Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181102
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
302 AM PST THU DEC 18 2014

.SYNOPSIS...AN APPROACHING FRONTAL SYSTEM WILL BRING A CHANCE OF
RAIN TODAY...FOLLOWED BY MORE CONTINUOUS RAIN TONIGHT AS THE COLD
FRONT MOVES INLAND. AN UPPER LEVEL TROUGH WILL BRING SCATTERED
SHOWERS ON FRIDAY. STRONG MOIST WEST FLOW ALOFT OVER THE NORTHEAST
PACIFIC WILL BRING A WINDY AND RAINY PATTERN THIS WEEKEND. A BROAD
UPPER LEVEL RIDGE WILL REMAIN OVER THE REGION EARLY NEXT WEEK FOR
NOT QUITE AS WET WEATHER.

&&

.SHORT TERM...A FRONTAL SYSTEM OFFSHORE APPROACHING 130W WILL BRING
A CHANCE OF RAIN TODAY...AND WILL SPREAD MORE CONTINUOUS RAIN OVER
THE AREA LATER THIS AFTERNOON. THE COLD FRONT DOES NOT LOOK ALL THAT
STRONG ON IR IMAGERY SO CHANCE POPS LOOK FINE FOR THIS MORNING AND
PROBABLY MUCH OF THE AFTERNOON. THE WETTEST PERIOD WILL BE TONIGHT
AS THE COLD FRONT CROSSES W WA. SNOW LEVELS ARE JUST LOW ENOUGH SO
THAT THE HIGHER ROADS TO MT BAKER AND MT RAINIER MAY RECEIVE
BORDERLINE SNOW ADVISORY AMOUNTS. STEVENS PASS MAY GET A FEW INCHES
OF SNOW LATE TONIGHT BUT SNOQUALMIE WILL PROBABLY JUST SEE RAIN.

THE TRAILING UPPER LEVEL TROUGH NEAR 140W-145W AT 09Z WILL CROSS W
WA ON FRIDAY. IR IMAGERY SHOWS THE AIR MASS IN THE TROUGH TO BE
NICELY UNSTABLE...WHICH WILL BE GOOD FOR SHOWER DEVELOPMENT AS IT
MOVES INLAND. LIKELY POPS FOR FRIDAY MORNING DECREASING TO CHANCE
POPS IN THE AFTERNOON AS THE TROUGH MOVES EAST LOOK FINE. LIFTED
INDICES ARE MARGINAL FOR THUNDERSTORMS OVER THE COASTAL WATERS. THE
LAND AREAS ARE COOL ENOUGH SO THAT THUNDER IS UNLIKELY.

VERY STRONG ZONAL FLOW DEVELOPS ACROSS THE NE PACIFIC FRIDAY NIGHT
AND CONTINUES THROUGH THE WEEKEND. 300 MB ISOTACHS ARE IN THE
160-170KT RANGE. A STRONG WET WARM FRONT ASSOCIATED WITH THE 130KT
NOSE OF THE JET STREAK WILL MOVE ACROSS W WA ON SATURDAY. MODELS ARE
IN FAIR AGREEMENT WITH THE STRENGTH AND GENERAL TIMING OF THIS
FEATURE. THIS WARM FRONT SHOULD GENERATE WIND ADVISORY LEVEL WINDS
OVER THE COAST AND N INTERIOR AND HEAVY RAIN OVER THE COAST AND
MOUNTAINS.

ALTHOUGH THERE WILL BE 2-3 INCHES OF HEAVY RAIN DURING THE 24 HOUR
PERIOD FROM 12Z SATURDAY-12Z SUNDAY...SO FAR IT LOOKS LIKE PRIMARILY
THE SKOKOMISH RIVER MAY BE AT RISK FOR FLOODING.

SNOW LEVELS START OUT NEAR 3000 FEET ON SATURDAY...AND WILL RISE TO
5500 TO 7000 FEET BY SATURDAY NIGHT. THERE COULD BE ENOUGH SNOW IN
THE TRANSITION TO PRODUCE BORDERLINE SNOW ADVISORY AMOUNTS OVER MT
BAKER AND MT RAINIER AGAIN. KAM

.LONG TERM...ECMWF AND GFS SHOW STRONG ZONAL FLOW CONTINUING INTO
EARLY NEXT WEEK WITH A BROAD FLAT UPPER LEVEL RIDGE OVER THE REGION.
LOTS OF FLOW THROUGH THE RIDGE WILL KEEP THIS A GENERALLY WET AND
BREEZY PERIOD THROUGH SUNDAY. THE MAIN PROBLEM WILL REMAIN HEAVY
RAIN SATURDAY NIGHT. FAIRLY STRONG WESTERLY 850 MB FLOW WILL REMAIN
30-45KT SATURDAY NIGHT INTO SUNDAY MORNING...THEN GRADUALLY DIMINISH
OVER THE NORTH AND CENTRAL PART OF W WA SUNDAY AFTERNOON. STRONGER W
FLOW WILL CONTINUE OVER NW OREGON AND THE SOUTHERN WA CASCADES.
THERE COULD BE ANOTHER 2-3 INCHES OF QPF OVER THE CENTRAL CASCADES
12Z SUNDAY THROUGH 12Z MONDAY WHICH COULD BRING SOME CASCADE RIVERS
CLOSE TO FLOOD STAGE.

THE BROAD FLAT RIDGE MAINTAINS STRONG SOMEWHAT MOIST W FLOW ALOFT
OVER THE AREA MONDAY AND TUESDAY...BUT THE AIR MASS IS DRIER FOR
MUCH LESS RAIN. GFS/ECMWF BOTH BRING AN UPPER LEVEL SHORTWAVE TROUGH
ACROSS THE AREA ON WEDNESDAY...THEN DEVELOP COOL DRY N FLOW ALOFT
BEHIND IT FOR THURSDAY. WE WILL SEE IF THIS MODEL AGREEMENT HOLDS.
KAM

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED THROUGH FRIDAY NIGHT. A
MOIST WESTERLY FLOW PATTERN WILL DEVELOP SATURDAY AND CONTINUE
THROUGH SUNDAY NIGHT WITH HEAVY RAIN AT TIMES IN THE MOUNTAINS. A
STRONG WARM FRONT WILL CROSS THE AREA SATURDAY WITH STRONG S FLOW
OVER THE OLYMPICS BECOMING W BY LATE AFTERNOON. THE STRONG W FLOW
WILL REACH THE CASCADES SATURDAY NIGHT. 2-3 INCHES OF RAIN ARE
EXPECTED OVER THE OLYMPICS AND PARTS OF THE CASCADES DURING THE 24
HOUR PERIOD FROM 12Z SATURDAY THROUGH 12Z SUNDAY. THIS COULD CAUSE
ANOTHER ROUND OF FLOODING ON THE SKOKOMISH RIVER AND PRIME SOME OF
THE CASCADE RIVERS.

LOW LEVEL W FLOW ALOFT OVER W WA WILL DIMINISH SUNDAY AND SUNDAY
NIGHT AS THE CORE OF STRONGEST WINDS SLIDES DOWN OVER NW OREGON AND
THE SOUTHERN WA CASCADES. STILL...ANOTHER 2-3 INCHES OF RAIN COULD
FALL OVER THE CENTRAL CASCADES BETWEEN 12Z SUNDAY AND 12Z MONDAY.
THIS ADDED RAINFALL COULD PUSH SOME OF THE MORE FLOOD PRONE CASCADE
RIVERS CLOSE TO FLOOD STAGE. KAM

&&

.AVIATION...A WEAK UPPER LEVEL TROUGH IS MOVING EAST OF THE CASCADES
EARLY THIS MORNING WITH LIGHT WESTERLY FLOW ALOFT. THE AIR MASS IS
MOIST AND STABLE WITH WEAK SURFACE GRADIENTS. A COLD FRONT ABOUT 500
NM WEST OF SEATTLE AT 09Z WILL PUSH INTO THE REGION THIS EVENING
WITH STRONG SOUTHWEST FLOW ALOFT.

AREAS OF LIGHT RAIN REMAIN ACROSS THE REGION THIS MORNING WITH
WIDESPREAD LOW MVFR AND AREAS OF IFR. SOME TEMPORARY IMPROVEMENT TO
VFR IS POSSIBLE MIDDAY TODAY AS A TRANSITORY UPPER RIDGE MOVES OVER
THE AREA AND SURFACE GRADIENTS BECOME INCREASINGLY OFFSHORE.
WIDESPREAD RAIN WILL RAPIDLY SPREAD BACK INTO THE AREA BY EARLY
EVENING AHEAD OF THE NEXT FRONT WITH CEILINGS FALLING BACK TO MVFR.

KSEA...LOW MVFR AND TEMPO IFR CEILINGS EXPECTED THIS MORNING WITH
LIGHT RAIN SHOWERS AND SURFACE VSBYS OF 3-5SM AT TIMES. VFR CEILINGS
AT OR ABOVE 5K FEET POSSIBLE 18Z-00Z BUT WILL DROP AGAIN WITH MORE
RAIN THIS EVENING. LIGHT SOUTH-SOUTHEAST WIND THIS
MORNING...BECOMING SOUTHEAST 7 TO 12 KNOTS IN THE AFTERNOON AND
EVENING. 27

&&

.MARINE...A FRONT ASSOCIATED WITH A 970-975 MB LOW WELL WEST OF
HAIDA GWAII WILL MOVE THROUGH THE OFFSHORE WATERS TODAY...THEN
INLAND TONIGHT. A PERIOD OF GALE FORCE WINDS CAN BE EXPECTED OVER
THE COASTAL WATERS MIDDAY TODAY THROUGH THIS EVENING WITH SMALL
CRAFT ADVISORY CONDITIONS FOR THE STRAIT...NORTHERN INLAND
WATERS...AND ADMIRALTY INLET. SOME MESOSCALE MODELS INDICATE A BRIEF
PERIOD OF GALE FORCE WIND POSSIBLE AT THE EAST ENTRANCE TO THE
STRAIT AND THE VICINITY OF THE SAN JUANS THIS EVENING JUST AHEAD OF
THE FRONT. INCREASING SOUTHERLY GRADIENT BEHIND THE FRONT WILL BRING
SMALL CRAFT ADVISORY WINDS TO PUGET SOUND AND HOOD CANAL TONIGHT. A
LONG PERIOD SWELL NEAR 22 FEET IS STILL EXPECTED ON THE COAST LATE
FRIDAY AFTERNOON OR FRIDAY NIGHT. TIDAL ANOMALIES ALONG THE COAST
ARE JUST OVER 1 FOOT ABOVE NORMAL AND HIGH TIDE OCCURS SATURDAY
MORNING. IF SWELL REMAINS HIGH INTO SATURDAY MORNING AS CURRENTLY
EXPECTED...SOME COASTAL FLOODING IS POSSIBLE. ANOTHER STRONG FRONT
WILL ARRIVE ON SATURDAY WITH GALE FORCE WINDS ONCE AGAIN LIKELY OVER
THE COASTAL WATERS...THE NORTHERN INLAND WATERS...AND THE STRAIT
ENTRANCES. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING THURSDAY AND THURSDAY NIGHT COASTAL WATERS.

&&

$$

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







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