Severe Weather Threat Day 1 Sun Jul 21

A Couple Of Changes Have Been Made To The Outlook For This Issuance. The First Change Is To Extend The Nrn Plains Slight Risk Area Wwd To Near The Mt-Nd State-Line To Account For Strong Convection South Of Williston. The Second Change To The Outlook Is To Remove The 5 Percent Wind Damage Probability From Parts Of Wv And Wrn Va. This Area Has Been Worked Over By A Line Of Strong Storms Currently Moving Across Cntrl Va.

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Models Indicate That A Seasonably Strong Mid/Upper Jet...Now
Extending From The Lower Great Lakes And St. Lawrence Valley Region Into The North Atlantic...Will Continue To Shift Eastward During This Forecast Period. As Deep Layer Flow Fields Weaken Across The Northeast...They Will Remain Generally Weak Elsewhere ...But Strongest In A Confluent...Largely Zonal...Regime Across The
Northeastern Pacific Through The Canadian/U.S. Border Area. A
Significant Embedded Short Wave Impulse That Supported Considerable Thunderstorm Activity Across Southern Alberta And Saskatchewan...As It Dug To The Lee Of The Canadian Rockies...Is Forecast To Remain Progressive And Gradually Turn Eastward Along And Just North Of The International Border...Into Western Ontario By Late Tonight.

The Primary Low-Level Baroclinic Zone Associated With The Departing Eastern Impulse Appears Likely To Stall Across Parts Of The Lower Great Lakes Into New England. But At Least Some Drying Associated With A Leading...Weak Convectively Enhanced Boundary Appears To Have Generally Shifted To The South Of The Stronger Residual Mid-Level Flow...Into The Ohio Valley And Northern Mid Atlantic Coast Region.

Steep Low To Mid-Level Lapse Rates Associated With Deep Boundary Layer Mixing Are Generally Confined Along And North Of Residual Subtropical Ridging Extending Along An East-West Axis Across The Central Pacific Coast Into The Central Rockies. But A Gradual Eastward Advection Of This Air Mass...As Strongly Capping Elevated Mixed Layer Air...Is Expected Across Much Of The Central Plains By Late Tonight.

Northern Plains...

Strengthening Large-Scale Forcing For Upward Vertical Motion And
Vertical Shear Near A Developing Frontal Wave Associated With The
Approaching Upper Impulse Will Contribute To Increasing Convective
Potential Later Today Into Tonight. Given Sufficient Destabilization ...The Environment Appears Likely To Become Conducive To Organized Storm Development...Including Supercells And An Upscale
Growing Mesoscale Convective System...Particularly Across Parts Of
Central And Eastern North Dakota Into Northwest Minnesota.

The Moisture Content Of The Air Mass Across This Region...However ...Is Somewhat Low /Surface Dew Points In The 50s/...And The Potential For Substantive Further Moistening Remains Unclear. Still ...Mid-Level Lapse Rates Appear Steep Enough...Given Marginal Moisture...To Support Slight Risk Probabilities...With Highest Probabilities Across North Central North Dakota...Mainly In 23-02z Time Frame.

Central Appalachians Into Northern Mid Atlantic Coast...

Weak Large-Scale Forcing For Upward Vertical Motion Supporting
Ongoing Storm Development Across The Central Appalachians Into The Tennessee Valley Is Expected To Develop Eastward Across The Northern Mid Atlantic Coast Region By This Evening. Despite The Weak Nature Of The Lower/Mid-Level Wind Fields...And Lapse Rates That Appear Modest At Best...Cape May Become Moderately Large... And Heavy Precipitation Loading In Stronger Cells May Support At Least Localized Potentially Damaging Wind Gusts This Afternoon.

Southern Plateau...

Thunderstorm Probabilities Will Remain Relatively High Today Across
Much Of The Region Associated With A Significant West Northwestward Migrating Upper Wave. Even If Substantial Early Day Cloud Cover Limits Surface Heating...Sub-Cloud Temperature/ Dew Point Spreads Will Probably Become Large Enough To Support The Risk For At Least Localized Strong Convective Surface Gusts To The North Of The Ongoing Storm Cluster Now Spreading Westward Through The Lower Colorado Valley.

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