US Preliminary Extended Weather thru Jan 13

During Days 3-5 Wed-Fri Expect A Potent Closed Low To Eject Newd
From Nrn Mexico/Rio Grande Valley While A Trof Approaching The
West Coast On Wed Will Steadily Amplify Into The West.

Fortunately Most Short Term Guidance Has Finally Converged On The
Development/Track Of The Closed Low Thru The Early Part Of The
Week Thus Leaving The Ultimate Track And Timing Of The Closed Low While It Ejects Newd As The Primary Fcst Problem In The Mid-Late Week Time Frame.

Some Of The Timing Diffs Seen In Latest Guidance Correspond To Diffs In Handling Of The Incoming Wrn Trof But The Correlation Is Far From Perfect. The 12z Ecmwf Trended Slower Than Consensus Of Prior Runs With The Closed Low While The 12z Ukmet Was Slowest.

Both Of These Model Runs Show A Sharper/More Positively Tilted Trof Coming Into The West Compared To Most Other Solns... Contributing To Slower Ejection Of The Closed Low. Gfs Runs Continue To Be In The Faster Half Of The Spread But Over The
Past Day Or So Have Exhibited A Trend Toward The More Stable Ecmwf Scenario. The 18z Gefs Mean Offers Added Support For Closed Low Timing Slower Than Gfs Runs... Though Not Quite As Slow As The 12z Ecmwf. At The Same Time 18z Gefs And 12z Ecmwf/Canadian Ensemble Guidance As A Whole Plus Gfs/Canadian Runs Indicate That The Sharp/Positively Tilted Wrn Trof In The 12z Ecmwf/Ukmet Is In The Lower Half Of Soln Probabilities.

Ultimate Preference Is For A Compromise Among The 00z/5 And 12z/5 Ecmwf Runs Along With The 12z Ecmwf Ensemble Mean To Give Somewhat More Weighting To Solns Other Than The 12z Ecmwf With The Wrn Trof... And Timing In The Slower Half Of The Spread For The Closed Low Tracking Newd Toward The Grtlks.

Stg Dynamics Assoc With The Closed Low Aloft And Leading Gulf
Inflow Will Support Potential For Hvy Rnfl Over Portions Of The
Srn Plains E/Ne Into The Ms Vly Around Wed-Thu. Less Intense
Activity Should Spread Farther Newd With Time.

Meanwhile As The Ern Pac Trof Begins To Move Into The West The Best Focus For Pcpn Should Be Over Favored Terrain From The Nrn Half Of The West Coast Into The Nrn Rockies Early In The Period With A Shield Of Relatively Lighter Activity Spreading Sewd Later In The Week.

By Days 6-7 Sat-Sun Operational Models Show Varying Distribution /Shape Of Energy Within An Agreeable Large Scale Amplified Wrn-Cntrl Noam Trof... Between Stg Ridges Over The Ern Pac And Wrn Atlc.

Gfs Runs Have Sporadically Been Indicating Potential For A Stg Sfc Low To Develop Within The Consensus Mean Frontal Zone From The Plains Newd. 12z Canadian And 00z/5 Ecmwf Runs Also Showed Decent Development Reaching The Grtlks During Next Weekend.

Though Not Especially Well Clustered... Ensembles Hint At The Midwest To Upr Grtlks Region As Being The Most Favored Region For Existence Of A Sfc Wave During Sat-Sun With Respective Means Near The Grtlks. Lack Of Confidence In A Specific Model Run... And In Particular The 12z Ecmwf That Lowered Ern Canada/Nern Us Hgts More Than Consensus... Leads To Favoring The Ensemble Means For The Days 6-7.

The 12z Ecmwf Mean Was Ultimately Used For The Manual Fcst As The Sfc Features In The 18z Gefs Mean Were Less Defined Than Desired.

Depending On Exact Evolution Of The Trof Aloft... The Leading Sfc
Front May Be Slow Moving With Persistent Gulf Inflow Aiding In The
Generation Of Some Mdt-Hvy Rnfl Over/Near An Area Including The
Mid-Lwr Ms Vly/Tn Vly Into The Oh Vly.

In Varying Ways Recent Gfs/Ecmwf Runs Show This Idea... Consistent With What Teleconnections Have Been Showing For The Fcst Pattern. The Gefs/Ecmwf Means Focus On This Region As Well Albeit In Typically Lgt Fashion.

The Front Will Serve As The Divide Between Well Above Normal Temps Over The East Versus Below Normal Readings Over The West Into The Plains. Except For Some Mstr Reaching The Pac
Nw... Most Of The West Should See Pcpn Become Fairly Lgt/Sct By
Next Weekend.


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