River Flood Warning 41, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
820 PM PST TUE NOV 11 2008

.SYNOPSIS...A WARM FRONT WILL LIFT NORTH ACROSS THE AREA TONIGHT
WITH RAIN AND WIND INCREASING ACROSS WESTERN WASHINGTON. SNOW LEVELS
WILL RISE ABOVE 8000 FEET AHEAD OF A COLD FRONT WHICH WILL BRING
ADDITIONAL HEAVY RAIN TO THE COAST AND MOUNTAINS ON WEDNESDAY.
WIDESPREAD RIVER FLOODING IS ANTICIPATED ALONG WITH SOME URBAN AND
SMALL STREAM FLOODING...ESPECIALLY OUTSIDE THE CENTRAL AND NORTH
PUGET SOUND AREA WHICH SHOULD EXPERIENCE SOME RAIN SHADOW EFFECTS.
RAIN WILL TAPER TO SHOWERS WEDNESDAY NIGHT FROM THE NORTH. WEAK HIGH
PRESSURE WILL BUILD FRIDAY INTO EARLY NEXT WEEK. HOWEVER..WEAK
WEATHER SYSTEMS WILL BRUSH THE AREA AT TIMES WITH A CHANCE OF LIGHT
RAIN OVER NORTHERN AREAS.

&&

.SHORT TERM...THE MOST SIGNIFICANT WEATHER IMPACT OVER WESTERN
WASHINGTON OVER THE NEXT 24 HOURS OR SO WILL BE HEAVY PRECIPITATION.
SMALL DEVIATIONS IN THE DURATION OF THE PRECIPITATION WILL HAVE A
SIGNIFICANT IMPACT ON THE EXTENT AND SEVERITY OF FLOODING THAT IS
EXPECTED ON AREA RIVERS OVER THE NEXT COUPLE DAYS.

SATELLITE IMAGERY CONTINUES TO SHOW AN IMPRESSIVE AND VERY MOIST
PLUME STREAMING FROM THE PACIFIC INTO THE AREA. THE PLUME EXTENDS
FROM THE WASHINGTON COAST WSW TO AROUND 34N/170W THEN WWD TO ALMOST
JAPAN. THE ORIGIN OF THIS MOISTURE IS TROPICAL CONVECTION FROM THE
SOUTH CHINA SEA AND AN OLD TROPICAL CYCLONE JUST W OF THE PHILIPPINE
ISLANDS. THE MOISTURE IS ALIGNED WELL WITH A STRONG 180+ KT 300 MB
JET STREAM...PRECIPITABLE WATER AMOUNTS RANGE FROM 1.5 TO 2.0 INCHES
IN THE TROPICAL MOISTURE PLUME...AND RAINFALL RATES PER SSMI AND
NOAA POLAR SATELLITES ARE UPWARD OF 0.45 INCHES PER HOUR AS REPORTED
BY THE NESDIS SATELLITE ANALYSIS BRANCH. OROGRAPHIC EFFECTS WILL
MAGNIFY THESE HOURLY AMOUNTS LOCALLY BY A FACTOR OF 2 OR SO IN THE
MOUNTAINS AS A WSW WIND 60-70KT AT 5000 FT SLAMS INTO THE MOUNTAINS.

18Z MODEL SOLUTIONS ARE A BIT FASTER IN BRINGING THE HIGH RAIN
RATES INTO THE OLYMPIC MOUNTAINS AND FAR NORTH CASCADES LATE TONIGHT
THAN THE EARLIER 12Z SOLUTIONS. BUT THE 18Z MODELS ARE GENERALLY
CONSISTENT IN BRINING THE HEAVIEST PRECIPITATION AMOUNTS OVER ABOUT
A 12 HOUR WINDOW FROM 12Z WED TO 00Z THU AS WHAT APPEARS TO BE A
WAVE BACK BETWEEN 135W AND 145W PUSHES ACROSS THE AREA. THE INCOMING
00Z SOLUTIONS ARE SLIGHTLY FASTER IN TAKING THE PRECIPITATION SOUTH
THROUGH THE AREA THAN THE 18Z RUNS...BUT LATEST MM5GFS PRECIPITATION
FORECASTS ARE IN LINE WITH CURRENT LOCAL AND RIVER FORECAST CENTER
QPF FORECASTS THAT WERE USED TO FORCE THE RIVERS. WILL NOT ISSUE AN
UPDATE TO THE QPF FORECASTS OR GRIDS THIS EVENING.

RAINFALL TOTALS UPWARD OF 10 INCHES ARE EXPECTED DURING A 12-HR
WINDOW IN THE MOUNTAINS WHILE THE LOWLANDS SEE 3-6 INCH TOTALS ALONG
THE COAST...2-5 INCH AMOUNTS IN THE SOUTH INTERIOR...1-3 INCH TOTALS
IN THE NORTH INTERIOR A FEW MILES E OF INTERSTATE 5...AND LESS THAN
2 INCHES IN AREAS SHADOWED BY THE OLYMPICS AND MOUNTAINS OF
VANCOUVER ISLAND. THESE AMOUNTS...CONCENTRATED IN A 12 HOUR
PERIOD...WILL CAUSE MOST RIVERS IN WESTERN WASHINGTON TO FLOOD. A
FLOOD WATCH IS IN EFFECT FOR THE AREA IN ALL BUT SAN JUAN AND ISLAND
COUNTIES AND WARNINGS WILL BE ISSUED ON MANY RIVERS OVERNIGHT
TONIGHT. SOME RIVERS MAY RISE TO MAJOR FLOOD STAGE WED...ESPECIALLY
IF THE BAND MOVES SOUTH SLIGHTLY SLOWER THAN NEW MODEL GUIDANCE.

RAIN WILL TAPER OFF FROM THE NORTH LATE WED AFTERNOON OR WED EVENING
AS A COLD FRONT SLIDES S THROUGH THE AREA AND INTO OREGON. BY THU
INTO THU NIGHT EXPECT HIGH PRES SLIDING E ACROSS THE AREA TO GIVE
WELCOME DRYING. A WEAK FRONTAL SYSTEM MAY CLIP NORTHERN PORTIONS OF
THE AREA FRI NIGHT INTO SAT...BUT WITH A WEAK RIDGE OF HIGH PRES
ALOFT EXPECT PRECIPITATION AMOUNTS TO BE RELATIVELY LIGHT.

THE STRONG FLOW ALOFT...INTERACTING WITH THE TERRAIN...WILL RESULT
IN WINDY CONDITIONS ONCE THE INITIAL WARM FRONT MOVES THROUGH THE
AREA IN THE EARLY MORNING HOURS WED. THE EXACT LOCATION OF THE WINDY
CONDITIONS WILL BE TOUGH TO PIN DOWN AS MESO-SCALE LOW PRESSURE
SYSTEMS FORMING TO THE LEE OF THE OLYMPICS AND VANCOUVER ISLAND
MOUNTAINS MOVE AROUND WITH THE SYNOPTIC SCALE FLOW. WHILE IT WILL
GET WINDY...NO AREAS ARE ANTICIPATED TO REACH HIGH WIND CRITERIA AND
ANY WIND PROBLEMS SUCH AS DOWN TREE BRANCHES OR POWER ISSUES SHOULD
BE QUITE LOCAL IN NATURE. A WIND ADVISORY WAS ISSUED FOR THE
INTERIOR LOWLANDS LATE TONIGHT THROUGH WED MORNING EARLIER TODAY AND
WILL BE MAINTAINED. ALBRECHT

.LONG TERM...PREVIOUS LONG TERM DISCUSSION FOLLOWS...HIGH PRESSURE
REMAINS OVER WESTERN WASHINGTON OVER THE WEEKEND INTO EARLY NEXT
WEEK. HOWEVER...SYSTEMS WILL RIDE OVER THE TOP OF THE RIDGE AND
OCCASIONALLY BRUSH THE AREA WITH SOME LIGHT RAIN. THE GFS SHOWS A
SYSTEM SKIRTING BY FRI NIGHT WITH A WEAK DECAYING FRONTAL BOUNDARY
STALLED OVER NRN WA OVER THE WEEKEND. KEPT A MENTION OF SHOWERS FROM
EVERETT NORTHWARD TO COVER THIS. EXTENDED MODELS VARY ON WHETHER OR
NOT THE RIDGE HOLDS IN PLACE EARLY NEXT WEEK. FOR NOW...DECIDED TO
GO WITH THE GFS SOLUTION WHICH IS MOSTLY DRY. MERCER

&&

.AVIATION...A STEADY STREAM OF SUBTROPICAL MOISTURE REMAINS AIMED AT
THE REGION AS A STRONG JET PUSHES A PAIR OF FRONT THROUGH THE AREA.
A WARM FRONT WILL LIFT NORTHWARD ACROSS THE AREA OVERNIGHT. LOWERING
CEILINGS AND INCREASING LOW LEVEL WINDS WILL ACCOMPANY THE FRONT.
SOUTHERLY WIND AT 2500 FEET WILL INCREASE TO 40 TO 50 KNOTS AFTER
THE WARM FRONTAL PASSAGE TONIGHT AROUND 6-8Z...AND LAST THROUGH AT
LEAST 12Z. THE TRAILING COLD FRONT WILL SAG SOUTHWARD ACROSS THE
PUGET SOUND TOMORROW AROUND MIDDAY. SURFACE WINDS WILL PICK UP
THROUGH PUGET SOUND EARLY WEDNESDAY MORNING AS SOUTHERLY GRADIENTS
TIGHTEN...WITH GUSTS OF 25 TO 30 KT POSSIBLE...ESPECIALLY 10Z TO
18Z. CEILINGS WILL MOSTLY REMAIN IN THE MVFR RANGE
OVERNIGHT...LOWERING OCCASIONALLY TO IFR IN MODERATE OR LOCALLY
HEAVY RAIN THROUGH LATE MORNING WEDNESDAY.

KSEA...CEILINGS GENERALLY OVC015-025 OVERNIGHT...OCCASIONALLY HIGHER
PRIOR TO WARM FRONTAL ARRIVAL THIS EVENING. SOUTHERLY WINDS TO 10
KTS THIS EVENING WILL BECOMING SOUTHWEST 12-18 KT WITH HIGHER GUSTS
AFTER 10Z. 27

&&

.MARINE...STRONG PACIFIC FRONTAL SYSTEM WILL BRING SW GALES TO THE
COAST AND SOUTHERLIES TO THE NORTHERN INTERIOR AND EAST ENTRANCE
AFTER MIDNIGHT. WESTERLY SURGE STILL EXPECTED THROUGH THE STRAIT AS
GRADIENTS BECOME ONSHORE AROUND 12Z. GALES IN EFFECT ALL WATERS
EXCEPT ADMIRALTY INLET WHERE A COMBINATION OF SOUTHERLIES THROUGH
PUGET SOUND AND WESTERLY PUSH THROUGH THE STRAIT MAKE IT A LESS
STRAIGHT FORWARD CALL. SEAS OVER COASTAL WATERS SHOULD FALL SHORT OF
ADVISORY CRITERIA...BUT WILL BE HAZARDOUS NONETHELESS IN THE 15 TO
18 FOOT RANGE OVERNIGHT THROUGH WEDNESDAY. SURFACE RIDGING WILL LEAD
TO MORE TRANQUIL CONDITIONS ACROSS THE WATERS THURSDAY AND FRIDAY. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH IN EFFECT FOR ALL OF WESTERN WASHINGTON EXCEPT
      ISLAND AND SAN JUAN COUNTIES.
    .WIND ADVISORY FOR THE INTERIOR LOWLANDS OF WESTERN
      WASHINGTON FROM 10 PM TONIGHT THROUGH 10 AM WEDNESDAY.
PZ...GALE WARNING FOR ALL WATERS EXCEPT ADMIRALTY INLET.
    .SMALL CRAFT ADVISORY ADMIRALTY INLET.
    .SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR.

WWW.WEATHER.GOV/SEATTLE

FOR THE ENHANCED AFD PLEASE VISIT

WWW.WEATHER.GOV/SEATTLE/GAFD/LATESTWEBAFD.HTML (ALL LOWERCASE)