anticyclone Will Remain Anchored Over Nm Through Day 2...With Some Height Rises Expected Monday Morning Across Central Mn And Nrn Wi As The Nern Extent Of The Ridge Builds Ewd Into The Great Lakes.

Meanwhile...Of Greater Significance To The Severe Weather Threat From Monday Afternoon To Tuesday Morning Is Models Agreeing With The Timing Of A Well-Defined Shortwave Trough Expected To Move Into The Central Dakotas Monday Afternoon. This Feature Is Currently Moving Nwd Through The Great Basin And Is Forecast To Accelerate Ewd Within The Srn Extent Of The Wlys From The Nrn Plains To The Upper Great Lakes Region By 12z Tuesday.

At The Surface...A Cyclone Should Become Centered Over Cntrl Sd Monday Afternoon With A Quasi-Stationary Front Near The Nd/Sd Border Into Nrn Mn. A Wind Shift Will Trail Wwd From The Surface Low To Invof The Mt/Wy Border...Where A Lower Severe Probability Is Expected. A 40-45 Kt Swly Llj Is Expected To Extend Into Nrn Ia/Srn Mn Monday Night To Early Tuesday Morning.

Upper Midwest...

Given The Degree Of Instability Expected Across Parts Of The Ern Dakotas Ewd To Wrn Wi Monday Afternoon/Evening /Mlcape Exceeding 4000 J Per Kg/ And The Model Agreement In Timing Of The Great Basin Shortwave Trough...Confidence Continues To Increase For The Potential Of A Forward-Propagating Mcs From Monday Evening To Early Tuesday Morning.

Severe Wind Gusts...Some At Hurricane-Force...Will Be Possible Across Nern Sd...Sern Nd...Much Of Central And Srn Mn Into Wrn Wi Resulting In An Upgrade To A Moderate Risk For This Region.

The Thermodynamic Setup Remains Similar To The Initial Day 2 Issuance...With Continued Evapotranspiration And Poleward Moisture Transport Yielding Increasingly Rich Boundary Layer Moisture Beneath An Eml Capping Inversion Across Ern Sd Into Cntrl/Srn Mn. Surface Dew Points Will Reach Into The 70s As Mean Mixing Ratios Approach 18-20 G/Kg. With Very Steep Mid-Level Lapse Rates Around 7.5-9.0 Deg C/Km In The 700-500 Mb Layer...An Extremely Unstable Air Mass Should Develop.

Large-Scale Ascent Attendant To The Progressive Shortwave Trough Will Move Into Nd/Nrn Sd Monday Afternoon. This Ascent Combined With Isentropic Lift Should Result In A Mix Of Scattered Elevated And Surface-Based Storms By Mid-Late Afternoon. Although Stronger Mid-Level Wlys Will Be Confined To The Canadian Prairies... Sufficient Deep-Layer Shear Will Exist For Multicells / Transient Supercells Supporting All Severe Hazards With The Initial Storms.

A Continued Likelihood Exists For Mlcin Being Minimized Near The Surface Cyclone And Adjacent Warm Sector Where Surface Temperatures Should Reach At Least The Upper 90s. Thus...An Amalgamation Of Updrafts Should Yield A Deepening Cold Pool... With An Upscale Growth Into An Mcs --- Producing Svr Wind Gusts --- Appearing To Be More Likely for The Ern Dakotas Into Mn And Wrn Wi Through Monday Night/Early Tuesday Morning.

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Tue Jul 22 Convective Outlook
Storm Prediction Center Norman Ok
No Svr Tstm Areas Forecast...

Isolated Strong To Severe Storms Are Possible Tuesday Afternoon Into Wednesday Morning From Parts Of The Western Great Lakes To Lower Missouri Valley And thru The Northern Rockies To Black Hills.

A 600 Dm Anticyclone At 500 Mb Will Remain Anchored Over The Srn Rockies. A Ridge Will Emanate N Across The Nrn High Plains...As A Shortwave Trough Tracks From S-Cntrl To Se Canada. An Upstream Shortwave Trough Should Approach The Wa/Bc Coast By Early Wed.

At The Surface...A Cold Front Will Push Se Across The Great Lakes To Lower Mo Valley As An Anticyclone Shifts From The Srn Prairie Provinces Into The Upper Midwest.

Upper Great Lakes To Lower Mo Valley...

A Decaying Mcs Is Anticipated To Be Ongoing At 12z/Tue Over Parts Of The Upper Ms Valley / Great Lakes. A Remnant Mcv/Mid-Level Perturbation Will Probably Continue E Over The Upper Great Lakes During The Day...Influenced By The Canadian Shortwave Trough. The Associated Belt Of Enhanced Mid-Level Wlys Should Become Increasingly Divorced From The Strong To Extremely Unstable Air Mass Centered Over Ia.

The Degree Of Afternoon Convective Coverage Is Questionable As Predominately Wly 800-700 Mb Winds May Maintain A More Prominent Capping Inversion Than Depicted In Nam/Gfs Forecast Soundings. But At Least Isolated Tstms Appear Likely Along Portions Of The Cold Front /Modulated By Remnant Convective Outflow/. This Would Seem Most Probable Across Parts Of Cntrl/Srn Wi And Also Near The Surface Thermal Ridge Over The Lower Mo Valley. Modest Shear Should Largely Favor Multicell Clustering Which May Be Capable Of Isolated Severe Wind/Hail.

Portions Of The Region May Be Upgraded To A Low-End Slight Risk In Later Outlooks.

Nrn Rockies To Black Hills...

A Belt Of Enhanced Mid-Level W/Swlys Will Exist Between The Srn Rockies Anticyclone And Shortwave Trough Approaching Wa/Bc. Elongated/Straight-Line Hodographs Would Favor Potential For Splitting Supercells Amidst Isolated To Widely Scattered Diurnally-Driven Storms. However... With Mid-Level Heights/Temperatures Slowly Rising...Overall Buoyancy Will Likely Remain Weak... Limiting Sustenance/Intensity Of Updrafts. As A Sly Llj Strengthens Over The Nrn High Plains Tue Night...Richer Low-Level Moisture Will Be Advected N.

Large Spread In Guidance With Overnight Convective Development Beneath The Ridge Renders Low Confidence In Highlighting Slight Risk Probabilities This Outlook.

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Jul 23-27 Convective Outlook
Storm Prediction Center Norman Ok

N-Cntrl States On D4-6/Wed-Fri...

At Least Marginal Risks For Organized Severe Storms Are Evident Through D6/Fri Before Predictability Further Wanes Next Weekend.

On The Large-Scale...Guidance Is Fairly Consistent That A Shortwave Trough Initially Near The Wa Coast At 12z/Wed Should Track Towards The Srn Prairie Provinces. This Will Impinge On The Ridge Emanating N From The Stout Anticyclone Centered Invof The Four Corners. With A Lee Trough Over The Nrn High Plains...An Initially Confined Low-Level Moisture Plume Should Increase By Late Week.

A Swath Of Enhanced Mid-Level Wlys Between The Trough / Anticyclone Should Yield Daily Risks For Organized Severe Storms. Mesoscale Predictability At This Time Frame Is Too Low To Warrant Probabilities Aoa 30 Percent.


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