US Extended Forecast Updated thru Oct 5

...Overview And Preferences...

The Primary Fcst Problem During The Period Involves The Timing/ Distribution Of Energy Within A Trough Aloft Expected To Amplify As It Moves Into The West Days 4-5 Wed-Thu And Continue Ewd Thereafter. Current Guidance Diffs With This Feature Lead To
Meaningful Spread With The Track/Speed Of Sfc Low Pressure
Progressing From The Plains Through Grtlks During Thu-Sat And
Timing Of The Trailing Cold Front. Variability In Guidance Over Recent Days Combined With Ideas From Upstream Teleconnections
Favor An Intermediate Soln Best Represented By A 40/30/30 Blend Of
The 00z Ecmwf Mean/00z Naefs Mean/06z Gefs Mean.

...Guidance Evaluation...

Not Surprisingly Recent Ensemble Means Have Been More Stable Than Operational Runs With The Handling Of Trough Energy Moving Into The West And Tracking Into The Plains... Though Even The Means Have Varied Somewhat.

Over The Past 24 Hrs Gfs/Ecmwf/Cmc Runs Have Simultaneously Alternated Between Solns Holding Back Some Energy Over The Four Corners/Rockies Into Fri... In 00z Cycles... Versus The 12z/27 Runs That Brought The Primary Trough Axis Into The Nrn Plains/Upr Ms Vly At The Same Valid Time. Prior To The 12z/27 Cycle Guidance Had Generally Been Trending Toward The Slower/More Separated Scenario. The 12z/00z Gefs-Ecmwf Mean Runs Also Reflect Some Of The Operational Waffling. The 00z Gefs Mean Was Still Relatively Fast/Neutrally Tilted But The 06z Version Has Trended Slower Toward The 00z Cmc Mean That Is Modestly Faster/Weaker Than The 00z Ecmwf Mean That Represents The Slow Side Of The Spread.

It Is Interesting To Note That While There Is Some Timing Spread With The Upstream Trough Moving Into The Nern Pac... It Appears To Be Less Than What Is Fcst Downstream Over The Conus.

Recent Multi-Day Means Late In The Period Have Been Highlighting A
Core Of Positive Hgt Anomalies Over The Nwrn Pac With
Teleconnections Offering Potential For A Fairly Progressive Mean
Pattern Over The Conus With The Hint Of A Trough Axis Developing
Over The E-Cntrl Conus.

While This Relationship Suggests The Progressive Side Of The Soln Spread Cannot Be Ruled Out... The Overall Inconsistency Of Guidance Over Recent Days Favors An Intermediate Soln That Is In The Middle-Slower Part Of The Envelope And Preserves As Much Wpc Continuity As Possible While Waiting For Better Clustering/ Continuity. This Is Achieved By The Preferred Ensemble Mean Blend With Minor Detail Enhancements.

One Other Area Of Some Contention Is Over/Near The Gulf Of Mexico.

There Has Been Reasonable Agreement Upon A Sfc Trough Taking
Shape Over The Wrn Caribbean And Tracking Nwwd Into The Gulf.

00z/06z Gfs And 00z Cmc Runs Indicate Some Sfc Development Along This Trough Axis With The 00z Gfs Tracking The Sfc Low Farthest Newd. The 00z Ukmet/Gefs Mean Keep Any Sfc Wave Farther Wwd While The 06z Gefs Mean Is Less Defined.

Individual Ensemble Members Late In The Period Are Broadly Spread Across The Gulf And To The E/Ne. For Now Prefer To Hold Off Depicting Any Well Defined Sfc Low. Nhc/Wpc Coordination Will Be Reflected In The Afternoon Update.

...Sensible Weather Highlights...

Over The West... Pcpn Initially Over The Pac Nw/Nrn Rockies Should
Shift Sewd With Time As An Amplifying/Cooling Trough Aloft Moves
Into The West.

Under The Trough Some Locations May See Daytime Highs 10-20 F Below Normal For One Or More Days.

Snow Is Psbl In Higher Elevs Of The Nrn And Psbly Cntrl Rockies.

Most Areas Should Trend Drier/Warmer By Next Fri-Sat As The Trough Continues Ewd And An Upstream Ridge Moves In.

Sfc High Pressure Building Into The West During The Latter Half Of The Period Should Also Support Strengthening Offshore Winds Over Ca.

As The Upr Trough Emerges Over The Plains Expect A Broad Area Of Rnfl From The Plains Into The E-Cntrl Conus Later In The Week. Location/Timing Of Heaviest Rnfl Will Depend On Still Uncertain Details Aloft... With Best Potential Currently From The Nrn Plains Into The Mid-Upr Ms Vly And Psbly Grtlks.

Areas To The Nw Of Plains-Grtlks Low Pres Should See Well Below Normal Daytime Temps Thu-Sat While Most Of The Remainder Of The Cntrl/Ern Conus Should Be 5-15 F Above Normal Most Days.

One Exception May Be The Gulf Coast/Fl With Lingering Mstr/Ely Low Lvl Flow And Then With Enhanced Rnfl In Assoc With The Sfc Trough Expected To Emerge From The Nwrn Caribbean.

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