US Extended Forecast thru Oct 19

Overview And Preferences...

Most Guidance Agrees Upon A Strong Ern Pac/Nwrn Noam Mean Ridge With An Amplifying/Broadening Mean Trough Downstream Over Canada Into The Conus. However There Are Diffs In The Orientation Of The Mean Trough Over The Conus By The Latter Half Of The Period... And Some Spread With The Timing/Evolution Of A Leading System Fcst To Track From The Nrn Plains/Upr Ms Vly Through The Grtlks Into Sern Canada.

A Model/Ensemble Mean Compromise Best Represented By The
12z Gfs/Ecmwf And Their 12z Means Is Preferred To Maintain
Consistency For This Leading System From Day 3 Tue to Day 5 Thu.

By Days 6-7 Fri-Sat The Favored Soln Adjusts To An Even Blend Of
The Two Ensemble Means As Teleconnections Offer Support For Some Aspects Of Each.

Guidance Evaluation...

Evolution/Timing Of The System Tracking From The Nrn Plains Into
Sern Canada Will Depend On Rather Sensitive Aspects Of Srn Canada Flow And Interaction With The Nrn Conus Mid Lvl Low/Trough. The 18z-00z Gfs/Gefs Mean Runs Are On The Faster Side Of The Spread Due To Somewhat More Phasing Aloft... While Greater Separation In Ecmwf/Ukmet/Cmc Runs Allows A Ridge Aloft To Pass Briefly To The N Resulting In Slower Timing Until Upstream Flow Begins To Kick It Newd. The 12z Gfs/Gefs Mean Were Still Faster Than The Ecmwf/Ukmet/Cmc Average But Not As Fast As The 18z Gfs/Gefs Mean.

Thus The 12z Gfs/Gefs Mean Were Included In The Compromise With
The 12z Ecmwf/Ecmwf Mean... Maintaining Reasonable Continuity
While Awaiting Better Guidance Clustering.

Based On Continuity Of The Various Operational Models Suspect The Slower Half Of The Spread May Be A Little More Correct.

In Spite Of This Continued Spread There Is Now Better Agreement With The Ejection Of Trailing Wrn Energy Reaching The Plains/Ms Vly By Day 5 Thu. Current Consensus Indicates This Energy Should Support At Most Minor Waviness Along The Leading Sfc Cold Front.

D+8 Multi-Day Means Continue To Show A Core Of Positive Hgt
Anomalies Cntrd Near The Alaska Panhandle. Teleconnections
Relative To This Feature Favor Establishment Of A Mean Trough Axis
Over/S Of Hudson Bay With Broad Troughing Farther S Over The Conus But With Modest Neg Hgt Anomalies. There Is Also Support For Some Model/Ensemble Solns That Pull A Portion Of Amplifying Trough Energy Into The Swrn Conus And Vicinity... Not To The Extent Shown By The Past Couple Ecmwf Runs But With Lower Wrn Conus Hgts Than Gfs Runs. Also Gfs Runs Have Been Rather Erratic With Evolution Near The West Coast Over The Past 1-2 Days. Note That Ensemble Spread Becomes Significant Upstream From The Ern Pac Ridge By Next Fri-Sat With Likely Impacts On The Ridge Itself Late In The Period Or Shortly Thereafter. For The Conus Trough An Avg Of The 12z Gefs Mean And More Positively Tilted Ecmwf Mean Provides A
Reasonable Account For Teleconnection Guidance And Current Soln
Spread By The Latter Half Of The Fcst.

Sensible Weather Highlights...

Nrn Tier Low Pressure And Trailing Cold Front Will Spread Rnfl Of
Varying Intensity Across The Cntrl/Ern States... Likely Trending
Lighter Toward The Sern Third Of The Conus.

Currently The Best Potential For Heaviest Amts Is Over The Srn Plains Into The Lwr Ms Vly Tue-Wed As Some Mstr From The Tropical Ern Pac May Stream Newd And Enhance Rnfl Along The Front Crossing The Area.

Combination Of This System And A Trailing Canadian Cold Front Should Keep Daytime Temps Below To Well Below Normal Over Most Of The Rockies/Plains.

Uncertainty With Flow Details Aloft By The Latter Half Of The Period Leads To Low Confidence In Coverage/Intensity Of Rain/Hier Elev Snow Over The Rockies Into Cntrl-Srn Plains Late In The Week. Current Ensemble Mean Prefs Would Support A Compromise Between The Dry Gfs Runs And Wet Ecmwf Runs.

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