US Extended Forecast thru Dec 7

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
Valid 12Z Sat Nov 30 2013 - 12Z Mon Dec 02 2013

Saturday should be another day of light and scattered snow or rain showers for the Washington Cascades and Northern Rockies.

By Sunday however, a closed upper level vortex will begin dropping south from the Gulf of Alaska and precipitation will be on the increase across much of the Pacific Northwest and into the Northern Rockies.

Strong onshore flow will stream plenty of moisture inland to fuel moderate to heavy rain for coastal areas of Washington and Oregon overnight Saturday and into Sunday, and heavy accumulating snow for parts of the Northern Rockies by Sunday afternoon.

A few widely scattered snow showers also should not be ruled out for parts of the Northern Great Plains as the system moves farther inland, but little to no accumulation is expected there.

Across the rest of the nation, the weather should remain fairly quiet.

A clipper system moving into the Great Lakes region on Saturday could trigger a few scattered snow showers downwind of the lakes and across New England... with rain expected for the coastal regions.

With the exception of extreme Northern New England, any snow accumulation should remain north of the U.S./Canadian border.

Down south, northeasterly flow off the Atlantic could bring a few scattered showers or thunderstorms for the coastal Southeast U.S.

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Extended Forecast Discussion
Nws Weather Prediction Center College Park Md
Valid 12z Tue Dec 03 2013 - 12z Sat Dec 07 2013

...Pattern Overview...Guidance/Uncertainty Assessment...Sensible
Weather Highlights...

Arctic Surges And Amplifying Mid-Upper Level Flow From The Ern
Pacific Through The Wrn Us Will Be The Prevailing Weather Theme
Next Week.

A Potent Ridge Builds From The Nern Pacific Into Alaska As A Downstream Cold Mean Trough Setting Up Aloft Over The Wrn Us.

While This Pattern Now Seems Strongly Supported By Models And Ensembles...The Timing/Handling Of Embedded Energy Emphasis
Dug Into The Base Of The Wrn Mean Trough Then Out Across The
Rockies Into The Plains And Beyond Toward Ern Noam Has Poor
Guidance Run To Run Continuity/Predictabilty.

Even So...This Pattern Should Force Quite Cold Frontal Surges To Dig Down Through The Wrn Us Along With A Threat Of Widespread Winter Weather/Snows Swept Across The Region As Enhanced By Local Terrain.

This Flow Meanwhile Favors N-Central Us Cyclogensis That Will Focus Modest To Locally Heavy Theta-E Advection/Frontal Pcpn.

Max Support Likely Will Be Centered Over The E-Central Us Warm Sector With Deeper Influx Of Gulf Moisture Late Week.

Thre Will Be Also A Period Threat For Significant Wrapback Nrn Plains/Upper Midwest To Great Lake Snows As The Main Low Heads Ejects Newd Toward E-Central Canada.

Fierce Cold Air Will Also Plunge Down Through The Nrn Plains In The Wake Of A Cold Front Passage.

Downstream...The Flow Over/Out From The Wrn Us Will Lead To A
General Rising Height Field Pattern Downstream By Midweek In The
Wake Of Lingering Shorter Term Troffing From The Ern Us To Wrn
Atlantic.

After Modest Mid-Atlantic To New Eng Coastal Cyclogensis...Some Wrap-Back Pcpn Is Indicated Tue-Wed.

Enough Cold Air May Linger To Cause Some Winter Issues Across The Interior Nern Us Heading Into Next Weekend With Upstream Frontal System Approach.

Overall...While Pattern Predictability Seems To Be Gradually
Improving...Enough Lingering Uncertainty And Run To Run Continuity
Issues Remain...So Favor Leaning On More Compatable And Steady
Ensemble Means.

Accordingly..Wpc Surface Fronts/Pressures And 500 Mb Progs Have Been Primarily Derived From A 40-40 Percent Blend Of The 18 Utc Gfs Ensemble Mean And 12 Utc Ecmwf Ensemble Mean But 10
Percent Of The 18 Utc Gfs And 12 Utc Ecmwf Was Included.

This Provides A Bit More System Definition Along With Some Further Manual Forecaster Adjustments As Needed.


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