US Short Term and Extended Forecast thru July 23

Short Range Forecast Discussion
Valid 12Z Tue Jul 16 2013 - 12Z Thu Jul 18 2013

...Heat wave over the East will expand into the Plains by the middle of
the week...

...Cool and wet weather will continue from western Texas into New Mexico along with the potential for flash flooding...

An anomalous upper level low which has been slowly tracking west across the southern Plains is forecast to continue drifting westward, reaching the southern Arizona/New Mexico border by Thursday morning. The upper low is forecast to weaken, but the disturbance will combine with deep moisture emanating from the Gulf of Mexico via low level southeasterly flow to produce locally heavy rain with the possibility of flash flooding from west Texas into Arizona and New Mexico. While not everyone will see heavy rainfall from this system, most will see at least light rain with lots of cloud cover which will help to limit high temperatures to the 70's and 80's across the region, well below mid-July averages.

The other big weather story is the heat wave currently across the east. A large mid-level ridge centered over the Ohio Valley is responsible for the heat wave, and the ridge is forecast to expand westward through Wednesday. This will have the effect of increasing high temperatures past the 90 degree mark throughout most of the central and northern Plains by Wednesday. Scattered afternoon and evening thunderstorms could bring some temporary relief from the heat in and around the Great Lakes and Appalachians, but widespread heavy rain is not anticipated.

Looking north, a cold front will slowly push across the north-central
U.S., but with little southward progress. The front will serve as a focus
for thunderstorm activity with a possibility of severe storms, according
to the Storm Prediction Center, extending across a good portion of the
northern U.S. near the Canadian border for Wednesday.

Extended Forecast Discussion
Valid 12z Fri Jul 19 2013 - 12z Tue Jul 23 2013

...Overview And Preferences...

The Majority Of Guidance Agrees Fairly Well On A Mean Pattern
Consisting Of A Wrn Conus Ridge That May Extend Into The Cntrl-Srn
Plains By Late In The Period And An Ern Conus Mean Trough Whose
Core In Canada Should Be Strongest During The Weekend.

Meanwhile, Flow Around A Persistent Ridge Cntrd To The Sw Of The Alaska Mainland Will Encourage Development Of A Nern Pacific Trough. Diffs With The Exact Strength Of The Wrn Ridge Impact The Ern Conus Trough And Position Of The Assoc Sfc Front Over The
Cntrl-Ern States From The Weekend Onward... And Combined Diffs
With Ern Pac Through Srn Canada/Nrn Conus Flow Lead To Some
Uncertainty With Sfc Features Over The Nrn Tier Conus/Srn Canada
By Early Next Week. At The Present Time A 60/40 Blend Of The 00z
Ecmwf Mean/00z Gefs Mean Provides The Best Intermediate Soln
Sfc/Aloft And Has The Best Support From Teleconnections.

...Guidance Evaluation...

By Day 5 Sun Two Aspects Of Fcst Uncertainty Arise Over The Ern
Pac/Wrn Conus... Handling Of Initial Closed Low Energy Well
Offshore The Pac Nw And Strength Of The Wrn Conus Ridge... With
Impacts Continuing Through The Rest Of The Fcst.

The 00z Ecmwf And Slower 12z/15 Ecmwf Are The Only Operational Model Runs To Bring The Initial Ern Pac Closed Low Inland Rather Than Having It Retrograde To Become Part Of An E-Cntrl Rex Block. A Minority Of 00z Ensembles Share Some Variation Of The Op Ecmwf Idea Of Bringing This Energy Inland But Best Consensus Keeps Most Of The Energy Offshore. By Mid-Late Period 00z/06z Gfs Runs And 06z Gefs Mean Are Somewhat Stronger And Slightly Wwd Of Remaining Consensus With The Wrn Conus Ridge. Due To Upstream Diffs The 00z Ecmwf Pushes The Ridge Almost Entirely Into The Plains.

Multi-Day Means By D+8 Strongly Agree Upon A Core Of Positive Hgt
Anomalies Cntrd Near The Wrn Alaska Peninsula/Ern Aleutians.

Teleconnections Relative To This Feature Support The Evolving Nern
Pac Trough And Only A Modestly Amplified Conus Ridge With A Center Near The Srn Half Of The Rockies. The 00z Gefs/Ecmwf Means Correspond Best To This Teleconnection-Favored Pattern And Thus Are Favored For The Fcst Blend. The 00z Cmc Mean Is Similar As Well. Recent Gefs Mean Trends Toward The Slightly Stronger Ecmwf Mean With The Wrn Ridge During The First Half Of The Period Favor Somewhat More Weighting Of The Ecmwf Mean But The Gefs Mean Shows A Little More Definition Of Some Features Thus Meriting Its 40 Pct Weight.

By Mid-Late Period These Ern Pac Through Cntrl Conus Diffs Lead To
Widening Spread With The Swd Extend Of The Cold Front Settling
Over The Ern Half Of The Lower 48. The Stronger Wrn Ridge Aloft
In The Gfs Leads To A Farther Swd Push Of Cooler Air In Its Recent
Runs While The More Progressive Wrn-Plains Ridge In The 00z Ecmwf Leads To The Wrn Part Of The Front Retreating As A Warm Front Faster Than Other Solns. The Favored Blend Of 00z Means Does Bring The Front Farther Swd Than Wpc Continuity From Sun Onward But Appears Reasonable Given The Current Guidance Spread And Support From Teleconnections Aloft Later In The Period.

...Sensible Weather Highlights...

Expect The Cold Front Dropping Into The Cntrl-Ern States To Be
Accompanied By Convection Of Varying Intensity. Consult Spc
Outlooks For Further Info Regarding Severe Threats... With The
Current Day 4-8 Outlook Issued Early Tue Highlighting Potential
Over Parts Of The Nern Quadrant Of The Conus Days 4-5 12z Fri-12z
Sun. With Much Lower Confidence There Is The Possibility Of Some
Convection Over Parts Of The Nrn Tier And/Or Grtlks By Next Mon-Tue.

Meanwhile Expect Typical Diurnal Convection Over The Srn-Sern States... And Monsoonal Activity Across Portions Of The Srn Rockies And Southwest. In Both Cases Some Locally Hvy Amts Are Psbl.

Locations Over The Nrn 2/3 Of The East Will See A Significant Cooling Trend After Late This Week. Coolest Readings Relative To Normal Should Be Over The Upr Ms Vly/Grtlks During The Weekend.

Daytime Temps Should Be Up To 10-15 F Above Normal Over Parts Of The Nrn Half Of The Interior West Into The Weekend...Moderating Somewhat By Early Next Week As Hgts Aloft Gradually Decline.

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