Severe Weather Threat thru Aug 19

Tue Aug 12 Convective Outlook
Storm Prediction Center Norman Ok
Slgt Risk Svr Tstms for parts of Upper Ohio Valley & N Mid Atl Region...
Slgt Risk Svr Tstms for parts of Northern Intermountain Region...

A Few Strong To Severe Storms...With Potentially Damaging Wind Gusts The Primary Threat...Are Expected To Impact Portions Of Northern Oregon Into Central And Eastern Washington Late This Afternoon And Evening.

The Risk For Strong To Severe Storms Continues Across Parts Of The Upper Ohio Valley And Northern Mid Atlantic Coast Region.

Outlook Update...

Northern Intermountain Region...

A Belt Of 30-40+ Kt Cyclonic 500 Mb Flow East Of An Evolving Mid-Level Closed Low/Trough Near The Pacific Coast May Enhance Convective Development Across Much Of The Northern Intermountain
Region This Afternoon.

However...The Potential For Organized Storm Development With A More Substantive/Widespread Risk For Potentially Damaging Wind Gusts Still Seems To Exist With An Area Of Forcing Now Shifting Northward Through Parts Of Central/Eastern Oregon. This Is Associated With Remnants Of An Initial Weak Mid-Level Low Which Has Accelerated Inland Across The Pacific Coast...And Is Forecast To Continue Northward Toward The Canadian/U.S. Border Through The Remainder Of The Period.

Strongest Surface Heating Has Been Occurring In Advance Of This Forcing Across The Columbia Basin...And The Development Of A Hot And Deeply Mixed Boundary Layer Across Parts Of Northern Oregon Into Central And Eastern Washington Will Provide Potential For Strong Downbursts With Convection Overspreading The Region Later Today.

Upper Ohio Valley/Mid Atlantic Region...

No Substantive Changes. There Still Is A Window Of Opportunity For Strong/Severe Wind Gusts With The Evolving Convective Line Across Parts Of The Upper Ohio Valley And Adjacent Lower Great Lakes Region...Before It Encounters A Considerably More Stable Boundary Layer Air Mass /Due To Cloud Cover And Precip/ Across Central And Eastern Pennsylvania/New York.

The Environment In General Appears Only Marginally Sufficient...At Best...For Vigorous Storm Development Due Mainly To Seasonable Moisture Content And Moderate Cyclonic Mid-Level Flow. Modest Boundary Layer Instability...Even Where There Is Sunshine...Is A Limiting Factor...Particularly With Stronger 850 Mb Flow Largely Confined To Areas To The Cool/North Side Of The Effective Warm Front. This Also Appears To Be Mitigating The Severe Weather Potential Across Eastern Virginia And The Delmarva.

Interior Pac Nw This Afternoon/Evening...

The Previously Closed Low Off The Nrn Ca Coast Is Now In The Process Of Ejecting Nwd From Nrn Ca As An Upstream Trough Evolves Into Another Closed Low Just Offshore Of Nw Ca/Sw Ore.

Steep Lapse Rates Are Present In Advance Of The Ejecting Wave...As Well As The Nw Fringe Of The Monsoonal Moisture Plume Into The Great Basin. In The Wake Of Morning Clouds/Convection Spreading Nwd From Nw Nv/Ne Ca Into Ore...Development Of Additional Thunderstorm Clusters Is Likely This Afternoon. Some Enhancement To Deep-Layer Sly Shear Will Support Storm Organization ...While Steep Lapse Rates And Mlcape Of 500-1000 J/Kg Favor A Risk For Damaging Winds And Isolated Large Hail.

Ne Nv Into Srn Id This Afternoon...

A Few Clusters Of Storms Will Form And Move Nnewd From Ne Nv Into Srn Id This Afternoon. Moisture Increases Across This Area S To N And Deep Vertical Shear Will Increase Along SE Edge Of The Ejecting Wave From Nrn Ca...So A Few Organized Clusters With Damaging Winds Are Expected.

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Wed Aug 13 Convective Outlook
Storm Prediction Center Norman Ok
Slgt Risk Svr Tstms for Northern Intermountain Region...

A Few Severe Storms Capable Of Producing Large Hail And Damaging Wind Gusts Are Possible Wednesday Across The Northern Intermountain Region.

Strong Storms May Not Be Completely Out Of The Question Across Parts Of Southern New England...Accompanied By Some Risk For Potentially Damaging Wind Gusts And Perhaps An Isolated Tornado.

Models Suggest That A Closed Mid-Level Low Now Forming Within The Southern Branch Of The Split Polar Westerlies Will Only Slowly Migrate Inland Across The Oregon Coast And Cascades On Wednesday. This Movement Appears Likely To Be Largely In Response To The Progression Of A Significant Embedded Perturbation Around Its Southern And Eastern Periphery...As Upper Ridging Within The More Prominent Northern Branch Amplifies Across Much Of The Northeastern Pacific Into Western North America...And The Cyclonic Circulation Likely Is Cut-Off From The Westerlies.

Meanwhile...Downstream...The Progression Of A Mid-Level Closed Low Now Forming Over The Upper Great Lakes Region Will Be Hindered By Blocking Off The North Atlantic Coast. However...A Series Of Significant Impulses Pivoting Around Its Periphery May Contribute To Some Eastward Redevelopment Across Southern Ontario Into Southwestern Quebec By The End Of The Period.

It Appears That This Latter Feature Could Support Weak Surface Cyclogenesis To The South Of The Primary Cyclone...Along A Frontal Zone Across The Northern Mid Atlantic Coast Region Through Southern New England Later Tonight Through Wednesday. While The Trailing Cold Front Probably Will Clear Much Of The Mid Atlantic Coast Early In The Period...It May Not Advance Much Farther South Than South Atlantic And Gulf Coastal Areas Before Stalling.

With Seasonably High Moisture Content Air Generally Confined To The Warm Sector Of The Low... And South Of The Cold Front... Thunderstorm Potential Across Most Areas East Of The Rockies Is Expected To Be Relatively Low Wednesday. However... Moisture Return From The Lower Latitude Eastern Pacific Will Continue Through Much Of The Eastern Great Basin And Rockies Into The Northern Intermountain Region...Which Should Support Considerable Thunderstorm Activity.

Northern Intermountain Region...

Forcing For Upward Vertical Motion...Cool Mid-Level Temperatures And Enhanced Vertical Shear Associated With The Inland Migrating Mid-Level Closed Low Are Expected To Contribute To The Risk For Severe Storm Development Wednesday Afternoon And Evening. Mixed Layer Cape By The Afternoon Hours Is Generally Expected On The Order Of 500-1000 J/Kg...But Could Be Locally Higher ...And Should Be Sufficient For Supercells Beneath 30-50 Kt Cyclonic 500 Mb Flow.

Initiation Seems Probable By Around 21z...Particularly Across Parts Of Northeastern Oregon / Southeastern Washington... With Additional Development Likely The Remainder Of The Afternoon Over The Mountains Of Central Idaho To Parts Of The Snake River Valley.

Consolidation Of Storms Into An Upscale Growing Mesoscale Convective System Is Likely Across Parts Of Northern/Central Idaho Into Western Montana Before Convection Weakens Wednesday Night.

Southern New England...

Uncertainty Remains Concerning The Strength And Track Of The Developing Surface Low...And Associated Potential For Inland Boundary Layer Destabilization During The Day Wednesday.

However...Given The High Moisture Content Within The Warm Sector Of The Low... It Appears Possible Conditions May Become At Least Marginally Sufficient For The Development Of An Isolated Supercell Or Two With Some Risk For Damaging Wind Gusts Or A Tornado.

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Thu Aug 14 Convective Outlook
Storm Prediction Center Norman Ok
No Svr Tstm Areas Forecast...

Isolated Strong To Severe Storms Are Possible Thursday Afternoon And Evening Over The Northern Rocky Mountain Region With Primary Threats Being Isolated Downburst Winds And Large Hail.

Blocking Regime Will Remain In Place Thursday With An Upper Ridge Over The Cntrl U.S. Flanked By Upper Lows Over The Nern States And Pacific Nw Areas.

Nrn Rockies Area...

Steeping Lapse Rates Associated With Diabatic Warming Of The Boundary Layer Will Contribute To Modest 400-800 J/Kg Mlcape During The Day.

Storms Will Likely Redevelop Over The Higher Terrain And Within Zone Of Ascent/Upper Divergence Associated With The Nearly Stationary Upper Low Circulation. A Belt Of Stronger Winds Will Reside Se Of The Upper Low Center Resulting In 35-45 Kt Effective Bulk Shear.

Potential Will Exist For A Few Persistent And Organized Updrafts Thursday With A Threat For Isolated Damaging Wind And Hail.

A Portion Of This Area May Need To Be Included In A Slight Risk In Later Updates.

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Aug 15-19 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 Gradual Transition To A Progressive Regime Is Expected To Evolve During Days 5-7.

Significant Model Differences Persist With Regard To The Ejection Of The Nwrn U.S. Upper Trough As Well As A Weaker Vorticity Maximum Expected To Move Into The Cntrl Plains By Day 4 Or 5.

Predictability Remains Low This 4-8 Period Due To Transitional Nature Of The Pattern And Lack Of Agreement Among Ensemble Members.


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