US Preliminary Extended Weather thru Dec 9

The Fcst Challenge During The Medium Range Period Continues To Be The Handling Of Individual Systems Embedded Within A Stable And Agreeable Large Scale Noam Mean Trof.

Based On The Current Best Clustering Of Guidance... Leading Energy In The Process Of Amplifying Over The W-Cntrl Conus As Of Day 3 Sat Should Eject Ewd/Newd By Early Next Week As An Upstream Nrn Pac System Rounds The Strengthening Ern Pac Ridge And Descends Into The Conus.

It Is This Leading Energy That Is Expected To Develop A Fairly
Significant Low Pressure System Tracking From The Srn Plains Into
The Grtlks/Ern Canada Around Sun-Mon. Individual Mdls/Ensembles
Have Been Quite Varied With Timing/Intensity Depending On Exactly
How Much Swwd Elongation Occurs With The Mid Lvl System Early In
The Period. Teleconnections Relative To The Upstream Ridge Have
Favored Decent Swwd Elongation Of Flow Within The Overall Mean
Trof... And Some Prior Guidance Plus The 00z Ukmet Show This Idea
Which Would Lead To Slower Ejection.

However The Latest Majority Clustering Of Mdls/Ensembles Suggests That The Upstream Nrn Pac System Will Round The Ridge Sufficiently Quickly To Push Along The Leading System In A Faster Manner. Given This Trend Occurring Fairly Early In The Medr Period Such A Scenario Appears Slightly More Likely So Will Lean In That Direction For The Deterministic Fcst. With This System The 00z Ecmwf Leans To The Fast Side While The Gfs Is Slower/Deeper With An Avg Of Ensemble Means Between These Solns. At Some Fcst Hrs The Gefs Mean Is Somewhat Slow As Well Though.

Farther Upstream The Gfs Has One Of The More Amplified Depictions
Of Another Nrn Pac Shrtwv Trof Expected To Reach The Wrn Noam
Coast By Mid-Late Tue... Resulting In A Sharper Than Consensus
Shrtwv Over The West At That Time. By The Latter Part Of The Period The 00z Ecmwf Generally Compares Better To The Ensemble Means.

A 00z Gfs/Ecmwf Blend Provides A Reasonable Account For Latest
Guidance Clustering For Days 3-4 Sat-Sun. The Gfs/Ecmwf Weighting Was Reduced To 20 Pct Each By Day 5 As Those Solns Begin To Stray From Ensemble Mean Consensus For The System Tracking Thru The Grtlks Into Ern Canada. The Remainder Of The Blend Consists Of The 00z Gefs/12z Ecmwf Ensemble Means.

With This System Departing Thereafter And The Ecmwf Comparing Acceptably To The Ensemble Means Thru Day 7... A Nearly Even Blend Of The Ecmwf/Gefs Mean/12z Ecmwf Mean Serves As The Starting Point For Days 6-7 Tue-Wed.

Especially For The Plains To Ern Canada System The Resulting Fcst
Yields Fairly Good Continuity In Light Of Current/Recent Soln Spread And Run To Run Trends.

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