US Extended Forecast thru Jul 19

...Unseasonable Temperature Readings For Much Of The Lower 48...

Overview...
The North American Continent Remains Under A Highly-Amplified Flow Pattern Through Day 5...Then The Pattern At High Latitude Slowly Retrogrades Westward...Allowing The Amplified Ridge-Trough Pattern From The Aleutians To Hudson Bay To Shift Westward About 15 Degrees Of Longitude By Period's End. This Subtle Shift Should Re-Direct Cooler Pacific Air Into Southwestern Canada And The Northern Rockies.

Uncertainty Assessment...
Much Of The Uncertainty In This Medium Range Period And Pattern Remains At High Latitude (About 55n To 57n)...And Has Ties To A 'Retrograding' Upper-Level Ridge Along The West Coast Of Canada... And Upper-Level Low In The Northwest Gulf Of Alaska.

Where It Eventually Ties Into The Lower 48 Weather Forecast Has More To Do With The Projected 'Re-Positioning' Of The Longwave
Trough Over Canada And A Subtle Alteration In The Configuration Of The Anticyclone In The Great Basin. Both Are Not Necessarily Related To Each Other.

With The Upper-Level Trough Over Canada Currently Lodged Invof 90w Longitude... Cooler Air Has Been Able To Descend Directly Downwind Of The Ridge Through South Central And East Central Canada...And Into The Northern Plains And Great Lakes. But With A Westward Shift In The Upper-Level Ridge To An Offshore Position In The Gulf Of Alaska...The Current Flow Pattern Ends---East Of The Divide. This May Only Be A Brief Period Of Change...But Enough To Establish The Longwave Trough Closer To 100w-105w Over Canada By Day 7. The Axis Of Monsoonal Moisture Will Also Attempt To Briefly Re-Position Itself...But In Relationship To The Subtle Shift Of The Anticyclone In The Four Corners And Flow Across Northern Mexico.

And In Essence...The Three Means---Ecens...Gefs And Naefs All Seem To Handle This Gradual Transition In The Day 6-7 Period.

Guidance Maintained Noteworthy Anomalies At Various Levels Beneath The Upper-Level Ridge And Upper-Level Trough Into Day 5...Giving Added Confidence That The Lower 48 Will Be Experiencing Some Temperature Distributions That Will Produce "Extraordinary" Values For The Third Week In July (Mainly Days 3-5).

Although The Deterministic Run-To-Run Depiction Of The Surface Low Tracking Across Southern Canada Has Been Less Than Stellar... Think A Sub 996 Mb Surface Low In Eastern Canada Around Day 3... Weakening By Day 5 Invof Northeast Quebec...Should Allow The Canadian Cold Front To Progress Through The Northeastern Quarter Of The Nation During Days 4-5.

The Baroclinic Contrasts Along The Front Over The East Central US Warrant The Spc Outlook Projection For Severe Weather Along The Front.

Sensible Weather Highlights...
The Rarity Of This Dry Canadian Airmass For July Just Might Approach Temperature Records (Low/Mins And Low/Maxes) For A Vast Majority Of The Eastern Half Of The Nation.

High Temperature Readings Are Likely To Be 15f-25f Above Normal In The Pacific Northwest (Washington...Oregon...Idaho) Beneath An Upper-Level Ridge Of High Pressure Extending From The Continental Divide Westward To The Pacific And Gulf Of Alaska.

As The Upper-Level Ridge Migrates Slowly Northwestward...The Thermal Trough And Monsoonal Moisture Will Move With It...Bringing Mid-Level Moisture And The Threat Of Convection To California And The Cascades.

The Less Humid---Almost Autumn-Like Feel Of The Airmass Originating In Canada--- Will Eventually Spread All The Way To The I-10 Corridor From West Texas Eastward Into Louisiana---And To The
I-20 Corridor From Mississippi To Georgia And South Carolina. Here The High Temperatures Will Be Closer To 10-20f Below Normal.

During This Medium Range Period...Active Weather---From A Precipitation Perspective---Will Be Concentrated Along The Northern And Central Divide Where Confluent Flow---A Mid-Level West To Northwesterly Wind---Will Be Carrying Residual Monsoonal Moisture And Convective Debris From The Mountains Into The High Plains.

A Secondary Focus For Showers And Thunderstorms Will Be Directed Along And Immediately Ahead Of The Day 3-5 Progression Of The Canadian Cold Front Through The Eastern Half Of The Nation.


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