US Extended Forecast thru Dec 28

...Overview And Preferences...

Guidance Is Stable With The Large Scale Pattern... A North American Mean Trough Downstream From An Ern Pac Mean Ridge.

The Ridge May Briefly Nudge Its Way Into The Wrn Conus Late Next Week Before Weakening And Rebuilding Back Over The Ern Pac.

The Primary Uncertainties In This Flow Will Be The Ultimate Evolution
Of Multiple Pieces Of Energy Contained Within A Trough Moving Into
The Cntrl-Ern States And The Amplitude Of An Upstream Pac Shrtwv
Reaching The Northwest Late In The Period.

Overall A Blend Of The 12z Gefs/Ecmwf Means Best Addresses Current Guidance Spread Which Appears To Have Changed Little Over The Past Day.

...Guidance Evaluation...

During The First Half Of The Period Detail Diffs Aloft Have A Relatively Minor Impact On The Sfc Pattern.

By Later In The Week Diffs In The Way Shrtwvs Evolve Within The Cntrl-Ern Trough Aloft Lead To Major Diffs In Sfc Evolution Near The East Coast.

So Far Two Primary Clusters Have Existed. The Majority Group That
Consists Of Gfs Runs And Gefs/Ecmwf/Cmc Means Keeps The Overall Trough Broad Enough To Minimize Sfc Development... While
Operational Ecmwf Runs Have Been Trying To Develop Some
Combination Of Low Pressure Along The East Coast And/Or
Grtlks/Northeast Low Pressure Due To Greater Sharpness Of At Least
One Piece Of Energy.

Either Scenario Fits Within The Expected Mean Pattern So Will Stick With The Majority Cluster As Best Represented By The Ensemble Means Until Guidance Trends In Such A Way To Suggest An Alternative Scenario Is More Likely.

Late In The Week Models/Ensembles Still Differ With The Extent To
Which Ern Pac Energy May Separate Before Reaching The West
Coast/Amplitude Of The Resulting Shrtwv Along And Inland From The
Coast.

Recent Gfs Runs Incl The New 00z Version As Well As The 18z Gefs Mean Are Somewhat More Amplified With The Incoming Shrtwv Than Other Solns.

Trends Over The Past 1-2 Days Have Been Toward A Stronger Ridge Reaching The West Before The Shrtwv Arrives... Favoring A Weaker Shrtwv. Thus The 12z Gefs Mean Is Preferred For Inclusion With The 12z Ecmwf Mean In This Fcst.

...Sensible Weather Highlights...

Albeit With Some Potential For Exceptions Mid-Late Period Over Some Srn/Ern Areas... Pcpn Over The Lower 48 Should Be Relatively
Low In Coverage And On The Lgt Side In Most Cases.

An Area Of Lgt Snow Should Progress Ewd From The Nrn Rockies/ Plains With Periods Of Lake Effect Providing Additional Snow Over The Grtlks.

Another Modest Area Of Mstr Should Arrive In The Northwest By Fri-Sat
Though Perhaps To A Lesser Extent Than Seen In Some Gfs Runs.

Locations Across The Upr Ms Vly Into The Wrn Grtlks Will Likely See The Core Of Coldest Temps Relative To Normal During The Overall Fcst Period While California Should Be Most Consistently On The Warm Side Of The Spectrum.


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