US Preliminary Extended Forecast thru Aug 3

The North American Continent Will Be Influenced By Strong Ridging
At High Latitudes In Northwest Canada And In A More Seasonal
Position Along 35n In The Southern Conus During This Medium Range Period. Downstream...An Active Upper-Level Trough Begins To
Establish Itself Across Eastern Canada...The Great Lakes And The
Northeast Us By Days 3/4 And Broadens Across Much Of Eastern North America By Day 7.

Blocky Flow Pattern In The Northern Pacific Is Expected To Breakdown With The Key Feature Of The Blocky Pattern Being The
"Omega-Like" Upper Level Ridge Configuration Over Southern
Interior Alaska And The Northern Gulf Of Alaska. This Upper-Level
Ridge Will Gradually Migrate Eastward Into Western Canada At High
Latitude (57n-58n Along 120w Longitude) With An Upper-Level Low
Emerging In The Bering Sea And Anchoring Over The Central
Aleutians By Day 7. This Feature And The Fast-Zonal Westerly Flow
Associated With It...Undercuts The Omega-Like Ridge In The
Northeast Pacific And Establishes An Active Northern Stream Flow
Downstream Across The Pacific Northwest...Northern Rockies And
Northeast Quarter Of The Nation.

To The South Of The Fast-Zonal Flow--Across The Southern High
Plains And Rio Grande Valley---A Quasi-Stationary Subtropical Ridge Produces Above Normal Temperatures And Little If Any Significant Precipitation Through Much Of The Period.

The Highest Degree Of Model Spread Concerns The Establishment Of The Broadening Upper-Level Trough In Eastern North America As An Active Storm Track Sets Up Over South Central Canada. The
Uncertainty Lies Within The Arctic Vortex And The Phasing Of
Energy Riding The Downwind Of The Omega-Like Ridge From Northern Alberta To The Upper Great Lakes And Eastward Into The St Lawrence Valley. The Push Of Shallow Canadian Air Into The Lower 48 And Its Depth Through Time Becomes A Forecast Challenge On Day 5 Through Day 7...Particularly Along Its Western Flanks...From The Continental Divide Eastward Into The Central Plains And Mid-Ms And Ohio Valleys.

Model Preferences...

As A Starting Point...The 26/12z Deterministic Ecmwf/Gfs And
Canadian Were In Good Agreement At The Surface And Aloft With The
Gradual Transitioning Of The Flow Pattern In The Eastern Pacific... The Progression Of The Day 3 Front Through The Eastern Seaboard ...And The Emergence Of A Second Mid-Level Cyclone Lee Of The Southern Canadian Rockies.

The Second Cyclone And Its Attendant Cold Front Begin To Carve Out The Base Of The Upper-Level Trough Over Southeast Canada And Across The Midwest...To The Appalachians And East Coast. However...These 26/12z Pieces Of Deterministic Guidance Differ Somewhat On How To Get The Surface Front From The Central Plains/Midwest To The Western Atlantic And Through A Rather Formidable Subtropical Ridge Between Day 5 And Day 7. To Mitigate The Deterministic Differences Aloft In Progression Of This Wave Through The St Lawrence Valley...The Naefs...Gefs And Ec Ensemble Mean Were In Very Good Agreement And Blended For Day 6 And 7.

Much Of The Difference Begins With Energy And Precipitation
Developing At The Base Of The Second Mid-Level Cyclone And Along
Its Surface Front Marching Into The Northern And Central High
Plains On Tuesday Night And Wednesday Morning. For This...The
26/12z Deterministic Gfs/Ecmwf Served As Guidance Through The
Lower Missouri Valley And Downstream Into The East Central States
And Carolinas.

For Day 5...There Again Was Enough Variance With How The Precipitation Influenced The Progression Of The Frontal Boundary Invof The Ohio Valley...South Central Appalachians And Piedmont...To Conclude That The Leading Edge Of The Front Stalls...And A Secondary Cold Front (Coupled With Height Falls Aloft) Will Emerge On Day 6 Over The Eastern Lakes. The Slowed/Stalled Progression Of The Day 5 Front Along The Appalachians Should Allow A Weak Surface Wave To Develop In The Carolinas And Ride North-Northeastward Along The Eastern Seaboard And Into The Canadian Maritimes During Day6-7 Period.

Sensible Weather Highlights...

In The West...A Significant Marine Push Begins Day 5 Time Frame
And Continues Through Day 7...With Mid-Level Energy Moving Periods
Of Pacific Moisture Orographically Into The Continental Divide Of
Montana...Idaho And Wyoming. The Establishment Of A Zonal Westerly Flow Should Deliver Decent Rainfall And Cooler Temperatures Downstream Into The Northern Plains And Upper Midwest.

Across The Lower Missouri Valley...Portions Of The Ozarks And
Southeast...Scattered Convection Is Anticipated Along A Series Of
Decaying Canadian Frontal Boundaries...Carving Out A Weak
Baroclinic Zone Beneath The Broad Subtropical Ridge. This Ridge
Will Be Weakening Considerably In The Southern Piedmont.

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