US Extended Forecast thru Jan 8

Extended Forecast Discussion
Nws Weather Prediction Center College Park Md
Valid 12z Sat Jan 04 2014 - 12z Wed Jan 08 2014

...Overview And Preferences...

The Dominant Focus Of The Medium Range Fcst Will Be On The Sfc
Evolution Ahead Of An Increasingly Deep Trough Aloft Heading Into
The Ern Conus By Early Next Week And Trailing Very Cold Air
Expected To Affect The Cntrl-Ern States.

Given The Multiple Pieces Of Energy Affecting Sfc Development There Is Still A Fair Amount Of Uncertainty In The Details But Guidance Spread Is Less Extreme Than Seen 24 Hrs Ago.

As This System Departs... Ern Pac/Wrn Conus Flow Is Expected To Flatten At Least Briefly By Next Tue-Wed With The Typically Poor Agreement On Shrtwv Details.

During The Weekend A Compromise Among The 12z Ecmwf Mean And 12z Ecmwf/12z-18z Gfs Runs Yields A Good Intermediate Fcst.

As Sfc Low Pres Intensifies Around Day 5 Mon The 12z Ecmwf Mean Is Preferred On Its Own Based On Its Continuity And Good Fit Within
The Entire Soln Spread.

Once This System Departs... Days 6-7 Tue-Wed Include 30 Pct Weighting Of The 18z Gefs Mean Along With The Ecmwf Mean To Account For Increasing Detail Uncertainty As Overall Mean Flow Becomes Less Amplified.

...Guidance Evaluation...

The Signal Is Still Present For Significant Sfc Development In
Response To The Interaction Of An Initial W-Cntrl Conus Shrtwv And
Amplifying Srn Canada/Nrn Conus Flow.

Compared To 24 Hrs Ago Latest Guidance Is Displaying Somewhat More Agreement With The Idea Of A Wave Tracking From The Srn Plains Into Oh Vly During The Weekend With Greatest Deepening Occurring Around Sun Night-Mon As The System Tracks Into The Nern Conus/Ern Canada.

Thus Far The Ecmwf Mean Has Been The Most Stable Piece Of Guidance For This System Especially Over/Near The Northeast.

This Favors Trending A Model/Ensmean Consensus Days 3-4 Sat-Sun To The 12z Ecmwf Mean Exclusively Around Day 5 Mon.

After Some Runs Of Hardly Depicting Any System Due To Upstream Issues Over The Nern Pac/Nwrn Noam... The 12/18/00z Gfs Runs Have Trended Toward A More Vigorous Nern Conus System Albeit With A Track A Bit On The Ern Side Of The Spread.

The 12z Ecmwf Is Closer To The Ecmwf Mean Than Most Prior Runs Due To Its Srn Canada Closed Low Remaining N Of The Border.

The 12z Ecmwf And New 00z Ukmet/Cmc Are In The Slower/Wrn Half Of The Spread For Track Though.

Typical Difficulty That Models Have With Higher Latitude Flow Suggests That It May Take A While To Resolve Exactly How The Srn Canada/Nrn Conus Trough Will Interact With The Initial W-Cntrl Conus Trough That Should Have Higher Predictability.

Upstream There Is Reasonable Continuity With The Idea That Ern
Pac/Wrn Conus Flow Should At Least Temporarily Flatten As One Or
More Pieces Of Shrtwv Energy Pass Through The Long Term Mean
Ridge... Ultimately Forming A Weak Positively Tilted Trough From
The Plains Into The Srn Rockies By The Middle Of Next Week.

Deterministic Details Are Too Varied Within A Particular Cycle Or On A Run To Run Basis To Include At This Time.

Therefore Prefer An Ensemble Mean Blend By Days 6-7 Tue-Wed Until Shrtwv Details Become More Agreeable.

This May Also Take Some Time Due To The Small Scale Nature Of The Energy Involved.

...Sensible Weather Highlights...

System Affecting The Ern Half Of The Conus From The Weekend Into
Early Next Week Should Generate A Band Of Snow To The N/W Of The
Sfc Low Track And Rain In The Warm Sector.

Placement And Intensity Of Pcpn Will Depend On Exact Track And Timing/Depth.

There Is Higher Confidence In The Very Cold Air Trailing This System ... With Many Locations Over The Ern Half Of The Conus Seeing One Or More Days Of Temps 10-30 F Below Normal.

Winds Between Departing Low Pressure And The Sfc High Sliding Sewd Across The Plains Into The East Will Also Lead To Extremely Low Wind Chill Values Over Some Areas... And Periods Of Lake Effect

Meanwhile The Wrn States Should Remain Mild With Near To Above Normal Temps.

Expect A Modest Increase Of Mstr Over The West By Next Tue-Wed With Assoc Pcpn Mostly On The Lighter Side Due To Relatively Weak Dynamics Aloft And Lack Of Onshore Flow Near The Sfc.

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