US Final Extended Forecast thru May 18

...Overview And Preferences...

Low Amplitude Pacific/Wrn Noam Flow As Of Early Tue Is Expected To
Evolve Into Modest Mean Troughs Over The Wrn Conus And Ern Noam By Late In The Period... With A Mean Ridge Building Into The Plains.

An Upr Low Ejecting From Nwrn Mexico Should Become Part Of The
Overall Ern Trough By The Latter Half Of The Period Though With
Continued Uncertainty Over The Degree Of Stream Interaction.

Overall There Is Decent Agreement With The Large Scale Pattern But
Typical Detail Diffs. A Compromise Among The 06z Gfs/00z Gefs
Mean/00z Ecmwf Mean Offers The Best Option For Resolving Diffs With The Srn Canada System And Front Trailing Into The Nrn Conus Days 3-5 Tue-Thu... While The 00z Cmc Mean And Teleconnections Relative To The Most Stable Mean Features Favor A Soln Close To A 70/30 Blend Of The 00z Ecmwf Mean/00z Gefs Mean For Days 6-7.

...Guidance Evaluation...

After Day 3 Tue Some Models/Ensembles Have Been Inconsistent With The Timing Of Low Pressure Crossing Srn Canada. Among Latest Solns The 00z Ecmwf Is On The Fast Side Of The Spread While The 00z Ecmwf Mean Is Not As Fast But Still Ahead Of The 00z/06z Gfs And Gefs/Cmc Means. Operational Ecmwf Runs Have Been Particularly Inconsistent With This System Incl The Trailing Front. Gfs Runs Have Been More Stable Albeit With A Slightly Faster Trend Over The Past Couple Days. Gefs And To Some Extent Ecmwf Means Have Provided Decent Stability Over 12-Hrly Runs.

A Compromise Among The 06z Gfs/00z Gefs Mean/00z Ecmwf Mean Is Closest To The Middle Of The Full Ensemble Spread By Day 4 Wed And Remains A Good Intermediate Soln As Individual Members Diverge By Day 5 Thu.

By The Latter Half Of The Period Solns Differ With The Amplitude Of Ern Noam Troughing... Impacting The Srn Extent Of The Sfc Front Over The East. Gfs/Gefs Mean Runs Have Tended To Have The More
Amplified Troughs While Ecmwf Means Have Been Flatter. On The
Other Hand The Past Two Operational Ecmwf Runs Have Indicated Ern Conus Hgts Higher Than A Considerable Majority Of Ensembles.

Multi-Day Means Continue To Emphasize Positive Hgt Anomalies Over The Nrn Atlc And Negative Anomalies N/Nnw Of Alaska. Both
Features Teleconnect To A Trough Near Or Just W Of The East Coast.

Closer Proximity Of The Nrn Atlc Pos Anomaly That Favors A
Somewhat Flatter/Ewd Trough Than The Neg Anomaly N Of Ak... Plus
The 00z Cmc Mean... Seem To Recommend Leaning 2/3 Toward The
Flatter 00z Ecmwf Mean Versus The Gefs Mean. This Soln Leads To
An Ern Conus Cold Front Reaching Well Sewd Of Ecmwf Runs But Not To The Extent Of The Gfs.

Yet To Be Resolved Smaller Scale Shrtwv Issues Over The Ern Pac
Early In The Period Temper Confidence With Exact Details Of The
Trough Fcst To Move Into The West By Late Next Week.

The 00z Ecmwf Starts To Differ From What Consensus Exists By Day 4 Wed While The 06z Gfs Leans Faster Than The Gefs/Ec/Cmc Means By Sat.

By Days 6-7 Fri-Sat Teleconnections Mentioned Above Hint At The
Developing Split In The Flow Near The West Coast With Modest
Amplitude Of Any Trough. Thus The 2/3 Weighting Of The 00z Ecmwf
Mean Versus The More Amplified Gefs Mean Appears Reasonable Here As Well.

...Sensible Weather Highlights...

Expect The Frontal System Crossing The Nrn Half Of The Lower 48 To
Be Accompanied By Rnfl Of Varying Intensity.

An Area From The Mid Ms Vly Into The Oh Vly Or Lower Grtlks Region Currently Has The Best Potential To See Highest Totals During This Event But Confidence In Specific Amts/Locations Is Moderate At Best.

Meanwhile Mstr Ahead Of The Upr Low Ejecting From Nwrn Mexico
Should Spread Newd From The Rio Grande Vly During The First Part
Of The Period With Some Localized Mdt-Hvy Activity.

Over The Northwest Mstr Should Be Confined To Nrn Areas Tue-Wed Before A Broader Area Of Mstr Spreads Across The Region And Into The Nrn Plains Where Interaction With The Trailing End Of The N-Cntrl Conus Front May Enhance Rnfl Totals By Fri Or Sat.

Tue Will Feature The Greatest Temperature Extremes With Morning Lows Psbly Approaching Daily Records Over Parts Of The East And Much Above Normal Highs From The Cntrl/N-Cntrl Plains Into The Upr Ms Vly.

Trough Aloft Eventually Moving Into The West Will Lead To A Steady
Cooling Trend Toward Normal Over The Region... While Cntrl-Ern
Temps Should Be Near To Above Normal Wed Onward.

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