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US Extended Forecast thru Sep 22

...Overview And Preferences...

An Ern Pacific/West Coast Mean Trough Will Be The Most Prominent
Large Scale Feature During The Medium Range Period.

Within This Pattern A Leading Shrtwv Departing From The West Will Move Into Ern Noam... While The Next Trough In The Series Should Head Into The West Next Weekend In Response To An Upstream Bundle Of Energy Tracking Across The Aleutians Into The Nern Pac.

Primary Issues Seen In Latest Guidance Involve Evolution Of Energy Within The Trough Crossing Cntrl-Ern Noam And Impact On Timing Of The Leading Sfc Front... With Diffs For Energy Moving Into The West During The Weekend Depending On The Timing/Degree Of Nern Pac Amplification.

Enough Consensus Exists To Trend Continuity Toward A Blend Of 06z
Gfs/Gefs Mean And 00z Ecmwf/Ecmwf Mean Solns Into Early Day 5 Fri.

From Late Fri Onward Operational Model Solns Diverge Sufficiently
To Favor A 60/40 Blend Of The 00z Ecmwf Mean/06z Gefs Mean To
Resolve What Diffs Exist And Maintain Fcst Continuity Where
Feasible.

...Guidance Evaluation...

Latest Model/Ensemble Solns Indicate Fairly High Confidence In A
Farther Nwd Track For Sfc Low Pressure Fcst To Deepen Near The
Us-Canadian Border Mid-Late Week In Response To The Leading Mid Lvl Shrtwv Ejecting From The West.

Guidance Has Shown Some Waffling Over The Past 1-2 Days So Further Adjustments Are Certainly Psbl But The 00z Gfs/Ecmwf/Ukmet/Cmc/Navgem All Support A Moderately Nrn Track Which Seems Reasonable Based On Fcst Deepening Of The Core Of Mid Lvl Energy.

Other Aspects Of Evolution Are Still Uncertain. The 00z-06z Gfs/ Gefs-Based Guidance Leans Toward A Slower More Positively Tilted Mid Lvl Trough And Thus Slower Sfc Front... While The 00z Ecmwf Becomes More Negatively Tilted Aloft Versus Consensus Around Fri-Sat Leading To A Fast Moving Front. Based On The Full Array Of
Model/Ens Solns Over Recent Runs A Compromise Appears Most
Reasonable And Is Well Represented By The Preferred Blend. The
Ecmwf Mean Is Given Slight Majority Weighting Based On 00z Cmc
Mean Support. The 06z Gefs Mean Is Favored Over The 00z Version
Due To Being Closer To The Stronger Model Solns For Srn Canada Low Pres And A Slightly Faster Trend With The Ern Noam Trough Later In The Period.

With The Trough Nearing The West Coast Late This Week There Is
Fairly Good Agreement Into Day 6 Sat Aside From The 00z Cmc Being
Fast With Leading Hgt Falls. However During The Weekend Spread
Increases With Respect To How Quickly The Trough Crosses The West Due To Diffs In How Much Amplification Occurs Upstream. At The Very Least The 00z Ecmwf Appears To Have One Of The Lower
Probability Solns With Its Closed Low Entering The Nrn High Plains
On Sun As By 00z Mon The Model Brings A Ridge Axis To The West
Coast Where Consensus Shows A Mean Trough.

Gfs/Gefs Mean Runs Since About 12z Sat Have Indicated More Pronounced Nern Pac Trough Amplification Sat-Sun Leading To Faster Wrn Conus Trough Ejection During That Time... Though The 00z Ecmwf Mean Does Show A Somewhat Deeper Trend Over The Nern Pac By Sun As Well.

Versus The 00z Run The 06z Gfs Has Adjusted Back Toward The Guidance Avg Over The Nern Pac With A Slower Wrn Trough Ejection More Similar To The Ecmwf Mean. Some Degree Of Nern Pac Amplification Is Consistent With The Established Mean Pattern So The Preferred 60/40 Ecmwf Mean/Gefs Mean Blend Provides A Reasonable Intermediate Soln Over The Ern Pac/Wrn Conus Late In The Period. Note That The 00z Cmc Mean Is Still Not As Amplified With The Nern Pac Flow And Slower With The Wrn Trough.

...Sensible Weather Highlights...

The West Will Trend Drier/Warmer During The First Half Of The
Period. Then The Region Will Trend Much Cooler For The Weekend
With Pcpn Extending From The Nrn Half Of The West Coast Into The
Nrn Rockies As An Ern Pac Trough Aloft Progresses Into The West.
Teleconnections Relative To An Agreeably Fcst Core Of Nern Pac Neg
Hgt Anomalies By The D+8 Time Frame Favor Pcpn Along The Nrn Half Of The West Coast As Well.

Meanwhile The Cold Front Expected To Progress From The Plains To East Coast May Be Accompanied By Some Areas Of Lcly Hvy Rnfl With Psbl Input Of Initial Wrn Conus Monsoonal Mstr As Well As Nrn Mexico/Tx Mstr That May Be Enhanced By Hurcn Ingrid Tracking Into Mexico. Consult Latest Nhc Products For Further Info Regarding Ingrid.

Currently Confidence Is Fairly Low With Pinpointing A Favored Area For Heaviest Rnfl During The Period.

Ahead Of The Front Expect Well Above Normal Temps With Some Anomalies Of At Least Plus 10-15f Across Nrn Areas... But Trailing Cooler Air Is Not Expected To Bring Temps Too Far Below Normal.


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