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US Final Extended Weather thru Apr 27

...Overview And Preferences...

Latest Medium Range Guidance Continues To Show A Broad Mean Trough Covering Most Of Noam... Supported By A Fairly Strong Ridge Over The Ern Pac And Another Ridge That Builds Over The Nrn Atlantic During The Period.

These Strong Ridges And Overall Amplified Flow Regime Seems Supported By Lead-In Water Vapor Imagery And Recent Pattern History. Within The Mean Trough The Most Agreeable Solutions Suggest There Should Be Three Discrete Main Shortwaves.

Wavelength Spacing Effects Between These And Smaller Scale
Impulses Offers Increased Uncertainty With Respect To The Extent
Of Amplification Of Each Of These Features. At Least In The Large Scale There Is Good Agreement Into Mid-Late Week...But Models And Ensembles Diverge More Significantly With Aspects Of Ern Pac/Wrn Noam Flow By Days 6-7.

A Blend Of Mid-Larger Scale Compatible Guidance From The Last Two Ecmwf Runs Along With The 00 Utc Ukmet...06 Utc Gfs Ensemble Mean...And 00 Utc Ecmwf Ensemble Mean Seems To Provide A Good Intermediate Solution For Individual Systems Days 3-4 Tue-Wed. After That Time Prefer To Transition By Fri To A 70/30 Weighting In A Blend Of The 00 Utc Ecmwf Ensemble Mean And 06 Utc Gefs Mean As An Initial Base Map To Account For The Increased Detail Uncertainty During The Latter Half Of The Medium Range Period.

...Guidance Evaluation...

During Days 3-4 Tue-Thu The Primary Features Of Interest Will Be A
Surface Low Tracking Newd From The Great Lakes Into Ern Canada ...The Trailing Cold Front Crossing The Ern States And Dug/Trailing Back Through The West-Central Us...And A Lead Low Slated To Track Nwd Just Off The Us East Coast And Into The Canadian Maritimes.

While The Various Determinitsic Models Offer Some Question Marks With Their Respective Solutions For Each Feature... An Average From A Blend Of Clustered Guidance From The Recent Ecmwf/Ukmet Runs And To A Lesser Extent Ecmwf/Gfs Ensemble Means Overall Yields A Good Intermediate Starting Point And Keeps Continuity Changes Fairly Modest.

Continuity Is Also Good With A Weak Frontal System Expected To
Affect Areas From The Nrn Plains Into Great Lakes Wed-Thu...Though
Solutions Differ With The Track An Anchoring Low. Best Indications Point Toward A Surface Low Track Near The Upper Great Lakes.

By The Latter Half Of The Period Guidance Continues To Diverge
With Ern Pac/Wrn Noam Flow Details...Related To How Quickly Energy Within An East-Central Pac Closed Low Ejects Ewd. Given
Uncertainty But With An Eye To Maintaining Continuity And Amplified Flow...Prefer A 70/30 Blend Of The 00 Utc Ecmwf Ensemble Mean And 06 Utc Gfs Ensemble Mean.

Favored Blends Next Week Hold Onto The Middle/Slower Portion Of
The Spread With The Upper Low Reaching Srn Ca/Nrn Baja...Given A
Continued Lack Of A Pronounced Upstream Kicker.

...Sensible Weather Highlights...

Most Locations Over The West Should Remain Warm And Dry.

The Plains Will See Below Normal Temps During The Period...With The Most Extreme Anomalies For Daytime Highs Of 20-30 F Below Normal Occurring At Some Locations Tue.

Some Of This Cool Air Will Spread Ewd But In Modified Form.

Early In The Fcst There Is Still Uncertainty With The Wwd Extent Of Moisture Associated With The Wave Tracking Nwd Off The East Coast...Favoring An Intermediate Solution. Great Lakes To Plains Front Heading Toward The East Coast Tue-Thu Will Be Accompanied By Some Areas Of Locally Moderate-Heavy Rainfall.

A Trailing System Brushing The Nrn Tier Should Have Only Modest Moisture ...But Then Enough Low Level Flow From The Gulf May Become Established To Support Development Of Another Area Of Rainfall Over The Plains/East-Central Conus By Next Fri-Sat.


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