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US Extended Forecast thru Oct 12

...Widespread Rainfall Expected In The Wake Of 'Karen'...
...Intense Cyclone To Migrate Through The Great Lakes...
...Intense Upper-Level Pacific System To Descend On California And
The Southwest...

Very Active Pattern Across Both Coast This Medium Range Period... As The Remnants Of 'Karen' Are Swept Northeastward Through The Mid Atlantic States And Appalachians.

Seemingly Lost In The Shuffle...Will Be An Active 'Trowal' And Cold Core Aloft...Currently In The Northern High Plains...Which Will Migrate
Across The Great Lakes...And Anchor The Western End Of The
Occlusion And Frontal Boundary Sweeping East-Northeastward Across The Northeast.

Out West...Deep Cutoff Circulation Digs Wetter...Windy And Much Cooler Conditions Across The Great Basin...California And The Desert Southwest...Before Ejecting Series Of Strong Shortwaves Across The Central Rockies And Central-Northern Plains.

Manual Surface Graphics Attempted To Provide Separation Between
The Active Trowal Across The Great Lakes And The Cold Front/Triple
Low Invof The St Lawrence Valley And Eastern Seaboard On Day 3 And Day 4.

Remnant Moisture And Dynamics Eject North-Northeastward Along The Front...With The 4/12z Deterministic Ecmwf And Gfs In Very Good Agreement With The Trowal And Marching This Closed H5 Feature Across The Great Lakes And Northeast...As A Separate Entity...Apart From The Cold Front Dragging Slowly Off The East Coast...Day 3-4.

The 4/12z Ecens Means Were Uncharacteristically Slower And Considerably Weaker At The Surface Than The 4/12z Naefs And Gefs Means In The Northeast As The Trowal Grinds Across The Eastern Great Lakes Day 4.

The Secondary Surface Low/Circulation Developing Off Nova Scotia
As The Trowal Works Its Way Into The Canadian Maritimes Was The
Primary Reason To Blend Out The Ecens Means From The Equation On Day 4-5 In The Northeast And Southeast Canada. Its Surface High
Looked Out Of Place Beneath A Very Deep Closed H5 Circulation.

Likewise...The 4/12z Ecens Means Were Uncharacteristically Slower
With The Shortwave Energy Ejecting Northeastward From The Front
Range Of The Rockies Day 5...And Much Slower With The Closed H5
Low Tracking Through The Desert Southwest And Four Corners Region. That Probably Is Not A Bad Thing...Given Most Of The Deeper
Systems Digging Through The West Will Slow Down And The Cold Core Aloft Will Contain Steep Mid-Level Lapse Rates Capable Of
Generating Intense Convection Across The Southern Sierra And
Downstream Into Mid-Colorado River Valley.

But Ignoring The Day 5 Shortwave Or Shortwaves Exiting The Desert Southwest For The South Central Rockies And Plains (Because It Is Much Slower) Might Be An Oversight That Ends Up Developing Into A Significant Wind Event And Scattered Convection...For Colorado... New Mexico And The North Central High Plains...Given The 40-60kt 700 Mb Southwesterly Flow Progged To Move Out Ahead Of The Cold Pool Aloft. The Largest Spread Is At The Surface And Over The Central Plains...With The Depth Of The Surface Reflection Tracking Out Of Northeast Colorado.

The 4/12z Deterministic Gfs Was Closer To The 4/12 Ecens/Gefs And
Naefs Than The Deterministic Ecmwf Over The Central And Northern
Plains By Days 5-7. And Overall...The Means Do Not Do The System
Moving Through The West...Great Basin And South Central Rockies
Justice. Tried To Incorporate Some Of The Finer Details Using The
4/12z Gfs. But Confidence For A Prefect Prog At The Surface (Days
6-7) In The Plains Is Below Average.

Last But Not Least...The Remnants Of 'Karen' Will Slide Along The
Day 3-4 Frontal Boundary In The Piedmont Region. I Was Skeptical
That The Spot Low (Per The 4/12 Gfs/Ecmwf) Would Ever Be Tied To
The Front For Very Long On Day 3---And Invof Of Srn Va. Cannot
Imagine There Will Be Any Phasing With A 'Trowal' Feature In The
Great Lakes. The Trowal Should Be A Completely Separate Entity.

The Likely Scenario Is That A Well-Defined Dry Slot Develops Along
The Spine Of The Appalachians And What Surface Wave Develops... Does Generate A Brief Period Of Southeasterly Upslope Flow Along The Eastern Slopes Of The Blue Ridge And Allegheny...But Quickly Loses Its Identity As The Front Sweeps Through The Northern Atlantic States.


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