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US Final Extended Weather thru Dec 30

Medium Range Guidance Is Indicating A Gradual Transition From A
Pattern Consisting Of Progressive And Short-Medium Wavelength
Systems Crossing The Ern Pacific And Conus Toward Slower Moving
Mean Features With Greater Wavelength.

This Change Corresponds To A Broad/Deep Mean Trof Fcst To Become Established Over The Central Pac/Bering Sea With A Downstream Ridge Eventually Building Into Extreme Wrn Canada And Ern Pac.

Thru Day 7 Next Sun The 00z Gefs/Ecmwf/Canadian Ensemble Means Agree Well In Principle Sfc/Aloft Though Some Individual Ensemble Members And Operational Models Display Meaningful Diffs. A Compromise Among The Operational/Ensemble Mean Solns Generally Provides A Reasonable Starting Point To Resolve Detail Diffs Into Day 5 Fri... While The Ensemble Means Best Reflect Aspects Of The Fcst That Have Highest Predictability By Days 6-7.

On Avg This Pattern Is Expected To Support Below Normal Highs Over A Majority Of The Lower 48.

The Primary Focus Early In The Period Will Be The Storm Fcst To
Track Newd From The Ms/Oh Valley Region During Wed-Fri.

The Avg Of All Guidance Has Adjusted A Little Nwwd Over The Past Day... Led To Some Degree By Ydays 12z Gfs/Ecmwf. The 00z
Ecmwf/Canadian/Ukmet Are Now On The Srn Side Of The Model
Spread... While The 06z Gfs Strays To The Nwrn Side Of The Spread.

Albeit Not In Perfectly Steady Fashion The Cmc/Ukmet Have Been
Trending Nwwd From Farther Sern Tracks Over Multiple Days So
Confidence Is Not That Great In The Sern Side Of The Spread.

There Is Still The Possibility Of Some Day-To-Day Waffling In The
Track So Preference Is To Incorporate Slightly Less Than Half Of
The Recent Nwwd Trends With Prior Consensus Continuity.

Continue To Expect A Band Of Significant Snowfall To The Nw Of The
System... Most Likely From Near The Lower Grtlks Into Interior New
England. Locally Hvy Rnfl Is Psbl In The Warm Sector With A
Period Of Wintry Mix In-Between.

Upstream A Trof Near The West Coast As Of Early Day 3 Wed Will
Make Steady Ewd Progress With Time. There Is Slightly Better
Agreement Today With Regard To Potential For Some Nrn Stream
Interaction By The Latter Half Of The Period... Helping To Aid In
The Development Of Another Decent Ern Conus And/Or Wrn Atlc System By Next Weekend. As Early As Day 3 The 00z Ecmwf Is Rather Amplified With The Trof Aloft Compared To The Majority Of Ensemble Guidance Including Ecmwf Members. Likely Due In Part To Issues Upstream The 00z Ecmwf Becomes Even More Extreme Relative To Its Ensemble Members And Other Guidance Over The Ern Conus.

Teleconnections Relative To The 00z Ecmwf D+8 Core Of Positive Hgt
Anomalies Just S Of The Ak Panhandle Also Favor A Core Of Ern Noam Negative Hgt Anomalies That Is Well Nwd Of The 00z Ecmwf. For The Most Part The 00z Gfs Compares More Favorably To Ensemble Mean Consensus Than The 06z Gfs Run Due To The Latter Having Questionable Handling Of Srn Canada/Nrn Tier Conus Flow By Next Sat-Sun.

Meanwhile The 00z Ecmwf/Canadian Ensemble Means Are Both
Closer To The Coast With Low Pressure Fcst To Be Over Or Offshore
The East Coast By Early Sun. The 00z Gefs Mean Is More Offshore
But Not To The Extent Of The 00z Gfs. Leaning About 2/3 Toward
The 00z Ecmwf Mean Versus The Gefs Mean Appears Most Reasonable By Next Weekend.

This System Will Likely Spread A Broad Shield Of Pcpn Across The Conus... But At Most Locations Intensity Should Be Somewhat Less Than Seen With The Preceding System. There Is Significant Spread Regarding How Hvy Rnfl Along The Gulf Coast May Be Around Fri-Sat Though.

With The Trof Approaching The West Coast By Fri... Guidance Is Still Undecided Over How Much Flow Separation Will Occur. Gfs Runs Maintain Decent Continuity With At Least Temporarily Closing A Low Just Off Ca Though The 06z Gfs Is A Lower Probability Scenario By Day 7 Sun As It Incorporates Upstream Energy In A Way Not Seen In Most Other Solns. The 00z Gfs Could Be A Little Deep But Otherwise Fits In With Ensembles.

Even If Not Quite A Majority There Are Enough Ensemble Sfc Low Centers Offshore Ca Early Day 6 Sat To Suggest At Least A Dissipating System At That Time. The Past Two Ecmwf Runs Pull The Closed Low Well Wwd Of Consensus. In Recent Weeks Operational Ecmwf Runs Have Not Verified Well When Showing Slower And More Amplified/Separated Solns Than Corresponding Ensemble Means... So Confidence In The 00z Ecmwf Is Fairly Low At This Point. On The Other Hand The 00z Cmc Is A Fast Extreme.

For Days 3-5 Wed-Fri The Fcst Starts With A Blend That Leans
Slightly Toward The 00z Ecmwf/Ecmwf Mean Versus The 00z Gfs/Gefs
Mean. This Yields The Desired Compromise Between Continuity And
Recent Trends With The Early Period Ern System And An Intermediate
Soln For The Upstream Wrn-Cntrl Conus Trof Aloft. Still The Ecmwf
Weighting Is Sufficiently Modest To Prevent Adverse Impact Of Its
Amplified Wrn-Cntrl Conus Trof Soln On The Fcst.

Days 6-7 Sat-Sun Use A 70/30 Weighting Of The 00z Ecmwf Mean/Gefs Mean To Reflect Prefs For The System Near The East Coast At That Time... With Diffs Between The Two Means Fairly Minor Elsewhere. This Blend Was Manually Adjusted To Give Slight Account For The 00z Gfs Off The Ca Coast On Day 6.


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