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US Extended Weather Forecast thru July 27

Short Range Forecast Discussion
Valid 00Z Sun Jul 21 2013 - 00Z Tue Jul 23 2013

...Showers and thunderstorms continue from the Mid-Atlantic/ Southeast to the Plains states...

...Showers and thunderstorms possible from the Gulf Coast to the Southwest U.S....

A cold front extending from the Mid-Atlantic through the Ohio Valley and
into the Central Plains will drift slowly southward before stalling across the Mid-Atlantic states on Sunday morning.

The chance for showers and thunderstorms will remain in the forecast along and ahead of the boundary from the Mid-Atlantic to the Middle Mississippi Valley and Central Plains through at least Monday.

Weak impulses of upper level energy could also enhance precipitation just along the front in the Ohio Valley as well as in the Central Plains.

Farther north, another area of upper level energy over southwest Canada will dive southeastward into and across south central Canada.

At the surface, a system over the Canadian Rockies will strengthen and accelerate as it drops into the Northern Plains on Sunday and lifts into the Upper Midwest/Northern Great Lakes on Monday.

Showers and thunderstorms will develop ahead of the warm front on Sunday across the Northern Plains and into the Upper Great Lakes by Monday.

Elsewhere, more upper level energy will induce showers and storms at the surface across the central Gulf Coast states and into the Southeast.

Across the southwest, monsoonal moisture and the weak upper low over Mexico will keep the scattered showers and thunderstorms across the region on Sunday but should begin to dissipate by Monday morning.
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Extended Forecast Discussion
Valid 12z Tue Jul 23 2013 - 12z Sat Jul 27 2013

...Overview And Preferences...

Albeit With Daily Adjustments In Some Details... The Expected Mean
Pattern Remains Unchanged With An Ern Noam Trough... Wrn Conus
Ridge Heading Toward The Plains... And Nern Pac Trough Drifting
Toward The West Coast.

Within This Mean Flow The Primary Uncertainties Involve Timing Of Ern Canada Low Pres And Trailing Front Early In The Period... Amplitude/Timing Of A Psbl Hudson Bay Into Ms Vly/Grtlks Shrtwv... And Impact Of The Upr Low Currently Over Extreme Nwrn Mexico As It Lifts Up The Ca Coast Through The Nrn Half Of The West And Then Into The Plains.

There Is Still Enough Variability In Model Details To Favor A Blend Of The 00z Gefs/Ecmwf Ensemble Means As A Starting Point For The Fcst While Adding A Little Detail Where Sufficiently Agreed Upon By
Operational Models.

...Guidance Evaluation...

With The Sfc Low Tracking Into Ern Canada Early In The Period The
Past Couple Ecmwf Runs Have Been On The Faster Side Of The
Envelope. On The Other Hand Multi-Day Trends/Continuity Do Not
Appear To Favor The Slower Adjustment Of The 06z Gfs/Gefs Mean.
The Preferred Blend Of 00z Gefs/Ecmwf Means Offers A Good
Intermediate Soln With The Sfc Low And Trailing Front.

A Decent Proportion Of Guidance Suggests That Cntrl Canada Energy
Should Amplify Into A Hudson Bay/Ontario Into Ms Vly/Grtlks Shrtwv
By Days 6-7 Fri-Sat. Thus Far Solns That Have Been On The
Amplified/Sharp Side Of The Spread Have Not Maintained Continuity
So Among Current Solns Would Tend To Lean Away From The 06z Gfs In Particular. The 00z Ecmwf Becomes More Amplified Than The
Guidance Avg But Not To The Point Of Being Extreme.

Latest Solns Have Converged Fairly Well With The Nern Pac Trough/ Embedded Closed Low. Note However That The 06z Gfs Strays To The N Of Consensus With The Closed Low Late In The Period And
Is Weak With The Trough Moving Into The Pac Nw. In Light Of This ... Would Also Lean Away From The 06z Gfs That Ultimately Tracks Day 3 Energy Along/Near The Ca Coast Farther Nwd Than Most Other Solns By Thu.

What Kind Of Sfc Reflection May Be Supported By The West Coast
Into Nrn Rockies Energy Ejecting Into The Plains Will Depend On
Evolution Of The Nrn Stream Flow And Convective Influences. The
00z Ecmwf Wave From The Plains Into Mid Ms/Lwr Oh Vly Could Be A
Little Overdone Due To Feedback But By Day 7 Sat Recent Ecmwf Runs As Well As Gfs Runs Back To 12z/19 Do Offer Support For Somewhat Lower Sfc Pressures From The Grtlks Into S-Cntrl Plains Relative To The Gefs/Cmc Means.

...Sensible Weather Highlights...

Expect Shwrs/Tstms Along A Front That Should Extend From The
Grtlks Into Cntrl Plains Early Tue And Then Decelerate/Stall As It
Reaches The East Coast/Southeast.

By The Latter Half Of The Period Coverage And Intensity Of Rnfl Along The East Coast Will Depend On Shrtwv Details Aloft/Assoc Frontal Waves That Are Typically Not Well Resolved More Than A Couple Days In Advance.

Currently The Greatest Spread Between Fairly Wet And Much Drier
Solns Exists Around Days 5-6 Thu-Fri.

Meanwhile Energy Lifting Nwd Along The Ca Coast And Then Flowing Across The Nrn Rockies And Sewd From There Will Likely Generate Areas Of Locally Hvy Convection From The Interior West/Rockies Into The Plains. The Combination Of This Energy And An Evolving Shrtwv Extending From Hudson Bay/Ontario Into The Conus Support The Best Potential For Some Hvy Rnfl From The Nrn Half Of The Plains Into The Grtlks-Mid Ms Vly During The Thu-Sat Time Frame.

The Nrn Plains Should See Below Normal Daytime Temps Most Of The Period. The Nwrn Conus/Nrn-Cntrl Rockies May See Highs Up To 10f Or So Above Normal Tue Before Moderating. Otherwise Expect Readings Within A Few Degs Of Normal Most Days.


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