Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 172249
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST WED DEC 17 2014

.SYNOPSIS...A WEAK FRONT WILL DISSIPATE OVER THE REGION TONIGHT AND
EARLY THURSDAY. THE NEXT STRONGER PACIFIC FRONTAL SYSTEM WILL MOVE
ASHORE THURSDAY NIGHT WITH MORE SIGNIFICANT RAIN AND SOME MOUNTAIN
SNOW. ANOTHER WET WEATHER SYSTEM IS FORECAST FOR THIS WEEKEND WITH
HEAVIER RAIN AMOUNTS ALONG THE COAST AND IN THE MOUNTAINS. COOLER
AND MORE SHOWERY WEATHER MAY DEVELOP BY THE MIDDLE OF NEXT WEEK.

&&

.SHORT TERM...MOIST SW FLOW ALOFT CONTINUES OVER THE PAC NW AHEAD OF
AN APPROACHING SPLIT NRN STREAM TROUGH. PRECIPITATION WAS LIGHT
TODAY WITH MOST SPOTS RECEIVING LESS THAN ONE TENTH OF AN INCH. THE
NEXT BATCH OF RAIN WILL ALSO BE LIGHT THIS EVENING WITH JUST MINOR
MEASURABLE AMOUNTS FOR MOST PLACES. SATELLITE IMAGERY SHOWS MORE
PRONOUNCED SPLITTING OF THE FRONTAL BAND STARTING TO MOVE INLAND.
HRRR AND NAM12 MODELS STILL KEEP LIGHT RAIN AROUND THE AREA...AT
LEAST THROUGH THE EVENING HOURS BEFORE DECREASING. WILL MAINTAIN THE
CHANCE POPS WITH MORE UNCERTAINTY THAN USUAL FOR THE FIRST PERIOD
FORECAST.

LIGHT SHOWERS MAY LINGER INTO THURSDAY MORNING BUT AN UPPER LEVEL
RIDGE WILL BE MOVING ACROSS WRN WA DURING THE DAY. THE NAM12 SHOWS
SOME LIGHT SPOTTY QPF OVER THE AREA UNTIL THE NEXT STRONGER FRONTAL
SYSTEM ARRIVES LATER IN THE DAY. CONSIDERATION WAS GIVEN TO DRYING
OUT THE MORNING PERIOD FOR INLAND AREAS BUT GIVEN THE NAM12...SOME
LIGHT PRECIPITATION CANNOT BE RULED OUT AND CHANCE POPS WILL BE
MAINTAINED.

MODELS AGREE THAT RAIN WILL DEVELOP ON THE COAST LATE IN THE DAY
WITH A WEAK WARM FRONTAL FEATURE. THE TRAILING COLD FRONT WILL
QUICKLY FOLLOW...PUSHING THROUGH WRN WA THURSDAY NIGHT. THIS SYSTEM
WILL BRING MORE RAIN THAN PREVIOUS SYSTEMS AND ALSO SOME COOLER AIR
WITH LOWERING SNOW LEVELS. A FEW INCHES OF SNOW ARE STILL EXPECTED
IN THE MOUNTAINS ABOVE ABOVE 4000 FEET ONCE THE SNOW LEVELS DROPS
OFF THURSDAY NIGHT. ABOVE 5500 FEET...UP TO 6 INCHES COULD
FALL...MAINLY ALONG S/SW FACING SLOPES OF MOUNT BAKER AND MOUNT
RAINIER.

RAIN WILL CHANGE OVER TO SHOWERS IN THE LOWLANDS FRIDAY AND SNOW
WILL TAPER OFF IN THE MOUNTAINS. A LULL IN PRECIPITATION IS EXPECTED
BETWEEN STORM SYSTEMS POSSIBLY THROUGH MUCH OF FRIDAY NIGHT BUT
MODELS DIFFER SLIGHTLY ON THE TIMING OF THE NEXT FRONT.

MODELS HOW A STRONG WARM FRONT TO SPREAD RAIN INTO WRN WA BY
SATURDAY MORNING. A MOIST WESTERLY FLOW PATTERN WILL BE DIRECTED
INTO WRN WA/WRN OREGON BY SATURDAY NIGHT AND SUNDAY. THE 12Z ECMWF
HAS TRENDED TOWARD THE PAST 2-3 GFS40 RUNS SAGGING THE MOISTURE PLUM
SWD ON SUNDAY AFTERNOON. THIS WOULD SHIFT HEAVIER MOUNTAIN RAIN INTO
THE CASCADES OF SRN WA/AND OREGON WITH LIGHTER FURTHER N OVER THE
CENTRAL/N WA CASCADES. WHILE MODELS HAVE TRENDED TOWARD LESS
PRECIPITATION OVER THE WA CASCADES THE MODELS HAVE YET TO LATCH ON
TO ONE PARTICULAR SOLUTION SO UNCERTAINTY REMAINS HIGH ON WHERE THE
HEAVIEST RAIN WILL FALL. SEE THE HYDRO DISCUSSION BELOW FOR FURTHER
DETAILS.

.LONG TERM...MODELS NOW SHOW A BREAK OR AT LEAST DECREASE IN
RAINFALL RATES SUNDAY NIGHT AS WEAK HIGH PRESSURE TEMPORARILY BUILDS
OFFSHORE. THE RIDGE WILL LIFT THE FLOW NWD SO MOISTURE DIRECTED INTO
SRN WA/OREGON WILL LIFT BACK OVER WRN WA ON MONDAY. THE RIDGING IS
QUITE A BIT STRONGER ON MOST OF TODAY`S MODEL RUNS WHICH WOULD SHUT
OFF PRECIPITATION BY LATE MONDAY OR MONDAY NIGHT. THE GOOD NEWS IS
THAT IF HEAVY RAIN DOES DEVELOP IT APPEARS THERE WILL BE SOME
SHIFTING BETWEEN THE CASCADES OF WRN WA/WRN OREGON. THIS SHOULD
LIMIT POTENTIAL QPF IN ANY ONE PARTICULAR AREA. MODELS NOW SHOW 24
TO 36 HOUR QPF AMOUNTS OF 4 TO 6 INCHES MORE EVENLY DISTRIBUTED
ALONG THE WA/OREGON CASCADES. THIS IS STILL ENOUGH FOR SOME MINOR
FLOODING ON THE MORE PRONE RIVERS SO FORECASTS SHOULD BE MONITORED
CLOSELY.

ANOTHER SURFACE LOW WILL TRACK INTO WRN WA OR NRN OREGON AROUND
TUESDAY. MODELS NO LONGER TAP THIS LOW FOR A CONTINUOUS MOISTURE
STREAM WHICH INSTEAD ARRIVES AFTER A BREAK IN RAINFALL. MODELS ARE
ALSO TRENDING MUCH WEAKER WITH THIS LOW SO ADDITIONAL RAIN AMOUNTS
LOOK TO BE LIGHT AND NOT HYDROLOGICALLY SIGNIFICANT.

LONG RANGE MODELS ARE INDICATING A POSSIBLE PATTERN CHANGE BY THE
MIDDLE OF NEXT WEEK. HOWEVER...MODELS DIFFER ON THE POSITIONING OF A
HIGH OVER KONA LOW PATTERN OVER THE PACIFIC WHICH WILL DETERMINE THE
STRENGTH OF NLY FLOW OVER WRN WA. THE 12/18Z GFS IS FURTHER E WITH
THE LOW AND DUMPS MOST THE COLD AIR INTO THE ROCKIES. COLDER AIR
WOULD ARRIVE ACROSS WRN WA IF THE ECMWF SOLUTION VERIFIED WITH THE
LOW FURTHER W. EITHER WAY...A TRANSITION TO COOLER TEMPERATURES IS
LIKELY ENOUGH TO TREND HIGH/LOW TEMPS DOWNWARD A BIT FOR WEDNESDAY.
MERCER

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED THROUGH SATURDAY. A
MOIST WESTERLY FLOW PATTERN WILL DEVELOP SATURDAY NIGHT AND CONTINUE
THROUGH SUNDAY NIGHT WITH HEAVY RAIN AT TIMES IN THE MOUNTAINS. THE
LATEST 12Z GFS/ECMWF ARE FASTER WITH THE TRAILING FRONTAL BAND AND
PUSHES IT SWD INTO OREGON BY MONDAY. THIS COULD HELP MITIGATE ANY
RIVER FLOODING THAT DEVELOPS BUT DOES NOT END THE THREAT AS MODELS
HAVE NOT ALL SETTLED ON A PARTICULAR SOLUTION. 24 TO 36 HOUR RAIN
AMOUNTS ARE STILL AVERAGING ABOUT 4 TO 5 INCHES OVER THE WA CASCADES
WHICH COULD CAUSE FLOODING ON PRONE RIVERS. AT THIS TIME...PRIMARY
CONCERN FOR ANY RIVER RISING ABOVE FLOOD STAGE IS THE SKOKOMISH BUT
ALSO NEED TO KEEP AN EYE ON OTHER CASCADE FLOOD PRONE STREAMS. MERCER

&&

.AVIATION...A STRETCHED OUT AND WEAKENING UPPER LEVEL TROUGH ALONG
THE COAST WILL MOVE ONSHORE TONIGHT. A STRONG PACIFIC FRONT
APPROACHING 140W THIS AFTERNOON WILL APPROACH THE COASTAL WATERS
THURSDAY AFTERNOON. AT THE SURFACE...HIGH PRES E OF THE CASCADES
COMBINED WITH LOWER PRES OVER THE COASTAL WATERS IS GIVING LIGHT
OFFSHORE FLOW. FLOW WILL BECOME LIGHT TO WEAKLY ONSHORE TONIGHT AS
THE TROUGH OFF THE COAST MOVES INLAND. FLOW ALOFT IS LIGHT SWLY AND
IS MOIST AND STABLE.

PACIFIC NW RADARS CONTINUE TO SHOW BANDS OF LIGHT RAIN SHOWERS
EXTENDING FROM THE NORTH INTERIOR SOUTHWESTWARD TO THE NORTHWEST
OREGON COASTAL WATERS. THIS MOISTURE IS MOVING NE AHEAD OF THE
INCOMING WEAKENING TROUGHS OFFSHORE. MOST TERMINALS IN THE INTERIOR
ARE SEEING VFR CEILINGS AND VISIBILITIES DESPITE THE LIGHT RAIN AS A
RESULT OF THE OFFSHORE PRES GRADIENTS. THE EXCEPTIONS ARE ON THE
NORTH COAST WHERE MVFR CONDITIONS ARE OCCURRING AND AT KOLM WHERE
SOME SCT-BKN006 IS PRODUCING OCCASIONAL IFR CONDITIONS THIS
AFTERNOON. WITH PRES GRADIENTS CONTINUING TO RELAX
OVERNIGHT...EXPECT CEILINGS TO BECOME MOSTLY MVFR LATER THIS EVENING
THROUGH THU MORNING. LOCAL IFR CONDITIONS CAN BE EXPECTED IN THE S
INTERIOR LATE TONIGHT INTO THU MORNING. ALBRECHT

KSEA...EXPECT CIGS AROUND 045 THROUGH THIS EVENING WITH PERIODS OF
LIGHT RAIN. CEILINGS WILL FALL TO 015-020 AFTER 05Z THIS EVENING AND
CONTINUE UNTIL 18Z. SOME IMPROVEMENT CAN BE EXPECTED THU AFTERNOON
AS OFFSHORE PRES GRADIENTS INCREASE IN RESPONSE TO THE NEXT
APPROACHING FRONT. LIGHT SE WINDS GENERALLY LESS THAN 6 KT. ALBRECHT

&&

.MARINE...A WEAK TROUGH OF LOW PRES OVER THE COASTAL WATERS WILL
MOVE INLAND OVERNIGHT. EXPECT LIGHT FLOW. SEAS OVER THE COASTAL
WATERS ARE EXPECTED TO DIMINISH TO BELOW 10 FEET LATE THIS
AFTERNOON...SO THE SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS FOR THE
COASTAL WATERS AND THE SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR
BAR WILL BE ALLOWED TO EXPIRE.

A STRONG PACIFIC FRONT ASSOCIATED WITH A 960 MB LOW CENTERED NEAR
50N 156W IS APPROACHING 142W THIS AFTERNOON. THIS FRONT WILL MOVE
THROUGH THE OFFSHORE WATERS ON THU...THEN MOVE INLAND THU NIGHT. A
PERIOD OF GALE FORCE WINDS CAN BE EXPECTED OVER THE COASTAL WATERS
THU AFTERNOON THRU THU NIGHT AHEAD OF AND WITH THE FRONT. THE GALE
WATCH FOR THE COASTAL WATERS WAS UPGRADED TO A GALE WARNING. EXPECT
SMALL CRAFT ADVISORY CONDITIONS LATER THURSDAY OR THURSDAY NIGHT AT
THE ENTRANCES TO THE STRAIT OF JUAN DE FUCA...THE NORTHERN INLAND
WATERS...AND ADMIRALTY INLET.

THE FETCH AREA TO THE SOUTH OF THE LOW NEAR 50N 156W IS MOVING WITH
THE WAVE FIELD IT IS PRODUCING. THE WAVEWATCH MODEL CONTINUES TO
FORECAST SEAS NEAR 45 FT TO THE S OF THIS LOW TONIGHT...AND THESE
WAVES ARE EXPECTED TO DECAY INTO AN ENERGETIC 22 FT AT 20 SEC SWELL
ON THE WASHINGTON COAST LATE FRI AFTERNOON AND FRI NIGHT. AS IT
TURNS OUT...TIDAL ANOMALIES ALONG THE COAST ARE JUST OVER 1 FT ABOVE
NORMAL AND HIGH TIDE OCCURS SAT MORNING. IF SWELL REMAINS HIGH INTO
SAT MORNING AS CURRENTLY EXPECTED...SOME COASTAL FLOODING IS
POSSIBLE ALONG PORTIONS OF THE COASTAL WATERS AROUND HIGH TIDE ON
SATURDAY. IT IS TOO EARLY TO TELL AT THIS POINT AND TOO EARLY TO
ISSUE ANY COASTAL FLOOD WATCHES...BUT INTERESTS ON THE COAST ARE
URGED TO KEEP UP WITH LATEST FORECASTS.

ANOTHER STRONG WARM FRONT WILL ARRIVE ON SAT AND WILL LIKELY RESULT
IN A PERIOD OF GALE FORCE WINDS OVER THE COASTAL WATERS...THE NORTH
INLAND WATERS...AND THE ENTRANCES TO THE STRAIT OF JUAN DE FUCA.
THEN ONSHORE FLOW WILL SLOWLY EASE ON SUN. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS COASTAL WATERS UNTIL 4
      PM PST THIS AFTERNOON.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR UNTIL 4 PM
      PST THIS AFTERNOON.
     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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