Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 270407
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
909 PM PDT FRI SEP 26 2014

.SYNOPSIS...SHOWERS WILL COME TO AN END THIS EVENING AS AN UPPER
TROUGH OVER THE AREA SHIFTS EAST OF THE CASCADES. WEAK HIGH PRESSURE
WILL BUILD INTO THE REGION OVER THE WEEKEND. A FRONT WILL BRING SOME
RAIN TO THE AREA MONDAY NIGHT. AN UPPER RIDGE MAY BUILD TOWARD THE
END OF NEXT WEEK.


&&

.SHORT TERM...A LINE OF SHOWERS THAT EXTENDED FROM CENTRAL WHATCOM
COUNTY SOUTHWARD TO AROUND TOLEDO IN LEWIS COUNTY AT 8 PM IS
GRADUALLY BREAKING UP AS IT MOVES EAST INTO THE CASCADES. THIS LINE
FORMED EAST OF AN UPPER TROUGH THAT IS SITTING OVER THE OFFSHORE
WATERS THIS EVENING. THE UPPER TROUGH IS EXPECTED TO PASS EAST OF
THE AREA EARLY SATURDAY MORNING. AT THE SURFACE...A RIDGE OF HIGH
PRES IS BUILDING RIGHT OVER THE AREA AND PRES GRADIENTS ARE BECOMING
LIGHT. WEAK TO LIGHT OFFSHORE FLOW EARLY SATURDAY WILL RESULT IN THE
FORMATION OF AREAS OF FOG THAT WILL LIKELY PERSIST INTO MIDDAY.

SHORT TERM MODELS SHOW FOG LIFTING AND SKIES SCATTERING OUT SHORTLY
AFTER NOON ON SATURDAY. THEN A RIDGE OF HIGH PRES ALOFT BUILDS FROM
THE OREGON OFFSHORE WATERS NE INTO SOUTHERN BRITISH COLUMBIA SAT
NIGHT AND SUN. WITH A CONTINUATION OF LIGHT OFFSHORE FLOW...A
DEVELOPING INVERSION AROUND 3000 FT...AND LONGER LATE SEP
NIGHTS...EXPECT AREAS OF FOG AGAIN DURING THE MORNING HOURS ON SUN.

HIGH TEMPERATURES OVER THE WEEKEND WILL GENERALLY BE IN THE MID 60S
TO LOWER 70S...NICE FOR LATE SEPTEMBER. LOWS WILL GENERALLY BE IN
THE 50S TONIGHT AND IN THE MID 40S TO LOWER 50S SUN MORNING.

A QUICK UPDATE WAS ISSUED TO THE GRIDDED AND ZONE FORECASTS THIS
EVENING TO END PRECIPITATION FROM THE WEST...TO INCREASE FOG
COVERAGE TONIGHT INTO SAT MORNING...AND TO MAKE WINDS LIGHT THIS
EVENING. THE REMAINDER OF THE FORECAST IS IN GOOD SHAPE. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...A COLD FRONT
WILL MOVE THROUGH WESTERN WA MONDAY NIGHT. 850MB TEMPS COOL OFF TO
AROUND +4C AS THE UPPER TROUGH MOVES THROUGH THE REGION MONDAY
NIGHT. THE ECMWF HAS A SECOND UPPER SHORTWAVE TROUGH INTO THE REGION
WEDNESDAY WHILE THE GFS KEEPS IT NORTH OF THE AREA. THE FORECAST IS
A LITTLE UP IN THE AIR THROUGH THE MIDDLE OF NEXT WEEK. TOWARD THE
END OF THE WEEK THE GLOBAL MODELS ARE COMING INTO AGREEMENT ON AN
UPPER RIDGE BUILDING OVER THE PACNW.

&&

.AVIATION...WEAK LOW PRESSURE ALOFT OVER THE AREA WILL MOVE OVER
EASTERN WA SATURDAY. LIGHT WLY FLOW ALOFT WILL BECOME NLY ON
SATURDAY. NEARLY FLAT SURFACE FLOW WILL PREVAIL OVERNIGHT INTO
SATURDAY MORNING.

ANTICIPATE AREAS OF CIGS BLO 1K FT AND VSBYS AOB 3SM /DUE TO FOG/
OVERNIGHT. THE FOG/STRATUS WILL PROBABLY BECOME MORE WIDESPREAD
LATE TONIGHT OR EARLY SATURDAY BUT IT WILL DEPEND ON HOW MUCH
CLEARING TAKES PLACE.

KSEA...THE FORECAST WILL BE TRICKY OVERNIGHT. IF THERE IS ANY
CLEARING...FOG AND STRATUS WILL BE LIKELY OVER THE TERMINAL.
IF NOT...VFR CONDITIONS CAN BE ANTICIPATED. FOR NOW...WILL STICK
WITH THE IDEA OF CLEARING LEADING TO FOG LATE TONIGHT OR EARLY
SATURDAY MORNING.

&&

.MARINE...
A 1020 MB HIGH WILL DEVELOP OVER SW BRITISH COLUMBIA OVERNIGHT AND
THEN PERSIST THROUGH SATURDAY MORNING. WEAK HIGH PRES OFFSHORE WITH
LOWER PRES OVER EASTERN WASHINGTON WILL RESULT IN LIGHT ONSHORE FLOW
SATURDAY AFTERNOON THROUGH SUNDAY. A WEAK COLD OR OCCLUDED FRONT
WILL MOVE ACROSS THE AREA ON MON.


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML








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