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US Final Extended Weather thru Mar 8

...Pattern Overview...

The Large-Scale Flow Across North America Is Expected To Deamplify Entering The Upcoming Medium Range Period...Day 3/Mon...Then Remain Moderately Amplified And Thus More Susceptible To Smaller-Scale Influences For 2 Or 3 Days...Before Reamplifying Toward The End Of The Period...Day 7/Fri.

Teleconnections With Positive And Negative Multi-Day Mean Height Anomalies Over The Atlantic Supports Reamplification Toward The End Of The Period...Especially With An Upper Trough Forming Across The West...Which Provides Better Support For The More Amplified Gefs Mean Over The Flatter Ecmwf Ensemble Mean.

...Guidance Choices/Uncertainty Evaluation/Sensible Weather
Impacts...

In Addition To Teleconnections Helping To Direct Model Choices
Toward The End Of The Period...Much Of The Guidance Spread And
Uncertainty Appear To Derive From Systems Currently Well Over The
Pacific...Specifically A Complex Shortwave Trough Near 40n/150w... And An Emerging Wave Packet Over East Asia.

The Result Is For 2 Major Systems To Affect The United States Days 3-7 That Must Be Resolved.

By Day 3/Mon...The Trough Near 40n/150w Is Expected To Reach The Northern Plains...With The 00z Ecmwf Stronger And Farther North Than The 00-06z Gfs...With Differences Appearing To Originate From Various Degrees Of Phasing With A Portion Of The Trough Entering Canada And The Tail End Of The Trough Moving Into California.

The Ecmwf Scenario Suggests Considerably More Phasing With Both Portions Of The Trough... And Thus Is Considerably Slower/Stronger Than The Gfs As It Reaches The East Coast By Day 5/Wed. The Gfs Shows Greater Separation With The Trough Crossing California And Thus Has Less Phasing...And In Fact Instead Helps To Accelerate The Trough Across The Southeast.

The Result Is A Faster And Weaker Solution Along The East Coast. Many Reasons Support A More Gfs-Like Solution However...Including Better Agreement With The Ensemble Means...Stronger Run-To-Run Continuity...And That So Many Small-Scale Interactions Need To Occur For The Ecmwf To Verify...Lending Itself To A Low But Not Zero Probability Outcome.

Therefore...The Model Choice For This System Is A 3-Way Blend Of
The 00z Gfs Along With The Gefs/Ecmwf Ensemble Means Through Day 5...Before Switching To A Predominant Gefs Mean Solution.

This System Will Likely Result In A Swatch Of Light To Moderate
Precipitation...Some In The Form Of Snow From The Northern Rockies Southeastward Toward The Tennessee River Vally/Central And Southern Appalachians And Possibly Portions Of The Carolinas.

If The Ecmwf Verifies...Precipitation Will Become More Widespread And Influence Areas Farther North To Possibly Include The Mid-Atlantic Region And Interior Northeast. Confidence Is Near To Slightly Below Average Due To The Complexity.

A Strong Shortwave Trough Possibly In The Form Of A Closed Low Is
Expected To Move Into The Pacific Northwest And Northern California Coast By Day 4/Tue...Before Moving Inland And Weakening.

The Wave Packet Leaving East Asia Should Begin To Influence The Eastern Pacific Flow By This Period...Resulting In Greater Amplification In General...Which Tends To Discredit The Fast Ecmwf Solution...Including Its Questionable Integration Of A Strong Shortwave Trough Entering Its Base Along The Pacific Northwest Coast By Day 6/Thu...Which If True Would Probably Mean An Occluded Frontal System.

Given These Conditions...A More Gfs/Gefs Mean-Like Solution Is Preferred Instead With Only Minimal Weighting Given To The Ecmwf Ensemble Mean For Continuity.

This Approach However...Eliminates The Occluded Frontal System Nearing The Coast By Day 6 That Was Depicted On The Earlier Forecast. While Some Timing/Strength Differences Continue...This System Should Result In Heavy Coastal Rains/Mountains Snows For The Pacific Northwest And Northern California Before Weakening Inland.

The Associated Cold Front Should Reach The Plains By Day 7/Fri... With The Gfs/Gefs Mean Approach Favoring Slightly Stronger Cold Air
Advection Behind The Front Including Colder Temperatures And Less
Precipitation Across The West Compared To The Ecmwf And Its Own
Ensemble Mean For Days 6/7.

Confidence In The Details For This System Is Near Average Through Day 5/Wed...Then Below Average Thereafter.


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