Gale Warning 1, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
904 AM PST SAT JAN 3 2015

.SYNOPSIS...AN UPPER LEVEL RIDGE WILL REMAIN JUST OFFSHORE TODAY FOR
DRY BUT MOSTLY CLOUDY WEATHER. A STRONG WARM FRONT WILL BRING RAIN
TO WESTERN WASHINGTON LATE TONIGHT THROUGH MONDAY NIGHT...WITH HEAVY
RAIN AT TIMES SUNDAY NIGHT AND MONDAY. AN UPPER LEVEL RIDGE WILL
MOVE ACROSS THE REGION TUESDAY AND WEDNESDAY FOR MAINLY DRY WEATHER.
A COLD FRONT WILL PROBABLY MOVE INTO THE AREA THURSDAY OR FRIDAY.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS THE WEAKENING OFFSHORE RIDGE
NEAR 135W THIS MORNING WITH LOTS OF MOISTURE FLOWING THROUGH THE
RIDGE...AND HEADING TOWARD W WA IN NW FLOW ALOFT. THE UPPER LEVEL
RIDGE WILL SLIDE E OVER 130W THIS AFTERNOON AND FLATTEN FURTHER AS
THE TIGHT LITTLE UPPER LEVEL LOW NEAR 55N/145W MOVES E ACROSS THE
TOP OF THE RIDGE.

THE ABOVE MENTIONED LOW IS ACTUALLY A KEY FEATURE IN THE DEVELOPMENT
OF THE STRONG WARM FRONT EXPECTED TO RAIN ON THE AREA TONIGHT
THROUGH MONDAY. IR IMAGERY SHOWS VERY MOIST SOUTHERN BRANCH AIR
MOVING N INTO THE RIDGE THIS MORNING. THIS IS THE WARM AIR AND
MOISTURE SUPPLY FOR THE DEVELOPING WARM FRONT. THE UPPER LEVEL LOW
WILL SUPPLY THE DYNAMICS...WITH NORTHERN BRANCH COLD AIR COMING S
WITH AND BEHIND THE LOW AS IT MOVE ACROSS N B.C. ON SUNDAY. THE
RESULTING TEMPERATURE GRADIENTS SPIN UP A 150KT 300 MB JET STREAK S
OF THE UPPER LOW...AND 40-60 KT W WINDS AT 850 MB ACROSS W WA IN
THE WARM FRONT HEADING FOR W WA. OK...GOT THAT OUT OF MY SYSTEM.

BOTTOM LINE IS THE FORECAST HAS NOT CHANGED MUCH. RAIN WILL DEVELOP
THIS EVENING OVER THE COAST AND N THEN SPREAD ACROSS ALL W WA
OVERNIGHT. SNOW LEVELS WILL BE INITIALLY LOW ENOUGH OVER THE
MOUNTAINS FOR A LITTLE SNOW ON ALL THE PASSES. WEAK FRASER VALLEY
OUTFLOW WILL ALSO DEVELOP TONIGHT...KEEPING COLD AIR IN PLACE OVER
WHATCOM COUNTY. WRF-GFS MODEL KEEPS SNOW LEVELS DOWN AROUND 600 FEET
OVER WHATCOM COUNTY SO SOME RAIN AND SNOW WILL FALL LATE TONIGHT AND
DURING THE MORNING HOURS. LOCAL ACCUMULATIONS UP TO AN INCH ARE
POSSIBLE...BUT MAINLY OVER INTERIOR WHATCOM COUNTY. PRECIP WILL BE
CRANKING UP ON SUNDAY OVER THE MOUNTAINS WHILE SNOW LEVELS ARE STILL
LOW ENOUGH FOR SIGNIFICANT SNOW ACCUMULATIONS ON THE MAIN CASCADE
HIGHWAYS AND PASSES. SIX TO 11 INCHES OF SNOW ACCUMULATION IS
STILL EXPECTED BEFORE SNOW LEVELS RISE ABOVE CRITICAL HIGHWAY LEVELS
SUNDAY NIGHT.

THE HEAVIEST RAIN IS STILL EXPECTED SUNDAY NIGHT THROUGH MONDAY
MORNING...THEN GRADUALLY DIMINISH MONDAY AFTERNOON AND EVENING.
THIS IS THE PERIOD WHEN 40-60 KT W WINDS DEVELOP AT 850 MB...AND
SNOW LEVELS RISE TO 6000-8000 FEET. STORM TOTAL PRECIPITATION FOR
THE 48 HOUR PERIOD ENDING 10 PM MONDAY EVENING WILL BE 3 TO 7 INCHES
OVER MUCH OF THE CASCADES AND 2 TO 6 INCHES OVER THE OLYMPICS. THIS
IS ENOUGH TO CAUSE FLOODING ON SOME CASCADE AND POSSIBLY OLYMPIC
RIVERS. SEE THE HYDRO SECTION BELOW FOR MORE DETAIL.

THE OFFSHORE RIDGE WILL BUILD MORE ON MONDAY...FINALLY PUSHING THE
WARM FRONT N OF W WA MONDAY NIGHT. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...PRECIPITATION
WILL TAPER OFF MONDAY NIGHT AS THE WARM FRONT LIFTS NORTH INTO
BRITISH COLUMBIA. AN UPPER RIDGE WILL MOVE SLOWLY INLAND TUESDAY AND
WEDNESDAY. MODELS DISAGREE ON HOW DIRTY THE RIDGE WILL BE...BUT THE
WEATHER WILL BE DRY OR MOSTLY DRY AND ANY RAIN WILL BE
HYDROLOGICALLY INSIGNIFICANT. MODELS DISAGREE A BIT MORE LATER IN
THE WEEK...BUT THERE WILL PROBABLY BE A WEAK COLD FRONT MOVING INTO
WESTERN WASHINGTON.   MCDONNAL

&&

.HYDROLOGY...A WARM FRONT WILL BRING SIGNIFICANT PRECIPITATION
AMOUNTS TO WESTERN WASHINGTON LATE TONIGHT THROUGH MONDAY NIGHT.
DEEP MOISTURE AND RISING SNOW LEVELS WILL COMBINE WITH WESTERLY FLOW
OF 40 TO 60 KT AT 850 MB...PRODUCING PERIODS OF HEAVY PRECIPITATION
ESPECIALLY ALONG THE WEST SLOPES OF THE CASCADES AND OLYMPICS. THE
HEAVIEST PRECIPITATION WILL OCCUR SUNDAY NIGHT AND MONDAY MORNING AS
THE SNOW LEVEL RISES TO 6000 TO 8000 FT. FOR THE 48 HOUR PERIOD
ENDING 10 PM MONDAY EVENING...WE CAN EXPECT 3 TO 7 INCHES OF
PRECIPITATION IN THE CASCADES AND 2 TO 6 INCHES IN THE OLYMPICS. THE
LATEST EARLY MORNING QPF FORECAST PUSHED THE RIVER FORECASTS A
LITTLE HIGHER. WE WILL SEE HOW THE 12Z MODEL QPF AFFECTS THE RIVER
MODELS. CONFIDENCE IS BECOMING STRONGER IN FLOODING OCCURRING...BUT
REMAINS LOW TO MODERATE REGARDING HOW MUCH RAIN WILL FALL.

MUCH OF THIS WILL FALL AS RAIN...AND IT COULD DRIVE SOME RIVERS TO
FLOOD STAGE. A FLOOD WATCH WAS ISSUED WITH THE EARLY MORNING
FORECAST PACKAGE THAT INCLUDES ALL THE COUNTIES WITH RIVERS FLOWING
OFF THE WEST SLOPES OF THE CASCADES PLUS MASON COUNTY FOR THE
SKOKOMISH RIVER. IT IS TOO EARLY TO SAY WHERE THE HEAVIEST RAIN WILL
FALL IN THE CASCADES...BUT THE RIVERS FACING THE GREATEST THREAT OF
FLOODING WILL PROBABLY INCLUDE THE
STILLAGUAMISH...SKYKOMISH...SNOQUALMIE... TOLT...SNOHOMISH...THE
REACH OF THE PUYALLUP NEAR ORTING...AND THE UPPER COWLITZ. THE
THREAT FOR THE NOOKSACK AND SKAGIT RIVERS MIGHT BE A BIT LOWER --
DUE TO A LOWER SNOW LEVEL AND PERHAPS LESS RAIN -- BUT THEY CANNOT
BE RULED OUT...AND WHATCOM AND SKAGIT COUNTIES WILL BE INCLUDED IN
THE WATCH.

AN EPISODE OF WIDESPREAD MAJOR FLOODING LOOKS UNLIKELY...BUT SOME OF
THE MOST FLOOD-PRONE RIVERS COULD HAVE MINOR TO MODERATE FLOODING.
FLOODING COULD BEGIN AS EARLY AS LATE SUNDAY NIGHT OR MONDAY
MORNING...WITH RIVERS PROBABLY CRESTING LATE MONDAY AND MONDAY
NIGHT.   MCDONNAL/KAM

&&

.AVIATION...WEAK HIGH PRESSURE ALOFT OVER THE PAC NW WILL WEAKEN
TONIGHT. LIGHT NW FLOW ALOFT BECOMING MODERATE TO STRONG SW FLOW
ALOFT LATE TONIGHT AND SUNDAY. STABLE CONDITIONS. MOIST IN THE LOW
LEVELS AND SOMEWHAT MOIST ALOFT...BECOMING MOIST AT ALL LEVELS AS
RAIN INCREASES WITH A WARM FRONT EARLY SUNDAY.

CIGS WILL REMAIN HIGHLY VARIABLE THIS MORNING BUT PREDOMINATELY
MVFR. LOCAL IFR WITH CIGS BELOW 1SM THROUGH 17-18Z MAINLY SW
INTERIOR AND COAST...THEN IMPROVING. STRATUS MAINLY ABOVE 3K FT THIS
AFTERNOON.

KSEA...SE WIND 4-7 KT. CIGS VARIABLE MAINLY BETWEEN 2500-3500
FEET...THEN IMPROVING TO 3500-4500 FT THIS AFTERNOON. DTM

&&

.MARINE...LIGHT OFFSHORE GRADIENTS TODAY AND THIS EVENING WITH WINDS
BELOW 15 KT THROUGHOUT THE WATERS. A WARM FRONT WILL ARRIVE LATE
TONIGHT SUNDAY MORNING WITH SMALL CRAFT WINDS LIKELY TO DEVELOP OVER
ALL OF THE WATERS AND GALES POSSIBLE OVER THE COASTAL WATERS. A GALE
WATCH FOR THE COASTAL WATERS WAS POSTED FOR SUNDAY. SWLY WINDS WILL
BECOME STRONGEST IN PUGET SOUND LATER SUNDAY NIGHT AND COULD REACH
THE 20-30 KT RANGE AS A SURFACE LOW FORMS NE OF THE OLYMPICS. WINDS
WILL BEGIN TO EASE MONDAY AS THE FRONT BEGINS TO LIFT NORTH AND WILL
CONTINUE TO EASE MONDAY AND MONDAY NIGHT AS THE FRONT MOVES INTO
SOUTHERN BRITISH COLUMBIA. HIGH PRESSURE WILL REBUILD ON TUESDAY
WITH THE LOW LEVEL FLOW GOING OFFSHORE. FELTON/DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH FOR MUCH OF WESTERN WASHINGTON.
PZ...GALE WATCH COASTAL WATERS ON SUNDAY.

&&

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