Severe Weather Threat thru Aug 18

Mon Aug 11 Convective Outlook
Storm Prediction Center Norman Ok
No Svr Tstm Areas Forecast...

Thunderstorms With Gusty Winds May Occur From The Midwest/Ohio Valley Southwestward Across The Tennessee And Lower Mississippi Valleys Into East Texas. Strong Storms With A Threat For Locally Strong Wind Gusts And Some Hail Will Also Be Possible From Northern California And Nevada To Western Oregon.

Update...

Changes To The Categorical And Probabilistic Lines Are Mostly Minor ... And Mainly To Account For Diminishing Thunderstorm Risk In The Wake Of The Lead Short Wave Impulse Progressing Across The Ohio Valley Toward The Lower Great Lakes Region.

Although Weak Convective Development Is Evident On The Southeastern/Southern Periphery Of The Stronger Upstream Impulse Now Digging Into The Upper Midwest...Potential For Intensification Into Thunderstorms Across Parts Of Central / Eastern Missouri Into Illinois Seems Minimized By Weak Low-Level Convergence.

Weak Low-Level Convergence And Mid/Upper Support For Large-Scale Ascent Along The Initial Surface Front...Across Upper Portions Of The Lower Mississippi Valley Into The South Central Plains ... Also Seems To Minimize Thunderstorm Potential To The North Of Composite Outflow Boundaries Now Advancing Southward Across Parts Of Southern Mississippi/Northern Louisiana And Central Texas.

Prev Discussion...

Oh Valley To E Tx...

A Lead Shortwave Trough Will Eject Newd From The Oh Valley To The Lower Great Lakes...While A Stronger Upstream Wave Digs Sewd And Amplifies Invof Lake Mi By The End Of The Period. Surface Cyclogenesis Is Expected To Commence This Afternoon Across Nrn Indiana And Srn Lower Mi...And The Low Will Deepen And Move Nnewd To Lake Huron By Early Tuesday. Trailing The Surface Low...A Cold Front Will Progress Ewd From The Mid Ms Valley To The Lower Oh Valley...Swd Into Tx.

A Moist Environment Exists In Advance Of The Front With Boundary Layer Dewpoints Ranging From The Upper 60s To Lower 70s Across The Oh Valley...To The Mid-Upper 70s Across La/Se Tx. This Moisture And Daytime Heating In Cloud Breaks Will Result In Mlcape Values Of 2000-3000 J/Kg From Tn To E Tx /Outside Of Areas Of Nrn Ms Affected By Morning Convection/...With Lower Values Of 1000-1500 J/Kg Into Indiana/Oh. However...Lapse Rates Aloft Are Not Steep Based On Regional 12z Soundings...And Vertical Shear Is Weak Initially And Will Only Increase Slowly This Afternoon Through Tonight As The Midlevel Trough And Surface Low Slowly Deepen.

The Primary Concern Today Will Be For Isolated Strong/Damaging Outflow Gusts With The Semi-Organized Clusters From Tn/Ky Nwd Into Oh Where Midlevel Flow Is A Bit Stronger Now...As Well As With Thermodynamically-Driven Downbursts From The Mid South Into E Tx.

There Will Also Be Low-End Potential For Rotating Storms In The Zone Of Stronger Low-Level Waa Associated With The Lead Shortwave Crossing Se/E Central Oh This Evening Into Early Tonight. The Magnitude Of The Low-Level And Deep-Layer Shear Will Not Be Particularly Large Where It Coincides With Any Surface-Based Buoyancy...Thus Any Tornado Risk Should Remain Marginal At Most.

Nv/Ne Ca Into Ore This Afternoon/Evening...

A Midlevel Low Will Remain Quasi-Stationary Off The Nrn Ca Coast Today...Until It Begins To Eject Enewd With The Approach Of An Upstream Trough Near The End Of The Period. Morning Soundings
Confirm An Environment Supportive Of High-Based Thunderstorms With Strong Outflow Winds...And Storm Development Should Be Focused In The Band Of Ascent Which Will Coincide With The Nrn Sierras And The Cascade Range.

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Tue Aug 12 Convective Outlook
Storm Prediction Center Norman Ok
Slgt Risk Svr Tstms for Upper Ohio Valley & North Mid Atlantic Area...
Slgt Risk Svr Tstms for Northern Intermountain Region...

Scattered Strong To Severe Storms Are Expected Tuesday Afternoon Into Tuesday Night Across Parts Of The Upper Ohio Valley Into Northern Mid Atlantic Coast Region. Clusters Of Strong To Severe Storms Also Appear Possible Across Parts Of Northeastern Oregon... Central And Eastern Washington...And Adjacent Northern Idaho.

Further Amplification Of Large-Scale Upper Troughing Across The Eastern U.S. Appears Likely During This Forecast Period. At The Same Time...Some Weakening Of Upstream Ridging May Commence ... As The Remnants Of A Closed Low Accelerate Inland Across The Northern California Coast Through The Northern Intermountain Region. This Is Expected To Occur In Response To The Approach Of A Digging Upstream Short Wave Impulse...Which Is Forecast To Contribute To The Evolution Of A More Prominent Closed Low As It Approaches The Northern Pacific Coast.

Ahead Of This Latter Feature...And Around The Western Periphery Of The Upper Ridging... Thunderstorm Probabilities Will Increase Further Across Much Of The Intermountain Region Into The Pacific Northwest. East Of The Rockies...Thunderstorm Potential Will Remain Relatively Low Across Much Of The Northern And Central Plains Into The Upper Midwest...While Drying Associated With The Eastern Upper Troughing Also Contributes To Diminishing Convective Potential Across The Southern Plains East Northeastward Toward The Tennessee Valley.

The Risk For Thunderstorm Activity Will Remain Relatively High... However...Across Much Of The Southern And Middle Atlantic Seaboard ...And The Central Appalachians Northward Into The Lower Great Lakes Region.

Upper Ohio Valley Into Northern Mid Atlantic Coast Region...

Pre-Cold Frontal Boundary Layer Dew Points Appear Likely To Remain High Enough To Support Modest Boundary Layer Destabilization With At Least Some Insolation Across The Upper Ohio Valley And Adjacent Lower Great Lakes Region By Tuesday Afternoon.

This Will Be Aided By Mid-Level Cooling Associated With The Fairly Vigorous Short Wave Impulse Forecast To Turn Eastward Across The Ohio Valley...Toward The Mid Atlantic Coast Region.

With Mixed Layer Cape Expected To Approach Or Exceed 1500 J/Kg In The Presence Of Modestly Strong Cyclonic Mid/Upper Flow...The Environment Probably Will Become Conducive To The Development Of Scattered Bands Or Small Clusters Of Storms Capable Of Producing Potentially Damaging Wind Gusts And Perhaps Marginally Severe Hail.

Activity Is Expected To Spread Eastward Across Parts Of The Upper Ohio Valley And Appalachians Through The Late Afternoon And Evening Hours...With Additional Storm Development Becoming Focused Along A Warm Frontal Zone Across Parts Of The Delmarva Peninsula/Northern Mid Atlantic Coast Region.

Although The Stronger Southerly 850 Mb Flow Fields Are Expected Mostly North Of This Front...Above The Cooler And More Stable Boundary Layer Air...It Does Not Appear Out Of The Question That Low-Level Hodographs Near The Front May Be Supportive Of Supercells Capable Of Producing Damaging Wind Gusts Or An Isolated Tornado Or Two.

This Potential May Be Highest Late Tuesday Night...With The Approach Of The Upper Trough.

Northern Intermountain Region...

Most Model Guidance Does Suggest That An Area Of Enhanced Mid-Level Forcing For Ascent Will Overspread Portions Of Northeastern Oregon And Central/Eastern Washington During The Peak Late Afternoon Heating.

Despite Lingering Uncertainties Concerning The Degree Of Cape... Even Given The Presence Of Seasonably High Moisture Content... This Forcing Seems Likely To Be Sufficient To Support One Or Two Substantive Storm Clusters. Given This Development...Based Above A Fairly Hot And Deeply Mixed Boundary Layer...The Risk For Severe Wind Gusts Appears To Exist...Perhaps Aided By The Downward Transport Of Momentum Associated With A 30+ Kt Lower/Mid
Tropospheric Speed Maximum.

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Wed Aug 13 Convective Outlook
Storm Prediction Center Norman Ok
No Svr Tstm Areas Forecast...

A Few Strong Storms Will Be Possible From The Pacific Northwest Into The Northern Rocky Mountain Area With A Threat For Mostly Isolated Damaging Wind Gusts. In The East...A Very Marginal Risk For A Strong Storm Or Two Will Exist From Eastern New York Into Southern New England.

A Blocking Pattern Will Persist Wednesday With An Upper Ridge Over The Cntrl U.S. Flanked By Upper Lows Over The Eastern And Western States.

Pacific Nw Through Nrn Rockies...

Diabatic Warming Of The Boundary Layer Will Contribute To Steep Lapse Rates And Inverted-V Thermodynamic Profiles...But Instability Will Remain Weak /Aob 500 J/Kg/ Due To Limited Low-Level Moisture. Scattered Storms Will Likely Develop Over The Higher Terrain And Within Zone Of Upper Divergence East Of The Upper Low Center. Some Of The Storms Will Become Capable Of Isolated Downburst Winds From Late Afternoon Through Early Evening.

Ern Ny Through Srn New England...

Vorticity Maximum Within Base Of Negative Tilt Upper Trough Will Eject Newd Through The Nern U.S. During The Day.

An Occluded Front Will Continue Through The Nern U.S. Into New England... While A Warm Front Lifts Nwd...Possibly Reaching Srn New England Wednesday Afternoon. Widespread Clouds And Ongoing Rain Will Be Ongoing Early In The Day Over Much Of This Region. As This Activity Shifts Newd Some Potential For Limited Destabilization May Exist In A Narrow Corridor Just Ahead Of The Occluded Front Where Additional Storms May Develop During The Afternoon.

Weak Destabilization Is Also Possible Over Srn New England Depending On How Far North Warm Front Advances...But The More Unstable Warm Sector Will Likely Remain Offshore. Hodographs In Vicinity Of The Warm Front Will Favor A Conditional Risk For Updraft Rotation.

However...Overall Severe Potential Appears Limited In This Region Due To Likelihood Of A Very Marginal Thermodynamic Environment.

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Aug 14-18 Convective Outlook
Storm Prediction Center Norman Ok

Model Consensus Is That Synoptic Regime Will Remain Dominated By A Blocking Pattern With Upper Lows Over The Ern And Wrn U.S. Through At Least Day 4...After Which A Transition To A More Progressive Regime Is Expected To Evolve.

Severe Potential May Increase By Day 5 And Especially Day 6 Over The Nrn High Plains As Corridor Of Modified Cp Air Returns Nwd Into This Region Ahead Of The Ejecting Wrn U.S. Shortwave Trough.

Models Have Come Into Better Agreement Regarding Timing And Amplitude Of This Impulse...But Considerable Uncertainty Remains Regarding Extent Of Severe Threat With This Feature.


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