Severe Weather Threat Day 1 Sat Sep 28

Models Suggest That Mid/Upper Flow Across The Western Into Central U.S. Will Trend More Zonal During This Forecast Period...As A Strong Westerly Jet Noses Inland Across The Pacific Northwest/ Northern Intermountain Region And Northern Rockies.

As This Occurs...A Significant Downstream Short Wave Trough Appears Likely To Progress Northeast Of The North Central Plains... Through The Upper Midwest And Northern Ontario By Late Tonight. While Ridging To The East Of This Feature Shifts Across Quebec And Parts Of New England...A Closed Low Is Forecast To Develop To The South...Within Remnant Troughing Off The Mid Atlantic Coast.

Upper Mississippi Valley Great Lakes Region...

Significant Surface Cyclogenesis Is Forecast Today Across The Upper
Mississippi Valley Through Northern Ontario...Associated With The
Approaching Short Wave Trough. However...Cold Air Aloft Appears
Likely To Continue To Lag To The West Of The Trailing Cold Front
Advancing Across The Upper Mississippi Valley Into The Great Lakes
Region. And...Despite A Narrow Plume Of Seasonably High
Precipitable Water Advecting Eastward Ahead Of The Front...It
Appears That A Relatively Stable Mid-Level Environment Will Limit
The Depth Of Pre-Frontal Convection...And Generally Preclude The
Risk For Thunderstorm Activity. Even So...The Presence Of Even Weak Convection Within Lower/Mid Tropospheric Mean Flow On The Order Of 30-50+ Kt May Be Enough To Support A Few Strong Surface Gusts Approaching Severe Limits.

Sern Plains Into The Ozark Plateau And Lwr Mo Valley...

At Least Low Probabilities For Severe Wind Gusts May Continue Beyond 12z This Morning As The Evolving Pre-Frontal Squall Line...Now Beginning To Advance East Of The High Plains... Continues Eastward Into The Lower Central And Southern Plains. However...Stronger Mid/Upper Forcing For Upward Vertical Motion And Deep Layer Mean Flow Fields Are Forecast To Generally Shift Into The Upper Mississippi Valley By Mid Day...And Continuing Severe Weather Potential Across The Region Thereafter Becomes More Unclear.

Pacific Northwest Coast...

It Remains Unclear Whether Thermodynamic Profiles Will Be Supportive Of Lightning With Convection Advancing Inland Along A Cold Front During The Late Afternoon And Early Evening Hours. But Activity Appears Likely To Be Embedded Within Rather Strong Lower/Mid Tropospheric Mean Flow...On The Order Of 50-60 Kts...And Could Contribute To The Downward Transfer Of Higher Momentum To The Surface. Otherwise...Occasional Weak Thunderstorm Activity Seems More Probable As Mid-Levels Cool Above The More Moderate
Post-Frontal Low-Level Onshore Flow Later This Evening Into The
Overnight Hours.

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