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US Final Extended Weather thru Apr 18

...Overview And Preferences...

The Middle And Late Portion Of The Medium Range Fcst Continues To
Be Challenging With Significant Model/Ensemble Spread For Timing
Of The Trough Expected To Amplify Over The West Early Next Week
And Progress Ewd Thereafter.

As A Result Confidence Is Lower Than Desired With Pinpointing Timing/Location Of Potentially Significant Weather Impacts.

There Is Now Better Agreement For The Leading Vigorous System Tracking From The Nrn Plains Through The Upr Grtlks Sun Into Mon... But Solns Then Rapidly Diverge With The Sewd Extent Of The Trailing Front Over The East. Trends And Teleconnections Provide Some Conflicting Ideas With The Evolving Wrn Trough Making It Difficult To Fully Exclude Any Portion Of The Soln Spread At This Time. Therefore Prefs Lean To An Intermediate Soln... Best Represented By A 06z Gfs/00z Ecmwf And 00z Gefs/Ecmwf Mean Consensus Days 3-4 Sun-Mon... 00z Gefs/Ecmwf Mean Blend Days 5-6 Tue-Wed... And 00z Gfs/Gefs Mean/Ecmwf Mean Blend Day 7 Thu. The 06z Gefs Mean Could Be Substituted For The 00z Version Days 3-4.

...Guidance Evaluation...

With The Amplifying Wrn Trough That Ultimately Becomes The Most
Significant Fcst Problem Mid-Late Period... There Appears To Be
Some Correlation Between The Swd Progression Of A Compact Closed Low Over Alaska From Sun Onward And Timing Of The Trough Over The Conus.

The 00z/06z Gfs And Gefs Mean Runs Bring The Ak Closed Low Farthest Swd And Are Fastest With The Trough Over The Conus. The
06z Gfs/Gefs Mean Ultimately Become A Little Faster Than The 00z
Gfs/Gefs Mean. On The Other Hand The 00z Ecmwf/Ecmwf Mean Are
Slowest To Bring The Ak Closed Low Swd And Hold Back Energy On The Wrn Side Of The Trough.

Ecmwf/Ecmwf Mean Runs Have Been Trending Slower And The Overall Ern Pac/Wrn Conus Pattern By Day 5 Tue Is Fairly Amplified... Which Could Support Moderately Slow Trough Progression. Meanwhile Multi-Day Means By Late In The Period Show The Ern Pac Positive Hgt Anomalies Assoc With The Ern Pac Ridge Aloft But Place Greater Emphasis On A Core Of Positive Hgt Anomalies Over Or Just N Of The Nrn Bering Sea.

Interestingly Teleconnections Relative To This Latter Feature Support Low Amplitude Progressive Flow Across The Conus. Thus A Soln Faster Than The Ecmwf/Ecmwf Mean Is Also Plausible.

Ensembles From Latest Gefs/Ecmwf/Cmc Cycles Are Even More Diverse Than The Operational Models. 00z Cmc Ensembles As A Whole Are Quite Progressive... Gefs Members Range Between Faster And Slower Timing... And Ecmwf Members Are More In The Slower Half Of The Envelope But Still With Considerable Spread.

In Light Of These Varied Pieces Of Guidance And Trend/ Teleconnection Considerations... Prefer To Maintain An Intermediate Soln That Is Closest To A 00z Gefs/Ecmwf Mean Blend Days 5-6 Tue-Wed With A Small Amount Of The 00z Gfs Added To The Ensemble Means For Day 7 Thu. The 06z Gfs/Gefs Mean Become Faster And/Or More Developed Than Desired Over The East Late In The Period.

For The Leading System Tracking From The Nrn Plains Across The Upr Grtlks Sun Into Mon... Gfs/Gefs Trends Over The Past Day Yield
Much-Improved Agreement Among Guidance For This Part Of The Fcst. However By Day 5 Tue The 00z/06z Gfs Stray To The Fast Side Of The Envelope With Trailing Front Over The East... Corresponding To A More Amplified Trough Aloft. Wrn Conus Troughing Seems To Favor Leaning Away From Gfs Amplitude Aloft Over The Northeast.

Preference Leans To A Gefs/Ecmwf Mean Compromise Aloft Given
Similar Prefs Upstream Though It Would Not Be Surprising For The
Front To Extend A Little Farther S Along The E Coast Than This Blend At Some Fcst Hrs.

...Sensible Weather Highlights...

System Tracking Out Of The Nrn Plains/Upr Ms Vly On Sun Will Spread Pcpn From This Region Ewd Into The Northeast With Some Rnfl Also Extending Along The Trailing Front That Should Stall Over The S-Cntrl Plains. Snow Is Psbl Over Extreme Nrn Tier Locations To
The N Of The Sfc Low Track. Trough Aloft Amplifying Into The West Should Spread Mostly Lgt-Mdt Pcpn Over The Nrn Half To 2/3 Of The Region But Areas Near The Cntrl/N-Cntrl Rockies-High Plains May See A Period Of Significant Upslope Enhancement With At Least Some Pcpn In The Form Of Snow.

Farther Ewd... Heaviest Rnfl Early In The Period Should Be Concentrated Near The Wrn Half Of The Gulf Coast Into The Extreme Southeast In Assoc With An Ejecting Srn Stream Shrtwv Aloft And Advancing Warm Front At The Sfc.

Then By Wed-Thu The Combination Of Persistent Low Lvl Gulf Inflow And Wrn Trough Aloft Heading Into The Plains Should Increase Potential For Locally Hvy Rnfl Most Likely Within An Area From The Ern Plains To W Of The Aplchns.

Continue To Expect Consistently Chilly Daytime Temps Over The Nrn Half Of The Rockies/Plains With Some Locations At Least 20 F Below Normal One Or More Days. Trough Aloft Crossing The West Will Also Support Below Normal Readings W Of The Rockies To A Lesser Degree... Especially Around Mid-Period... While Ern Areas Should See Above Normal Temps For Most Of The Period.


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