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Area Forecast Discussion

Fxus66 Kmtr 241127

Area Forecast Discussion
National Weather Service San Francisco Ca
325 Am Pst Thu Nov 24 2005

.discussion...the First Signs Of A Major Pattern Shift Are Becoming
Evident On Latest Satellite Imagery This Morning As A Vigorous
Upper Low Centered Near 49n/141w Pushes A Trough Ever Closer To The
West Coast. Meanwhile...the Upper Ridge Which Has Provided The
Unseasonably Warm And Dry Weather To The District For The Past
Several Days Is Slowly Weakening. To Our East...a Very Cold Polar
Vortex Is Diving Southeastward From Hudson Bay Towards The Great
Lakes And New England. The Gfs Then Lifts The Core Of This System
Rapidly Northeastward...taking Along With It The Trough Axis Which
Has Resided Over The Eastern United States The Past Several Days.
Overall...this Movement Downstream Will Allow A More Progressive
Pattern To Develop Across North America...dislodging The Blocking
Ridge From The Pacific Northwest And Allowing Pacific Storm Systems
To Affect The West Coast.

Locally...the First Effect Of This Pattern Shift Is The Transition
To Onshore Flow. This Is Already Occurring Near The Coast...with
Coastal Low Clouds And Fog Spreading Into The North Bay Coastal
Valleys As Well As The Monterey Peninsula/salinas Valley Area.
Humidity Levels Are Much Higher In The Lower Elevations Than They
Were Last Night...and The Offshore Flow Of Wednesday Morning Is
Absent From The East Bay Hills Raws Obs Today. As The Flow Turns
Onshore...temperatures Should Be Noticeably Cooler In The Sf Bay
Area Today. Clouds Will Be Increasing Then Thickening This Afternoon
As A Cold Front Associated With The Upper Low Near 49n/141w
Approaches The Northern California Coast. Increased Pops To
Categorical Across The North Bay Tonight As Nam Mos Guidance Went
Near 100% For Both Sts And Apc. Rain Should Develop This Evening
Across The North Bay...then Spread Southward Across The Bay Area By
Friday Morning. Measurable Rain Is Likely North Of Monterey...with
Lesser Chances South. This System Does Not Appear To Be A Major
Rain Maker...and Much Of Its Strength Will Go Into Breaking Down The
West Coast Ridge. About A Quarter Inch Of Rain Should Fall North Of
The Golden Gate And Over The San Mateo Coast...with Lesser Amounts
Further South And Inland.

After This Front Moves Through...the Transition To A Much Cooler
Airmass Will Begin Friday. Highs Will Fall To Below Normal Levels
By Saturday With Our First Widespread Frost Likely Across The Inland
Valleys Saturday Night. The Coolest Spots Should Have No Problem
Dipping Down Into The Upper 20s With Clear Skies And Calm Winds.

For Sunday And Beyond...there Is Quite A Bit Of Uncertainty As The
00z Gfs Came In Much Drier Than Previous Runs For Sunday Night And
Monday. The 06z Gfs Came In Closer To The Earlier Runs...with
Higher Rain Chances As The Next Shortwave Comes In Sunday Night.
While The 00z Gfs Trended Drier...the 00z Ecmwf Trended Wetter...
Backing Off On The Ridging It Was Calling For Sunday And Monday. In
Fact...both The 00z Ecmwf And The 06z Gfs Are Much Closer To What
The Ukmet Has Been Saying For Quite Some Time Now...with A Very
Strong A Cold Upper Low Parked Over The Pacific Northwest And Cold
Onshore Flow Across California. If This Verifies...expect The
Unsettled Weather Pattern To Continue Through Wednesday...and
Temperatures Below Normal As Well. All In All...a Marked Change
From What We Have Grown Accustomed To Over The Past Couple Weeks.

&& Anticipated Local Patches Of Dense Fog Have Developed
Over The Last Few Hours In Area Such As Santa Rosa...watsonville...
Salinas...and Will Develop Shortly Here In Monterey. Expect Fog To
Burn Off Relatively Early In Most Location (check Taf Package For
Timing). Also Of Note Is The Thin Layer Of Stratus Hugging The
Coast. Expect Coastal Stratus To Persist Through The Day. Rain
Accompanied By Southeast Winds Will Affect Our Area Friday.


.mtr Watches/warnings/advisories...none

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