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US Extended Forecast thru Nov 23

...Gusty winds and heavy snows continue over the Cascades and Rockies...

...Strong to severe thunderstorms will be possible ahead of cold front
crossing the Eastern half of the Nation...

Strong winds and heavy snows will continue over the Cascades and Northern Rockies into Saturday morning...as a vigorous piece of energy aloft dives through the region.

The widespread snow shower activity should begin to spread southeastward across the Central Rockies...eventually leading to
snowfall accumulations approaching a foot over the favored peaks of the higher terrain.

Conditions should begin to improve across the Rockies for the later half of the weekend as the vigorous energy aloft progresses east ...but unsettled weather will continue over the Pacific Northwest
where flattening flow will keep Pacific moisture streaming inland.

Widespread showers and thunderstorms will begin to breakout ahead of a strengthening cold front pushing through the eastern half of the U.S. this weekend.

Increasing amounts of Gulf moisture funneling northward should fuel an organized axis of storms while the front crosses the Great Lakes
and Middle Mississippi Valley on Saturday...and then activity should shift east as the front approaches the Appalachians on Sunday.

The quick progression of the front should limit any flash flood threat ...however...heavy rains and severe weather are expected within
developing storms. Also...a surface low along the boundary will quickly intensify while lifting out of the Upper Mississippi Valley Sunday night into Monday...bringing strong and gusty winds across the Great Lakes region.
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Extended Forecast Discussion
Nws Weather Prediction Center College Park Md
141 Am Est Sat Nov 16 2013
Valid 12z Tue Nov 19 2013 - 12z Sat Nov 23 2013

...Overview And Preferences...

Individual Model And Ensemble Solns Continue To Display
Significant Spread And Run To Run Diffs For Some Details Within A
Large Scale Evolution That Should Trend From Low Amplitude Flow At
The Start Of The Period Toward An Ern Pac/West Coast Ridge And
Downstream Troughing.

In General Latest Operational Model Runs Differ Enough From Each Other And Ensemble Means To Favor Emphasizing The Means. Even The Latest Gefs/Ecmwf Means Differ Over Some Areas But Both Offer Some Plausible Ideas. Initial Prefs Start With A 60 Pct Weighting Of The 12z Ecmwf Mean And The Remainder Consisting Of The 18z Gefs/12z Naefs Means. Localized Detail Adjustments Were Made Where There Was Enough Support From Operational Guidance.

...Guidance Evaluation...

Guidance Already Begins To Diverge In The First Half Of The Period
As Energy Within A Trough Near The Bc Coast As Of Early Tue Flows
E/Ne Across Srn Canada And Psbly The Nrn Conus.

The 12z Ecmwf/Cmc Runs Were Most Aggressive With The Ewd Progression Of Hgt Falls Aloft And Leading Sfc Front. While Not Outliers Relative To The Full Ensemble Spread... Those Solns Are Somewhat On The Extreme Side.

There Have Been Indications Of Somewhat Faster Nrn Stream Flow Than Fcst By Recent Gfs Runs So The 12z Ecmwf Mean That Is Less Extreme Than The 12z Ecmwf/Cmc May Have Some Validity. The 12z Cmc Mean Also Shows More Stream Separation Than The 12z/18z
Gefs Means But Does Not Lower Hgts Across The Nern Quadrant Of The Conus As Much As The Ecmwf Mean.

Given These Considerations Prefer An Intermediate Soln As Given By A 60/20/20 Weighting Of The 12z Ecmwf Mean/12z Naefs Mean/18z Gefs Mean.

By The Latter Half Of The Period Partial Inclusion Of The Naefs/Gefs Means Helps To Pull The Faster-Trending Ecmwf Mean Back Toward Continuity With The Cold Front Heading Into The Ern Half Of The Conus. Through The Middle Of The Period The New 00z Gfs/Gefs Mean Runs Appear To Display A Faster Trend Toward The Preferred Soln.

Review Of D+8 Mean Charts And Teleconnections Relative To Positive
Hgt Anomaly Center Assoc With The Ridge Building Toward The West
Coast Suggest The 12z Ecmwf May Be The Least Probable Scenario By Late In The Period.

Regardless Of Exactly Where The Positive Anomaly Center Is Positioned Over The Nwrn Conus There Is Reasonable Support From Teleconnections Toward An Ern Conus Trough And A Core Of Above Normal Hgts Near The Canadian Maritimes.

The 12z Ecmwf Is The One Piece Of Guidance That Depicts Negative Hgt Anomalies Over The Latter Area. In Spite Of This Relative
Agreement In The Large Scale By The End Of The Period There Is Low Predictability With Important Shrtwv Details In Both Nrn And Srn
Streams That Will Impact Sfc Details Over The Ern Half Of The
Conus.

...Sensible Weather Highlights...

Low Amplitude Flow Early In The Period Will Favor Enhanced Pcpn
From The Cntrl-Nrn West Coast Into The Nrn Rockies... With Lesser
Amts Pushing Sewd Through The Rest Of The Week.

Some Mstr May Return To The Pac Nw By Late In The Period But With Little Agreement As To How Much At This Time.

As Evolving Conus Troughing And Leading Sfc Cold Front Head Toward The Cntrl/E-Cntrl States Expect Rnfl Of Varying Intensity To Develop From The Srn Plains Newd. There Is Sufficient Spread In Guidance Sfc/Aloft To Keep Confidence Fairly Low With Determining Coverage/Amts... Especially To The Ne Of The Mid Ms Vly.

Ely Low Lvl Flow At Lower Latitudes Will Likely Support Periods Of Rnfl Over The Fl Peninsula And Wrn Gulf Coast.

Expect The Rockies/ Plains To See The Greatest Temperature Change During The Period As Above Normal Highs Should Be Replaced By Rather Chilly Readings By Next Sat.


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