US Final Extended Weather thru Dec 24

High Latitude Blocking Over Northeastern Canada Will Combine With
Reinforced Ridging South Of The Aleutians To Keep An Active Southern Stream Through The Conus Over The Next Week.

These Northern Positive Height Anomalies Correlate Well With Each Other Through Teleconnections And Both Act To Support Robust Systems.

The Synoptic Differences Among The Guidance Are Relatively Small ... But The Details Will Be Important Down The Road. In Addition... Continuity Remains Good With Few Changes From Recent Hpc Forecasts.

For The First Part Of The Forecast... Thu-Sat/D3-5... The Northwest And Northeast Portion Of The Conus Will Be The Most Active.

In The Northeast Pacific... A Developing Omega Block Over Alaska Will Drop An Upper Low Out Of The Gulf Which Will Pause For A Few Days Off The Pac Nw. The Continued Onshore Flow Will Wring Out Several Inches Of Precipitation Along The Coast And Onto The
Terrain... Including Up To Several Feet Of Snow.

Downstream Over The East... A Deepening Cyclone Will Move Out Of The Mid-Ms Valley Into The Great Lakes And Then Into Northern New England As It Slows Its Exit Amid Amplifying Flow.

This Should Spread Increasingly Heavy Precip... With Snow On The Nw Side... Through The Central And Northeastern States Especially On Thursday Into Friday.

The Gfs Remains A Bit Quicker Than The Gefs Mean With The Progression Of This System... And Each Are Just Ahead Of The
Ecmwf/Ecens Mean. Given The Good Continuity Of The Ecmwf And A
Slower Trend In The Gfs... Based The First Part Of The Forecast On
The 00z Ecmwf.

By Sun-Mon/D6-7... The 00z Ecmwf Becomes Quite Slow To Exit The
System In The Northeast Compared To The Other Ecmwf/Gefs Ensemble Members As A Result Of A Very Blocky Forecast Both Over Northeast Canada And Over The Central Conus. The Gfs Remains Flatter With The Flow From The Northeast Pacific Into The West Coast While The Ecmwf And Canadian Maintain A Deeper Upper Low. The Gefs And Ecmwf Ensemble Means Fall In Between With The Former A Bit Flatter Than The Latter. Given The Tendency For The Gfs/Gefs To Underamplify The Flow... Opted To Gravitate Toward The 00z Ecmwf Ensemble Mean As A Hedged Intermediate Point.

This Will Set Up A System Just East Of The Southern Rockies By Early Next Week. The Precipitation Over The West May Not Let Up Much Until The Very End Of The Medium Range Once The Upper Trough Moves Inland.


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