US Extended Forecast thru Oct 6

...Overview And Preferences...

Operational Models And Individual Ensemble Members Still Display
Considerable Spread And Run To Run Variability With The Trough
Aloft Fcst To Move Into The West Wed-Thu And Continue Ewd
Thereafter... With Diffs In How Nern Pac Trough Energy Flows Into
Wrn Noam By Next Weekend Adding Further Complexity To The Fcst.
In Both Cases

The Ensemble Means Have Been More Consistent Thus Far With Ecmwf Means Most Stable Over The Past Four 12-Hrly Cycles.

By The Latter Half Of The Period The 06z Gefs Mean Has Adjusted Toward The 00z Ecmwf Mean Idea Of A More Positively Tilted Trough Over The Conus And Remains Fairly Close With Incoming Ern Pac/Wrn Noam Flow... So An Even Blend Those Two Solns Is Preferred For The Days 3-7 Wed-Sun Fcst.

...Guidance Evaluation...

Compared To The Full Guidance Spread The 00z Gfs Offers The Least Appealing Soln With The Conus Trough Aloft As It Becomes
Broader/Faster Already By Day 4 Thu And Remains On The Fast Side
Through The Rest Of The Period. The 06z Gfs Is A Little Faster
Than The 00z Ecmwf/Ukmet/Cmc Through Late Day 5 Fri But Is Still
Much Closer To What Consensus Exists At That Time.

Meanwhile The 00z Ecmwf Is On The Fast Side Of The Spread With A Leading Sfc Wave Tracking Through The Upr Grtlks Into Sern Canada Thu-Fri.

Low Confidence In Fcst Details Is Highlighted By The Full Ensemble
Spread By Early Day 7 Sun... With A Trough Axis Anywhere From The
Four Corners/Rockies To Near The East Coast. The Slower Part Of
This Spread Contains Mostly Ecmwf/Cmc Members While The Fast Side Has More Solns From The Gefs Though Also A Handful Of Ecmwf Members.

Upstream Diffs Become Most Pronounced By Day 7 Sun As The 00z/06z Gfs Runs Bring Significant Hgt Falls Into The Northwest In
Contrast To Other Guidance That Is Slower And/Or Flatter.
Although Not Quite Outliers Relative To The Full Ensemble
Spread... The Two Gfs Solns Appear Too Aggressive In Breaking Down The Leading Ridge That Builds Into The West.

Multi-Day Means Have Been Showing A Core Of Positive Hgt Anomalies Over The Nwrn Pac With Latest D+8 Charts Showing That This Center Should Retrograde To A Posn Just Sw Of Kamchatka. Closer To Noam These Means Are Now Highlighting A Core Of Negative Hgt Anomalies Heading Into Or Just N Of The Nrn Bering Sea.

Teleconnections Relative To Both Of These Features Correspond To Progressive Mean Flow Over The Conus With The Hint Of A Ridge Over The Interior West And Trough Over The Ern Half Of The Conus. This Favored Mean Pattern Would Support An Intermediate Timing For The Conus Trough And Relatively Low Amplitude For Any Nern Pac Energy Reaching Wrn Noam Late In The Period... A Soln Provided By The Preferred 06z Gefs/00z Ecmwf Means.

Guidance Continues To Indicate Potential For A Sfc Trough With A
Psbl Embedded Weak Sfc Low To Emerge From The Nwrn Caribbean Into The Gulf Of Mexico. As Is Typically The Case The 00z Cmc Has The Strongest Reflection. The 06z Gfs Has Trended Weaker Than The 00z Gfs And Faster/Nwd Ukmet. Track/Timing Of This Feature Will Depend Somewhat On The Evolution Of The Trough Crossing The Lower 48. Consult The Aftn Update For Todays Wpc/Nhc Coordinated Fcst.

...Sensible Weather Highlights...

As The Trough Aloft Moves Thru The West... Expect Pcpn To Extend
From The Northwest Into The Nrn-Cntrl Rockies With Some Snow Psbl In The Rockies. Also Some Locations Will See One Or More Days With Daytime Temps Of 10-20 F Or So Below Normal Before Moderating Next Weekend.

Once Mstr Reaches Just E Of The Rockies Late In The Week There May Be A Period Of Enhanced Pcpn Over/Near The N-Cntrl High Plains With The Aid Of Low Lvl Upslope Flow. The Most Agreeable Operational Mdls Emphasize Mdt-Hvy Activity Over This Area Around Thu Night-Fri. A Broad Shield Of Mstr Should Then Progress Ewd With Nrn Areas That Are Closest To The Lowest Hgts Aloft/Sfc Low Pressure Most Likely To See Highest Rnfl Totals.

Fl/Gulf Coast Areas May See One Or More Periods Of Mdt-Hvy Rnfl
Due To The Feature Emerging From The Nwrn Caribbean. Some Of This Mstr May Be Drawn Nwd Into The Ern States If The Conus Trough Is Sufficiently Sharp/Amplified But Confidence In Such A Scenario Is Modest At This Time.

Ahead Of The Advancing Cold Front... Temps Will Be Quite Warm With Some Highs 10-15 F Above Normal From The Midwest Into The Northeast. Late In The Period Flattening Flow Aloft Along The West Coast May Bring Some Mstr Back Into The Pac Nw/Extreme Nrn Rockies But Likely With Lighter Amts Than Fcst By 00z/06z Gfs Runs.


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