Gale Watch 37, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST SAT NOV 17 2012

.SYNOPSIS...WET AND LOCALLY WINDY WEATHER WILL CONTINUE TONIGHT AND
SUNDAY MORNING. TWO VIGOROUS FRONTS WILL MOVE THROUGH WESTERN
WASHINGTON QUICKLY LATE SUNDAY THROUGH MONDAY NIGHT BRINGING HEAVY
RAIN...HIGH MOUNTAIN SNOW...AND VERY WINDY CONDITIONS TO THE COAST
AND NORTH INTERIOR. FRONTS WILL CONTINUE TO MOVE THROUGH THE REGION
TUESDAY THROUGH SATURDAY FOR MORE RAIN...WIND...AND MOUNTAIN SNOW.

&&

.SHORT TERM...THE VERTICALLY STACKED LOW OF A MATURE OCCLUSION IS
TRACKING NORTH TOWARD HAIDA GWAII. ITS NORTH-SOUTH ORIENTED COLD
FRONT IS MOVING SLOWLY EASTWARD ACROSS WESTERN WASHINGTON. IT MOVED
THROUGH THE SEATTLE AREA AROUND 1 PM. RAIN WITH THE FRONT CONTINUES
TO SPREAD ACROSS THE FORECAST AREA...WITH WINDY CONDITIONS
ESPECIALLY AT THE COAST AND IN THE NORTH INTERIOR.

THE FRONT WILL MAKE ITS WAY ACROSS THE CASCADES THIS EVENING...AND
THE UPPER TROUGH WITH THIS SYSTEM WILL MOVE INTO WESTERN WASHINGTON
TONIGHT. IN GENERAL THE TROUGH WILL BRING MORE OF THE SAME...RAIN AT
TIMES AND BREEZY TO WINDY CONDITIONS...AND THAT WILL CONTINUE
THROUGH MIDDAY SUNDAY.

A CONVECTIVE AREA ASSOCIATED WITH THE SHARP VORTICITY NEAR THE BASE
OF THE TROUGH...CURRENTLY NEARING 125W OVER THE OFFSHORE WATERS...
COULD PRODUCE THUNDERSTORMS OVER THE COASTAL WATERS AND COAST ZONES.
IN ADDITION...I WILL CONSIDER A WIND ADVISORY FOR THE COAST WITH THE
ARRIVAL OF THIS FEATURE THIS EVENING. FOR THE MOUNTAINS...THE SNOW
LEVEL WILL FALL TO AROUND 3500 FT TONIGHT AND 3000 FT SUNDAY
MORNING. ABOUT 4 TO 8 INCHES OF SNOW IS LIKELY TONIGHT THROUGH NOON
SUNDAY FOR THE HIGHER MOUNTAIN ROADS LIKE MOUNT BAKER HIGHWAY...
WASHINGTON PASS...AND PARADISE IN MOUNT RAINIER. FOR THE INTERIOR
LOWLANDS...THIS PERIOD WILL BE A RELATIVE BREAK IN THE RAIN ALTHOUGH
A WEAK PUGET SOUND CONVERGENCE ZONE COULD FORM TONIGHT WITH THE
ARRIVAL OF THE UPPER TROUGH AXIS.

A MORE VIGOROUS SYSTEM WILL AFFECT THE AREA LATE SUNDAY AND SUNDAY
NIGHT...AND IT WILL BE FOLLOWED IMMEDIATELY BY ANOTHER STRONG SYSTEM
MONDAY. THIS PERIOD LOOKS TO BE THE MOST THREATENING WITH REGARD TO
HIGH WINDS AT THE COAST AND IN THE NORTHWEST INTERIOR...ENOUGH RAIN
TO PRODUCE A FLOOD ON THE SKOKOMISH RIVER...AND HEAVY SNOW AT THE
HIGHER MOUNTAIN SPOTS WITH A SLOPPY MIX OF PRECIPITATION IN THE
LOWER MOUNTAIN PASSES.

WE WILL ISSUE A HIGH WIND WATCH FOR THE COASTAL ZONES AND NORTH
INTERIOR ZONES FOR NOON SUNDAY THROUGH MIDNIGHT SUNDAY. THIS SHOULD
BE A FAIRLY BRIEF AND MARGINAL EVENT. MORE HIGH WIND IS POSSIBLE
MONDAY AND MONDAY NIGHT WITH THE SECOND FRONT...BUT FOR NOW THE
WATCH WILL JUST ADDRESS THE FIRST SYSTEM.

WE WILL ALSO ISSUE A WINTER STORM WATCH FOR THE NORTH AND CENTRAL
CASCADES ZONES FOR NOON SUNDAY THROUGH MIDNIGHT MONDAY. THE SNOW
LEVEL WILL RISE FROM ABOUT 3000 FT SUNDAY AFTERNOON INTO THE 5000 TO
6000 FT RANGE MONDAY. SNOWFALL AT THE HIGHER MOUNTAIN SPOTS SHOULD
BE IN THE 8 TO 18 INCH RANGE. THE LOWER PASSES LIKE STEVENS...
SNOQUALMIE...AND WHITE SHOULD START AS SNOW AND SWITCH TO RAIN WITH
A PERIOD OF FREEZING RAIN POSSIBLE DURING THE TRANSITION.

REGARDING RAINFALL...UP TO 2.5 INCHES OF RAIN WILL FALL IN THE
LOWLANDS WITH THE HEAVIEST AMOUNTS GENERALLY AT THE COAST AND IN THE
INTERIOR FROM AROUND EVERETT SOUTHWARD. THIS AMOUNT OF RAIN
JUSTIFIES CONCERN FOR LANDSLIDES...AND I WILL MENTION THAT IN OUR
SPECIAL WEATHER STATEMENT WHEN IT IS UPDATED THIS AFTERNOON. AROUND
2 TO 5 INCHES OF RAIN IS LIKELY IN THE MOUNTAINS BELOW THE SNOW
LEVEL. THIS SHOULD CAUSE THE SKOKOMISH TO FLOOD...AS DISCUSSED IN
THE HYDROLOGY SECTION BELOW.

ANOTHER FRONTAL SYSTEM SHOULD ARRIVE TUESDAY. IT WILL CONTINUE THE
ACTIVE PATTERN WITH RAIN AND WIND AND HIGH MOUNTAIN SNOW. AT THIS
TIME THOUGH IT DOES NOT LOOK LIKE A SYSTEM THAT WILL REQUIRE MORE
WARNINGS FOR LAND WINDS OR MOUNTAIN WEATHER.   MCDONNAL

.LONG TERM...ANOTHER GARDEN VARIETY LATE-NOVEMBER FRONT OR SURFACE
TROUGH WILL MOVE THROUGH WESTERN WASHINGTON SOMETIME WEDNESDAY. IT
LOOKS INCREASINGLY LIKELY THAT A WARMER AND WETTER PATTERN WILL
DEVELOP THANKSGIVING OR FRIDAY AND CONTINUE UNTIL AROUND SATURDAY.
TIMING AND OTHER DETAILS DIFFER OF COURSE BUT THE 00Z SAT ECMWF...
12Z SAT GFS...AND NOW THE 12Z SAT ECMWF ALL AGREE IN THE DEVELOPMENT
OF THIS PATTERN. THE MAIN CONCERN HERE IS THE POTENTIAL FOR RIVER
FLOODING.   MCDONNAL

&&

.HYDROLOGY...THE THREAT OF FLOODING IS LOW THROUGH SUNDAY...EVEN FOR
THE UNIQUELY AND EXCEPTIONALLY FLOOD-PRONE SKOKOMISH RIVER.

TWO SYSTEMS WILL MOVE THROUGH WESTERN WASHINGTON IN QUICK SUCCESSION
DURING THE LATE SUNDAY THROUGH MONDAY NIGHT PERIOD. THEY WILL BRING
MORE SIGNIFICANT RAINFALL. OUR QPF HAS A MEAN AREAL PRECIPITATION OF
AROUND 4.5 INCHES OVER THE SKOKOMISH BASIN DURING THAT 36-HOUR
PERIOD...AND THERE IS A GOOD CHANCE THE SKOKOMISH WILL FLOOD.

OTHER WESTERN WASHINGTON RIVERS PROBABLY WILL NOT FLOOD AS A RESULT
OF THE RAINFALL LATE SUNDAY THROUGH MONDAY NIGHT. IT IS NOT THE
CLASSIC FLOOD-PRODUCING WEATHER PATTERN. THE SNOW LEVEL WILL ONLY
RISE TO AROUND 5500 FT...AND THERE IS NO INDICATION THAT THE FLOW
WILL TAP INTO DEEP SUBTROPICAL MOISTURE. HOWEVER A POSSIBILITY OF
MINOR FLOODING CANNOT BE RULED OUT FOR A FEW OF THE MOST FLOOD-PRONE
RIVERS. WE WILL NEED TO KEEP A CLOSE EYE ON AT LEAST THE SATSOP...
NOOKSACK...AND STILLAGUAMISH RIVERS.

ENOUGH RAIN WILL FALL OVER THE WESTERN WASHINGTON LOWLANDS SUNDAY
NIGHT AND MONDAY TO POSE A THREAT OF LANDSLIDES...AND I WILL
HIGHLIGHT THAT IN OUR SPECIAL WEATHER STATEMENT THIS AFTERNOON.

IT NOW LOOKS LIKE A MORE CLASSIC WESTERN WASHINGTON FLOOD PATTERN
COULD DEVELOP AROUND THANKSGIVING OR FRIDAY OF NEXT WEEK. THE 00Z
SAT ECMWF...12Z SAT GFS...AND NOW THE 12Z SAT ECMWF ALL SHOW THIS.
FOR NOW WE ARE IN A PERIOD OF WATCHFUL WAITING TO SEE IF FUTURE
MODEL RUNS STRENGTHEN CONFIDENCE IN THIS SCENARIO. IF IT DOES
OCCUR...IT SHOULD BE NO GREAT SURPRISE. HISTORICALLY SPEAKING...THE
FREQUENCY OF RIVER FLOODING IN WESTERN WASHINGTON REACHES ITS PEAK
DURING THE WEEK OF THANKSGIVING.   MCDONNAL

&&

.AVIATION...AN UPPER TROUGH JUST OFF THE COAST WITH SOUTHERLY FLOW
ALOFT OVER WASHINGTON. THE TROUGH WILL MOVE ONSHORE AND WEAKEN
THROUGH SUNDAY. A COLD FRONT IN THE CASCADES WILL CONTINUE THROUGH
EASTERN WA THROUGH 12Z. AIR MASS MOIST AND UNSTABLE BEHIND THE FRONT.

RAIN WITH MVFR CEILINGS AND VISIBILITIES ASSOCIATED WITH THE COLD
FRONT WILL GIVE WAY TO BREAKS IN THE CLOUD COVER AND MORE VFR
CONDITIONS ALONG THE COAST BY 00Z AND THE INTERIOR 02Z OR SO.
SATELLITE IMAGES SHOW A BAND OF SHOWERS WITH THE UPPER TROUGH THAT
WILL MOVE ONSHORE LATER TONIGHT. THEN THE NEXT PACIFIC FRONTAL
SYSTEM WILL SPREAD MORE RAIN ONSHORE DURING THE DAY SUNDAY WITH
CEILINGS MAINLY MVFR.

KSEA...CEILINGS TO IMPROVE TO VFR ONCE THE BAND OF RAIN WITH THE
COLD FRONT MOVES EAST BY 00Z. THEN SOME HIT AND MISS SHOWERS WITH
VFR CEILINGS THROUGH THE NIGHT. SW WIND AROUND 10 KT TO BECOMING S
THIS EVENING. ANTICIPATE MVFR CEILINGS TO RETURN BY 12Z SUNDAY
AROUND 025. BUEHNER

&&

.MARINE...THE COLD FRONT MOVED INTO THE CASCADES EARLY THIS
AFTERNOON. SOUTHERLY PRESSURE GRADIENTS INCREASED THROUGH THE
INTERIOR WATERS WITH THE FRONT. A BAND OF SHOWERS ASSOCIATED WITH
THE UPPER TROUGH OFFSHORE SHOULD MOVE ONSHORE OVERNIGHT AND WILL
LIKELY HAVE A BRIEF SURGE OF STRONGER SOUTHERLIES WITH IT MAINLY
ALONG THE COAST AND NORTH INTERIOR.

THE NEXT PACIFIC FRONTAL SYSTEM APPROACHES THE REGION SUNDAY WITH
THE POTENTIAL OF A STRONG LOW MOVING INTO CENTRAL VANCOUVER ISLAND
BY SUNDAY EVENING. ANTICIPATE GALE FORCE WINDS ALONG THE COAST AND
IN MOST NORTHERN INTERIOR WATERS WITH THIS SYSTEM. THEN THE NEXT LOW
APPROACHES LATER ON MONDAY FOR ANOTHER BURST OF GALES. LOOKS LIKE AN
ACTIVE WEEK AHEAD AS WELL AS THE PARADE OF PACIFIC STORMS SWING INTO
THE AREA. BUEHNER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH REMAINS IN EFFECT FOR THE SKOKOMISH RIVER IN MASON
     COUNTY FROM SUNDAY AFTERNOON THROUGH TUESDAY AFTERNOON.
    .WIND ADVISORY FOR THE COAST ZONES THIS EVENING.
    .HIGH WIND WATCH FOR THE COAST AND NORTH INTERIOR ZONES SUNDAY
     AFTERNOON AND EVENING.
    .WINTER STORM WATCH FOR THE NORTH AND CENTRAL CASCADES SUNDAY
     AFTERNOON THROUGH MONDAY EVENING.

PZ...GALE WARNING COASTAL WATERS...ENTRANCES TO THE STRAIT OF JUAN
     DE FUCA AND THE NORTHERN INLAND WATERS.
     SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE REMAINING WATERS.

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