River Flood Warning 39, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 AM PST SAT DEC 11 2010

.SYNOPSIS...A BRIEF SPELL OF DRY WEATHER WILL END LATER TODAY...AS A
STRONG WARM FRONT MOVES NORTH ACROSS THE AREA. THIS WILL BRING AN
EPISODE OF VERY WET AND UNSEASONABLY WARM WEATHER TONIGHT THROUGH
SUNDAY NIGHT...WITH SOME FLOODING LIKELY. A COLD FRONT WILL MOVE
THROUGH SUNDAY NIGHT. OCCASIONAL RAIN AND COOLER WEATHER WILL
PREVAIL NEXT WEEK...WITH SNOW IN THE MOUNTAINS.

&&

.SHORT TERM...THE LEADING EDGE OF THE WARM FRONTAL PRECIPITATION IS
BEGINNING TO MOVE INTO FAR SOUTHWEST WASHINGTON THIS MORNING. RAIN
WILL CONTINUE TO SPREAD NORTH ACROSS THE AREA TODAY. ALL AREAS
SHOULD BE RAINING BY EVENING. RAIN WILL BECOME HEAVY IN SOME OF THE
SOUTHERN LOCATIONS THIS AFTERNOON.

SNOW LEVELS ARE STARTING OUT AROUND 2500-3000 FEET THIS MORNING.
THESE WILL RISE SHARPLY TO 7000 OR 8000 FEET THIS AFTERNOON AND
EVENING. LOW LEVEL CROSS-CASCADE PRESSURE GRADIENTS WILL BE EASTERLY
UNTIL AROUND MIDNIGHT TONIGHT...KEEPING A WINTERY MIX IN THE PASSES.
WITH HEFTY QPF AMOUNTS...THE PASSES COULD GET SEVERAL INCHES OF SNOW
AND POSSIBLY SOME FREEZING RAIN BEFORE TURNING TO RAIN. A WINTER
WEATHER ADISORY IS IN EFFECT FOR THIS.

THE BIG STORY OF COURSE IS THE HEAVY RAIN THAT WILL FALL TONIGHT
WITH THE WARM FRONT AND SUNDAY AS A BAROCLINIC JET CONTINUES TO
CONVEY SUB-TROPICAL MOISTURE INTO THE FORECAST AREA. RAIN WILL
GRADUALLY DECREASE SUNDAY NIGHT AS THE COLD FRONT MOVES EASTWARD
ACROSS WESTERN WASHINGTON...BRINGING AN END TO HYDROLOGICALLY
SIGNIFICANT PRECIPITATION.

OVERALL THE MODELS HAVE BEEN CONSISTENT OVER THE LAST COUPLE DAYS
WITH THE GFS...UW WRF-GFS...AND EUROPEAN MODEL SHOWING THE HEAVIEST
RAIN SOMEWHAT FURTHER NORTH THAN THE NAM AND UW MM5-NAM. I LEAN
TOWARD THE GFS SOLUTION IN THIS CASE...BUT ALL MODELS SHOW ENOUGH
PRECIPITATION TO JUSTIFY A FLOOD WATCH FOR OUR ENTIRE FORECAST AREA.
SOME DETAILS ARE GIVEN BELOW IN THE HYDROLOGY SECTION. THERE WILL
ALSO BE SOME WIND THIS WEEKEND BUT NOTHING DRAMATIC.

ON MONDAY A COOLER AIR MASS WILL START MOVING INTO WESTERN
WASHINGTON AS A DEEP UPPER TROUGH DIGS INTO THE OFFSHORE WATERS OF
BRITISH COLUMBIA. A WEAK SHORTWAVE AHEAD OF THE UPPER TROUGH WILL
BRING SOME SHOWERS TO THE FORECAST AREA. MODELS HAVE BEEN
CONSISTENTLY SHOWING A STRONG JET -- MAYBE 170 KT JET -- REACHING
THE BASE OF THE TROUGH...CAUSING AN INSTANT OCCLUSION TO DEVELOP AND
MOVE INTO WESTERN WASHINGTON MONDAY NIGHT.

THIS WILL BE A VIGOROUS FEATURE ITSELF. THE SNOW LEVEL WILL BE
FALLING TO AROUND 3000 FT MONDAY NIGHT...SO IT WILL NOT RENEW OR
WORSEN FLOODING. IT WILL BRING RAIN AND SOME WIND TO THE LOWLANDS
THOUGH...AND IT SHOULD BE A GOOD SNOW PRODUCER FOR THE MOUNTAINS.
SCHNEIDER

.LONG TERM...PREVIOUS DISCUSSION...MODELS DIFFER SOMEWHAT ON
DETAILS...ESPECIALLY WITH RESPECT TO TIMING OF FEATURES BEYOND
TUESDAY. CONFIDENCE IS FAIRLY HIGH HOWEVER THAT TUESDAY THROUGH
FRIDAY WILL BE RATHER COOL WITH A PROGRESSIVE PATTERN BRINGING
OCCASIONAL SHORTWAVE FEATURES THROUGH THE REGION. ONE OF THESE
SYSTEMS WILL PROBABLY MOVE THROUGH WEDNESDAY NIGHT...AND SOME DRYING
IS HINTED AT FOR THURSDAY NIGHT AND FRIDAY.  MCDONNAL

&&

.HYDROLOGY...NO SURPRISES WITH THE NEW MODELS COMING IN THIS
MORNING. THE LEADING EDGE OF THE WARM FRONTAL LIGHT RAIN HAS STARTED
TO MOVE INTO FAR SOUTHWEST WASHINGTON THIS MORNING AND WILL CONTINUE
TO SPREAD NORTH ACROSS WESTERN WASHINGTON TODAY. EVERYTHING APPEARS
TO BE ON TRACK. THE PREVIOUS HYDROLOGY DISCUSSION IS STILL VALID AND
FOLLOWS. SCHNEIDER

MODEL RUNS CONTINUE TO SUPPORT A FLOOD WATCH FOR OUR ENTIRE FORECAST
AREA...WITH AN EPISODE OF HEAVY RAIN AND HIGH SNOW LEVELS TONIGHT
THROUGH SUNDAY NIGHT. THE HEAVIEST RAIN WILL PROBABLY OCCUR TONIGHT
AND SUNDAY...AS A WARM FRONT MOVES NORTHWARD ACROSS WESTERN
WASHINGTON AND THEN A BAROCLINIC BAND HANGS OVER THE AREA. THE
ASSOCIATED COLD FRONT WILL MOVE ACROSS THE AREA FROM THE NORTHWEST
SUNDAY AFTERNOON AND EVENING WITH MORE HEAVY RAIN...THOUGH PROBABLY
NOT AS HEAVY AS WITH THE WARM FRONT. THE EPISODE COULD CONTINUE INTO
MONDAY MORNING FOR JUST THE SOUTHERN PORTION OF THE CASCADES IN OUR
FORECAST AREA.

WHILE ADDITIONAL RAIN IS IN THE FORECAST MONDAY AND BEYOND...THE
AMOUNTS ARE NOT EXPECTED TO BE HYDROLOGICALLY SIGNIFICANT AND THE
SNOW LEVEL WILL BE FALLING.

PRECIPITATION AMOUNTS FOR TONIGHT THROUGH SUNDAY NIGHT WILL BE
MAINLY 1 TO 3.5 INCHES OVER THE LOWLANDS...WITH 4 TO 9 INCHES OVER
THE MOUNTAINS. THE SNOW LEVEL WILL CLIMB INTO THE 7000 TO 8000 FOOT
RANGE TONIGHT. THE GFS...UW WRF-GFS...AND EUROPEAN MODELS SHOW THE
HEAVIEST RAIN SLIGHTLY FURTHER NORTH THAN THE NAM AND UW MM5-NAM.
HOWEVER...AS MENTIONED ABOVE...ALL MODELS JUSTIFY A FLOOD WATCH FOR
OUR ENTIRE FORECAST AREA. THE MOST FLOOD-PRONE RIVERS IN WESTERN
WASHINGTON COULD FLOOD IN THIS EVENT...WHETHER FLOWING OUT OF THE
OLYMPICS...OFF THE WEST SLOPES OF THE CASCADES...OR IN THE CHEHALIS
BASIN.

IN ADDITION...IT IS NOT JUST THE MAIN STEM RIVERS THAT ARE FLOOD
RISKS. LOCAL FLOODING FROM SMALL STREAMS AND URBAN FLOODING OF LOW
LYING STREETS IS ALSO POSSIBLE...AND THIS COULD HAPPEN ALMOST
ANYWHERE IN WESTERN WASHINGTON.

FINALLY...SOME OF THE NAM AND UW WRF-NAM RUNS HAVE SHOWN 4 INCHES OF
RAIN IN A 24 HOUR PERIOD OVER THE HOWARD HANSON DAM CATCHMENT.
HOWEVER RECENT MODELS HAVE TRENDED LOWER WITH THESE FORECASTS. IT
MUST BE NOTED THERE IS GREAT UNCERTAINTY IN THE RAINFALL FORECAST...
UT THERE IS DEFINITELY A CHANCE OF HEAVY RAIN OVER THE UPPER GREEN
RIVER BASIN.  MCDONNAL

&&

.AVIATION...A WARM FRONT WILL SWEEP NORTH ACROSS MOST OF THE AREA
THIS EVENING WITH GRADUALLY LOWERING CEILINGS AND VISIBILITIES LATER
TODAY.  THIS FRONT WILL STALL NEAR THE NORTHER BORDER INTO EARLY
SUNDAY THEN MOVE SOUTHEAST AS A COLD FRONT LATER SUNDAY.

IN ADVANCE OF THE WARM FRONT TONIGHT WOULD EXPECT THE CEILINGS TO
DROP BELOW 1000 FT MANY AREAS WITH VISIBILITIES 1-3 NM COMMON.  THE
RAIN ASSOCIATED WITH THIS FRONT WILL BE HEAVY AT TIMES LATE TODAY
INTO TONIGHT.  SOUTHERN TERMINALS LIKE KHQM...KOLM COULD IMPROVE
TOWARD SUNDAY MORNING WITH THE FRONT LIFTING NORTH.  IF IT OCCURS IT
WILL BE SHORT LIVED AS THE COLD FRONT WILL DESCEND ON THOSE AREAS
DURING THE DAY SUNDAY.  SOUTHERLY WINDS WILL INCREASE BEHIND THE
WARM FRONT AND IN ADVANCE OF THE COLD FRONT LATE TONIGHT THROUGH
SUNDAY.  CERNIGLIA

KSEA...EXPECT LIGHT E/SE FLOW TODAY WITH LOWERING CLOUDS.  LIGHT
LIGHT RAIN WILL DEVELOP BY 20Z AND BECOME HEAVY AT TIMES AFT 03Z.
VFR CIGS...DROPPING TO MVFR 20Z-22Z AND TO IFR AFT 04Z WITH MVFR-IFR
VISIBILITIES AFTER 00Z.  CERNIGLIA

&&

.MARINE...OFFSHORE FLOW WILL INCREASE ACROSS W WA TODAY AS A STRONG
WARM FRONT APPROACHES.  THIS FRONT WILL BEGIN TO LIFT NORTH THROUGH
THE AREA THIS AFTERNOON WITH STRONG SOUTHERLY WINDS DEVELOPING
ACROSS THE COASTAL WATERS AFTER THE FRONT PASSES.  THE WINDS OVER
THE COASTAL WATERS ARE EXPECTED TO REACH GALE FORCE AND I HAVE
UPGRADED THE GALE WATCH TO A GALE WARNING.  CURRENTLY WE HAVE A
SMALL CRAFT ADVISORY FOR THE WEST ENTRANCE OF JUAN DE FUCA AS
EASTERLY BUT WILL NEED TO WATCH THIS AS THE PRES GRADIENT MAXES AT
-5.1 KUIL-KBLI.

SLY WINDS INCREASE ON SUN AS THE BAROCLINIC BAND STALLS OVER W WA
WITH GUSTY WINDS INTERIOR.  MAY SEE SCA WINDS IN PUGET SOUND/HOOD
CANAL.  THE COLD FRONT WILL PUSH TROUGH THE AREA SUN AFTERNOON OR
EARLY EVENING.  ANOTHER COLD FRONT WILL AFFECT W WA MON NIGHT INTO
TUE...KEEPING THE PATTERN ACTIVE. CERNIGLIA

&&

.AVALANCHE...HEAVY PRECIPITATION HAS BEGUN OVER THE MT HOOD AREA AND
IS EXPECTED TO LIFT TO THE CASCADES THROUGH THE DAY ALONG WITH
SIGNIFICANT WARMING...ESPECIALLY TONIGHT. SNOW SHOULD CHANGE TO RAIN
BY LATE SATURDAY IN MOST AREAS WITH THE EXCEPTION OF THE CASCADE
PASSES DUE TO COOL EASTERLY SURFACE FLOW. THE CREST LEVEL WINDS
SHOULD BE VERY STRONG BY LATE SATURDAY AS WELL.

THIS WEATHER IS EXPECTED TO PRODUCE A SIGNIFICANT AVALANCHE CYCLE
CAUSING A HIGH DANGER THROUGHOUT THE OLYMPICS AND
CASCADES...ESPECIALLY SATURDAY NIGHT AND INTO SUNDAY. THE HIGH
DANGER IS INITIALLY EXPECTED AT MT HOOD STARTING SATURDAY AFTERNOON
AND IN THE OLYMPICS AND WASHINGTON CASCADES STARTING SATURDAY NIGHT.
AN AVALANCHE WARNING IS BEING ISSUED SATURDAY MORNING TO HIGHLIGHT
THE INCREASING DANGER. PLEASE VISIT WWW.NWAC.US FOR DETAILS. KRAMER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCHES AND/OR WARNINGS FOR PORTIONS OF THE
       FORECAST AREA.
    .WINTER WEATHER ADVISORY FOR THE NORTH AND CENTRAL CASCADES.
    .AVALANCHE WATCH FOR THE WESTERN SLOPES OF THE CASCADES.

PZ...SMALL CRAFT ADVISORY COAST...STRAIT OF JUAN DE FUCA...
      ADMIRALTY INLET AND NORTHERN INLAND WATERS.
    .GALE WARNING COASTAL WATERS TONIGHT THROUGH SUNDAY MORNING.
    .SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS.

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.