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US Extended Forecast thru Nov 27

Short Range Forecast Discussion
Valid 12Z Wed Nov 20 2013 - 12Z Fri Nov 22 2013

...Widespread rain and snow showers expected across the West...

...Conditions will turn blustery behind an arctic boundary plunging
through the Central U.S....

...Showers and thunderstorms possible over the Southern Plains and Middle Mississippi Valley as moisture returns to the South Central U.S....

Widespread shower activity will continue to accompany a cold front sinking through the Western U.S. Wednesday and Thursday.

The heaviest amounts should be focused along the favored terrain of California and the Great Basin...and temperatures will be low enough to support accumulating snows in the higher elevations.

Late in this period, the best chance for significant precipitation will shift farther south...where energy closing off aloft will combine with an abundance of Pacific moisture streaming inland to fuel moderate to heavy rains over the Southwest.

Temperatures will continue to plunge behind an arctic boundary dropping south and east through the Central U.S.. The falling temperatures and gusty winds will make for blustery conditions... especially where low level flow into the terrain should produce upslope snows along the Northern/Central Rockies and High Plains.

As the front dips farther south and east...moisture returning from the Gulf of Mexico will fuel widespread shower activity surrounding the front. Showers and thunderstorms containing heavy rains are expected ahead of the front across the Southern Plains and Middle Mississippi Valley...while light snows and even freezing rain will be possible in locations behind the boundary.

A lingering frontal boundary stretched through the Caribbean and Gulf of Mexico will help trigger scattered showers and thunderstorms across Florida and perhaps into the coastal Carolinas the next few days. Elsewhere over the eastern third of the Nation...sprawling high pressure will keep weather dry and quiet.

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Extended Forecast Discussion
Valid 12z Sat Nov 23 2013 - 12z Wed Nov 27 2013

...Overview And Preferences...

While Some Differences/Uncertainties Persist... Guidance Is Making
A Gradual Effort Toward Better Agreement After Multiple Days Of
Considerable Spread And Run To Run Variability.

The Most Significant Issues Have Been With Nrn Stream Flow Defining An Ern Noam Mean Trough That Will Support Very Chilly Weather Over Much Of The Ern 2/3 Of The Lower 48 Most Of The Period. With Some Moderate Scattering Of Solns There Has Generally Been Better Consistency With A Srn Stream Closed Low Ejecting Ewd From Near The Srn Ca Border. Exactly How The Two Streams Interact Over The East Late In The Period Will Ultimately Determine Sfc Evolution/Timing Near The East Coast.

A Mean Ridge Will Prevail Over The Nwrn Conus And Bc Coast Most Of The Period But Some Ern Pac Energy Is Expected To Amplify Toward The West Coast Around The Middle Of Next Week.

An Average Of Latest Global Models Yields A Good Compromise To
Resolve Remaining Detail/Timing Diffs For Significant Features
While Maintaining Better Definition Than The Ensemble Means During Days 3-5 Sat-Mon.

Day 6 Tue Transitions To A Model/Ensemble Mean Blend And Day 7 Wed To A 70/30 Weighting Of The 12z Ecmwf Mean/18z Gefs Mean As Operational Models And Particularly The 12z Ecmwf Begin To Stray From The Core Of Ensemble Solns.

...Guidance Evaluation...

Over The East Early In The Period... Recent Gfs Runs Have Been
Quicker/Wwd Versus Other Solns With Wrapping Up A Deep Low
Sfc/Aloft Over Ern Canada And On The Faster Side Of Guidance With
At Least Portions Of The Trailing Cold Front Near The East Coast.

On The Other Hand Ecmwf/Ecmwf Mean And Gefs Mean Runs Over The Past Day Or So Have Trended Faster With The 12z Ecmwf/Ec Mean Still A Tad On The Slower Side. Prefer A Compromise Given Some Skepticism In Either Extreme Of The Spread. New 00z Guidance Thus Far Appears Fairly Supportive Of The Resulting Soln.

It May Still Take Time To Resolve The Details But Confidence Is
Increasing Somewhat For The Wave Fcst To Brush The Upr Grtlks
Around Day 5 Mon... As Ensemble Members Have Come Close Enough Together For The Resulting Means To Show A Degree Of Definition More Similar To What Some Operational Model Runs Have Been Showing In Recent Days. Minor Shuffling Of Solns In The 00z Run Leads To Good Continuity With The Overall Consensus.

For The Most Part Operational Ecmwf Runs Have Been Most Consistent With The Handling Of The Closed Low Expected To Track Ewd From Near The Srn Ca Border Into The Srn Plains.

Currently There Is Better Than Avg Clustering Of Models/Ensembles Into Day 6 Tue. Spread Increases Thereafter As System Evolution Sfc/Aloft Depends On How Much Interaction There Is With Nrn Stream Energy Reaching The Ern Conus By The Middle Of Next Week.

Sharper/More Amplified Nrn Stream Shrtwvs In The Past Couple Ecmwf Runs And New 00z Gfs Lead To Faster Timing Versus Other Runs That Maintain Greater Stream Separation. It Is Worth Noting That The 18z Gefs/12z Ecmwf Means Maintain Slower Timing Even Though The Averaging Of Disparate Members Can Sometimes Lead To The Means Having Troughs That Are More Phased Than In Reality. Thus Prefer To Lean On The Ensemble Means By Day 7 Wed And More Toward The Ecmwf Mean Due To Its Slightly Stronger Sfc Reflection. The Result Is Actually Rather Close To The 4-Cycle Lagged Avg Of Ecmwf Runs.

The 12z Ecmwf Is Rather Extreme Relative To Other Model/Ensemble
Guidance As It Amplifies/Closes Off Energy To The W Of Ca Late In
The Fcst Period Versus A Flatter/More Progressive Consensus.
Gefs/Ecmwf Means Are Fairly Agreeable And Recent Gfs Runs Compare Well To The Means In Principle.

...Sensible Weather Highlights...

Expect Many Locations Over The Cntrl-Ern States To See One Or More
Days With Highs At Least 10-20 F Below Normal.

Nrn Areas Should See Some Moderation As Low Pressure Tracks From Canada Across The Upr Grtlks Toward Ern Canada.

Ern States Should Trend Drier For A While Aside From The Grtlks Which Should See Snow From Lake Effect Activity And Then Over Nrn Areas In Assoc With Low Pres Brushing The Upr Grtlks.

The Closed Low Aloft Near The Srn Ca Border At The Start Of The Weekend Will Support Locally Mdt/Hvy Pcpn In The Southwest And Srn Rockies Early In The Period With Snow In The Higher Elevs.

As The Upr Low Progresses... Mstr Will Spread Across The Srn Plains/Srn Tier Late Weekend Into The First Half Of Next Week With Some Hvy Rnfl Psbl Along/Inland From The Gulf Coast Mon-Tue.

Uncertainty With Stream Interaction Aloft Over The East Leads To Lower Confidence With Impacts/Timing Over The Southeast/East Coast By The Middle Of Next Week.

Enough Low Lvl Cold Air May Be In Place To Support Wintry Pcpn At Times Over The High Plains And In The Nrn Periphery Of The Pcpn Shield Crossing The Srn Tier.

Ridging Aloft Should Keep The Northwest Dry Most Of The Period. Some Mstr May Begin To Reach The West Coast By Next Wed As Hgt Falls Approach.


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