US Extended Forecast thru Sep 29

Overview...

Deep Longwave Trough Settles Invof 140w In The Gulf Of Alaska And
Northeast Pacific During This Medium Range Period...With The
Biggest Spread In The Ensemble Guidance Centered near 50n150w.

The Spread Of Which Lends Itself To The Volatile Nature Of The Shortwave Energy--Originating Within The Base Of This Broad
H5 Trough. A Focus Near...Just After The End Of The Medium Range ...Appears To Be The Details Associated With Tropical Energy In The Western Pacific Intruding Upon The Mid-Latitude Westerlies.

Over The Plains And Midwest...The Medium Range Forecast Becomes A Series Of 'Pacific' Fronts With Surface Cyclogenesis And
Downsloping 'Warmth' More Of The 'Post-Frontal' Result Of
Mid-Level West-Northwesterlies...Tending To Undercut The
'Blockiness' Downstream Over Labrador And Northeast Quebec.

What Precipitation Can Develop...Is Along The Path/Track Of The Primary And Secondary Surface Cyclones Tracking Northeastward Along The Upwind Side (In A Southerly To Southwesterly Low-Level Flow) Of The Amplifying Upper Ridge Between North Central Colorado And Northern Minnesota.

Over The East And Northwestern Atlantic ...A 'Blocky' Pattern
Emerging Over Greenland And The Davis Strait Allows The
Amplification Of An Upstream Upper-Level Ridge Over The East For
Days 6-7.

Preferences...

The 22/00z Cycle Was In Very Good Agreement Through 27/12z
Including The Deterministic Ukmet...Gfs...Ecmwf And Canadian.

Beyond This Period...The Deterministic Runs Differ Enough In The
North Central Plains With The Structure Of The H5 Trough Spreading
Across The Plains...To Start Tailoring The Forecast Into A 60/40
Blend Of The 22/00z Ecens-Naefs Means. The Choice Of Means For
Days 6-7 Captures The Nature Of The Increasing Volatility In The
Gulf Of Alaska And Southeast Gulf Of Alaska.

Manual Graphics Leaned A Bit More Towards The 22/00z Deterministic Details Of The Canadian/Ecmwf For The Structure Of The Cyclonic Flow Over Central And South Central Canada Between Days 5-7.

Of Note...The Ec... Gefs And Naefs Are Generally Deeper With The Undercutting Energy At H5 Over The Midwest And Great Lakes Around Day 6...Than Their Corresponding Operational/Deterministic Runs (Aside From The 22/00z-06z Gfs).

Sensible Weather Highlights...

Active Weather (Including High-Elevation Snowfall) Expected For The Northern Rocky Mountains With Areas Of Moderate Precipitation For The Upper Ms Valley.

Breezy To Windy Conditions...And Large Diurnal Temperature Swings Over The Central/Southern Plains And Mid-Ms Valley.

Early Wet Season Pattern For The Extreme Northwest Pacific Coast
Southward To South Central Oregon...With Well-Defined And
Seasonally-Intense Surges Of Surface Ridging Behind The Frontal
Passages.


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