Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 261104
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST THU FEB 26 2015

.SYNOPSIS...A WEAK WEATHER SYSTEM WILL PRODUCE SCATTERED SHOWERS
ACROSS WESTERN WASHINGTON TODAY THROUGH FRIDAY...ESPECIALLY AT THE
COAST AND IN THE SOUTHWEST. A FEW INCHES OF SNOW WILL FALL IN THE
MOUNTAINS. AN UPPER RIDGE AND OFFSHORE FLOW WILL BRING SUNNY DAYS
AND COOL NIGHTS THIS WEEKEND. A WEAK SYSTEM WILL MOVE THROUGH SUNDAY
NIGHT AND MONDAY...WITH DRY WEATHER AGAIN TUESDAY AND WEDNESDAY.

&&

.SHORT TERM...NORTHWEST FLOW ALOFT CONTINUES OVER WESTERN WASHINGTON
AHEAD OF A LARGE UPPER RIDGE CENTERED WELL OFFSHORE ALONG 140W. A
WEAK UPPER SHORTWAVE IS JUST OFFSHORE -- INSIDE OF 130W -- AND
MOVING TOWARD THE FORECAST AREA IN THE NORTHWEST FLOW. THE AIR MASS
IS MOIST FROM THE SURFACE TO AROUND 10000 FT...WITH LIGHT SPOTTY
PRECIPITATION AND PATCHY FOG. TEMPERATURES AT 2 AM WERE IN THE 40S.

THE SHORTWAVE OFFSHORE WILL MOVE SOUTHEAST INTO THE PACIFIC
NORTHWEST TODAY...WITH THE CORE OF THE SYSTEM MOVING ACROSS WESTERN
OREGON THIS AFTERNOON. THERE WILL BE SHOWERS ACROSS OUR FORECAST
AREA MAINLY AT THE COAST...IN THE MOUNTAINS...AND ACROSS THE
SOUTHWEST INTERIOR. THE PUGET SOUND REGION AND NORTHWEST INTERIOR
SHOULD HAVE RELATIVELY LITTLE PRECIPITATION TODAY...AND PARTS OF
THOSE REGIONS COULD REMAIN DRY.

AN UPPER LOW CURRENTLY JUST NORTHWEST OF HAIDA GWAII WILL DEEPEN
SOMEWHAT AS IT DIGS SOUTHEAST TODAY...MOVING TO A POSITION NEAR THE
WASHINGTON COAST LATE TONIGHT. IT WILL BRING MORE WIDESPREAD
SHOWERS...WITH POSSIBLY UP TO AN INCH OF PRECIPITATION AND
THUNDERSTORMS IN THE VICINITY OF THE UPPER LOW.

THE UPPER LOW WILL CONTINUE SOUTHEAST INTO SOUTH CENTRAL OREGON ON
FRIDAY...WITH THE FLOW ALOFT OVER WESTERN WASHINGTON VEERING TO
NORTHERLY FRIDAY AND FRIDAY NIGHT. IN ADDITION...NORTHERLY OFFSHORE
FLOW WILL DEVELOP AT THE SURFACE LATE FRIDAY AND FRIDAY NIGHT AS
SURFACE HIGH PRESSURE MOVES SOUTH ACROSS BRITISH COLUMBIA. SHOWERS
SHOULD DECREASE FROM NORTH TO SOUTH AS THE AIR MASS OVER WESTERN
WASHINGTON DRIES LATE FRIDAY AND FRIDAY NIGHT. THERE WILL ALSO BE
SOME BREEZY NORTH OR NORTHEAST WINDS...ESPECIALLY IN THE NORTH
INTERIOR DUE TO FRASER RIVER OUTFLOW.

THE SNOW LEVEL WILL BE IN THE 3000 TO 4000 FT RANGE AS MOST OF THIS
PRECIPITATION OCCURS. THE OLYMPICS AND THE MOUNT RAINIER AREA SHOULD
GET THE HEAVIEST AMOUNTS...WITH UP TO AN INCH OF PRECIPITATION IN
SPOTS. THE CENTRAL AND NORTH CASCADES WILL PROBABLY RECEIVE 0.25 TO
0.75 INCH PRECIPITATION DURING THE PERIOD.

SATURDAY AND SATURDAY NIGHT SHOULD BE CLEAR WITH DRY NORTHEAST FLOW.
LOW LEVEL OFFSHORE FLOW WILL EASE...WHICH WILL ALLOW TEMPERATURES TO
FALL BELOW FREEZING SATURDAY NIGHT IN THE WIND SHELTERED SPOTS.
MCDONNAL

.LONG TERM...SUNDAY SHOULD BE ANOTHER DRY AND SUNNY DAY...BUT THE
UPPER FLOW WILL BACK TO NORTHWEST AS AN UPPER SHORTWAVE STARTS
DIGGING SOUTH ALONG THE COAST OF BRITISH COLUMBIA. THAT SYSTEM WILL
MOVE ACROSS THE FORECAST AREA SUNDAY NIGHT AND MONDAY...PROBABLY
BRINGING AREAS OF LIGHT PRECIPITATION TO WESTERN WASHINGTON. THE
SNOW LEVEL WILL BE AROUND 2000 TO 3000 FT. DRY NORTHERLY FLOW WILL
RESUME TUESDAY AND WEDNESDAY. OF COURSE THERE ARE SOME DIFFERENCES
IN THE LONGER RANGE MODELS...BUT THEY ALL INDICATE THIS SAME BASIC
PATTERN.   MCDONNAL

&&

.HYDROLOGY...FLOODING IS UNLIKELY FOR THE NEXT 7 DAYS.

&&

.AVIATION...NORTHWEST FLOW ALOFT EARLY THIS MORNING WILL BECOME
WESTERLY TODAY...THEN BRIEFLY SOUTHERLY TONIGHT AS AN UPPER LEVEL
LOW WEST OF HAIDA GWAII DROPS SOUTH INTO THE COASTAL WATERS. THE
FLOW ALOFT WILL BECOME NORTHEASTERLY LATE TONIGHT INTO EARLY FRIDAY
AS THE LOW DROPS TO THE SOUTH OF THE AREA. THE AIR MASS IS MOIST AND
STABLE. THE AIR MASS WILL DESTABILIZE SOMEWHAT OVER THE COASTAL
WATERS TONIGHT AS THE UPPER LOW TO THE NORTHWEST DROPS SOUTHWARD.

THIS MORNING...LIGHT NW FLOW ALOFT IS INTERACTING WITH THE VANCOUVER
ISLAND MOUNTAINS AND THE OLYMPICS...PRODUCING BROAD AND WEAK
CONVERGENCE BANDS OVER THE INTERIOR. ONE AREA OF CONVERGENCE
IS SEEN FROM THE SAN JUAN ISLANDS AND BELLINGHAM DOWN TO WHIDBEY
ISLAND NAS AND EVERETT WHILE ANOTHER IS SEEN FROM THE KITSAP
PENINSULA SE TO KSEA THEN ESE TO KSMP. CONDITIONS ARE GENERALLY MVFR
EXCEPT IN THE CONVERGENCE ZONE AREAS WHERE THEY ARE IFR. 06Z NAM12
BUFR SOUNDINGS SHOW ABUNDANT MOISTURE CONTINUING AROUND THE AREA
TODAY. THE PREFERRED HRRR SOLUTIONS SHOW SOME SLOW AND GRADUAL
IMPROVEMENT TO MAINLY MVFR CEILINGS EARLY THIS AFTERNOON.

AFTER ABOUT 22Z...A WEAKENING FRONT WELL OFFSHORE WILL STALL AS IT
MOVES ONTO THE OLYMPIC PENINSULA. THEN LOW PRESSURE OVER HAIDA GWAII
WILL DROP SOUTHWARD DOWN THE WASHINGTON COASTAL WATERS TONIGHT. THE
INTERIOR LOWLANDS WILL LIKELY STAY MVFR TONIGHT AS LOW LEVEL
FLOW BECOMES WEAKLY OFFSHORE. PRECIPITATION WITH THE LOW WILL KEEP
CONDITIONS ALONG THE COAST MAINLY IFR LATE THIS AFTERNOON THROUGH
THIS EVENING. ALBRECHT

KSEA...A CONVERGENCE ZONE NEAR THE TERMINAL WILL KEEP CONDITIONS
P6SM SCT005 OVC015-020 THIS MORNING WITH 3-5SM -RA BR BKN005
POSSIBLE AT TIMES TIL 18Z. CONDITIONS WILL GRADUALLY IMPROVE TO P6SM
VCSH BKN025-030 OVC050 THIS AFTERNOON. IF THE LOW MOVING DOWN FROM
THE NW COMES CLOSER THAN ANTICIPATED TO THE TERMINAL LATE THIS
AFTERNOON THROUGH TONIGHT...PRECIPITATION MAY BE MORE PERSISTENT AND
IFR CONDITIONS PREVAIL TONIGHT. WINDS WILL BE VARIABLE TO 5 KT.
ALBRECHT

&&

.MARINE...A WEAK FRONTAL SYSTEM WILL STALL AS IT MOVES ONTO THE
COAST THIS EVENING. THEN A DIFFUSE AND BROAD 1008 MB LOW WILL DROP
SSE FROM THE QUEEN CHARLOTTE SOUND INTO THE COASTAL WATERS TONIGHT.
PRES GRADIENTS ARE EXPECTED TO REMAIN LIGHT OVER THE WATERS THROUGH
EARLY FRI.

HIGH PRES WILL BUILD OVER THE INTERIOR OF BRITISH COLUMBIA FRI. AS
THE LOW LATE TONIGHT PASSES SOUTH OF THE AREA...EXPECT N TO NE FLOW
TO INCREASE. SMALL CRAFT ADVISORY CONDITIONS ARE LIKELY OVER MOST OF
THE WATERS LATER FRI OR FRI NIGHT INTO SAT. THERE IS A SMALL CHANCE
THAT GALE FORCE WINDS WILL OCCUR AROUND THE WEST ENTRANCE TO THE
STRAIT OF JUAN DE FUCA AND OVER PORTIONS OF THE NORTHERN INLAND
WATERS FRI NIGHT.

HIGH PRES WILL MOVE INTO THE WATERS ON SUN GIVING ANOTHER PERIOD OF
LIGHT WINDS.

ANOTHER LOW WILL DROP SOUTHEAST ACROSS THE AREA SUN NIGHT INTO MON
AND WILL BE FOLLOWED BY STRONG N TO NE PRES GRADIENTS LATER MON OR
MON NIGHT. GLOBAL FORECAST MODELS SHOW OFFSHORE FLOW BEHIND THE
SECOND LOW ON MON OR MON NIGHT BEING STRONGER THAN THE OFFSHORE FLOW
FRI NIGHT INTO SAT...AND GALE CONDITIONS ARE POSSIBLE AROUND THE
WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA AND THE NORTHERN INLAND
WATERS. ALBRECHT

&&

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

&&

$$

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





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