US Extended Forecast thru Dec 6

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Valid 00Z Sat Nov 30 2013 - 00Z Mon Dec 02 2013

Another day of light and scattered snow showers are expected over the Washington Cascades and extreme Northern Rockies on Saturday...but as a closed upper vortex begins to drop south out of the Gulf of Alaska on Sunday...conditions will become increasingly active across the northwestern corner of the Nation.

Strong onshore flow bringing an onslaught of Pacific moisture inland will fuel heavy rains along the coastal ranges of Washington and Oregon and accumulating snows in the higher elevations of the Washington Cascades.

Enough moisture should spill inland to also produce heavy snows along the Northern Rockies on Sunday.

The weather will stay fairly quiet across the rest of the Nation for the
remainder of the holiday weekend.

A moisture starved system crossing the Great Lakes on Saturday and lifting through the Northeast on Sunday will trigger light snow showers...but any accumulations should stay north of the Canadian border.

In the southern northeasterly flow over relatively warm ocean waters could bring some scattered showers and thunderstorms into Florida's Atlantic coast.

Light shower activity over southern California should gradually come to an end Friday night as closed low aloft drops southwestward away from the California coast.


Extended Forecast Discussion
Nws Weather Prediction Center College Park Md
Valid 12z Mon Dec 02 2013 - 12z Fri Dec 06 2013


Arctic Surge And Amplifying Upper-Level Flow From The Eastern
Pacific To The Intermountain West Will Be The Prevailing Weather
Themes This Medium Range Package Across The Western Half Of The Continental United States.

In The East And Southeast...The Main Forecast Challenge Will Be To Determine The Outcome In The Western Atlantic--Of Southern Stream Energy Originating In Southern And Baja California.

...Model Preferences And Uncertainty Assessment...

Aspects Of The Deterministic 29/00z Gfs/Ecmwf And Their Respective
Means Should Handle The Larger-Scale Weather Features This Medium Range Forecast Period.

That Includes The Progression Of A Massive And Bitterly-Cold Arctic Intrusion Into The Lower 48 On Late Monday Into Tuesday.

The Biggest Forecast Challenge Is Timing The Southward Release Of The Airmass Through The Very Complex Terrain Of The Rockies And Great Basin.

Tied To This Release...A Series Of Mid-Level Shortwaves Will Rotate Through The Base Of The Broad-Scale Upper-Level Trough And Provide Some Over Running Moisture That Will Initially Modify The Leading Edge Of The Boundary.

This Sets Up A Long-Duration Winter Weather Event For A Large Portion Of The West...Including Montana...Idaho Portions Of Northern
Utah-Northern Colorado...Wyoming And The Dakotas.

Wpc Manually Adjusted The Surface Graphics To Fit Terrain And
Included A 40+ Millibar Surface Gradient Between Northwest Montana
And Southwest Kansas For Days 4-5.

This Implies Strong Wind... Coupled With Significant Snowfall.

Aloft...The Gefs...Naefs And Ecmwf Ensemble Guidance Prefer To
Maintain A Deep Trough Axis Over The Desert Southwest Invof 110w
Through The End Of This Medium Range Period...But Have Different
Solutions And Conclusions Downstream With An 'Initial Shortwave'
Exiting The Upper-Level System As Early As Day 4 (Into The
Northern High Plains).

The 29/00z Deterministic Cycle Offered A Mixed Bag Of Results... With The 29/00z Canadian Beginning To Take A Faster And More Southerly Solution Over The Plains By 4/00z With The Energy Tracking Over The Black Hills And Quickly Racing It Eastward Into Nebraska/Iowa By 4/18z.

Recommend A Blend Of The Deterministic 29/00z Ecmwf/Gfs To Sort
Out Some Of The Smaller Scale Details Surrounding The Digging
Upper-Level Trough Which Peels Slowly Southward And Draw The
Arctic Airmass West Of The Continental Divide And Deep Within The
Great Basin And Desert Southwest During The Latter Half Of This

Both Have A 'Lead' Shortwave Ejecting Northeastward In The Dakotas Around 5/12z...But Maintain The Bulk Of The Energy--And More Importantly--Cold Pool Aloft Over Idaho (The Ecmwf) And Eastern Nevada (The Gfs) When The Canadian Has The Lead Shortwave... And Much Of The Upper-Level Trough Racing Towards Southern Ontario.

...Sensible Weather Highlights...

Confidence Is Rising For Temperatures Ranging From 15f To 25-30+ F Below Normal Across The Western States And Northern Plains.

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