River Flood Advisory 10, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
345 AM PST FRI DEC 30 2011

.SYNOPSIS...THE LAST IN THIS SERIES OF VIGOROUS FRONTAL SYSTEMS WILL
MOVE THROUGH THE AREA EARLY THIS MORNING. WINDY CONDITIONS WILL
DEVELOP BEHIND THE FRONT LATER THIS MORNING. SNOW WILL CONTINUE ON
THE MOUNTAINS WITH GOOD OROGRAPHIC FLOW CONTINUING INTO THIS
EVENING. AN UPPER RIDGE WILL BUILD OVER THE REGION THIS WEEKEND FOR
DRIER WEATHER. A WEAK FRONT WILL MOVE INLAND EARLY NEXT WEEK FOR
MORE RAIN.

&&

.SHORT TERM...A WARM FRONT MOVED THROUGH WESTERN WASHINGTON LAST
NIGHT WITH THE CWA REMAINING IN THE WARM SECTOR BETWEEN THE WARM AND
THE COLD FRONT OVERNIGHT INTO THE EARLY MORNING HOURS. HARD TO PICK
UP THE COLD FRONT ON THE SATELLITE IMAGERY. AT 11Z THE LEADING EDGE
OF THE COLD AIR LOOKS TO BE JUST REACHING CAPE FLATTERY.
TEMPERATURES IN THE MID AND UPPER 40S AT 11Z WILL ACTUALLY DROP A
FEW DEGREES AS THE COLD AIR ARRIVES THIS MORNING AND THEN REMAIN IN
THE MID 40S FOR THE REMAINDER OF THE DAY. SURFACE GRADIENTS...WHICH
PEAKED WITH THE WARM FRONT AROUND MIDNIGHT WILL INCREASE ONCE AGAIN
THIS MORNING WITH THE WINDS INCREASING UP INTO THE 20 TO 30 MPH
RANGE FOR MOST OF THE AREA. FOR THE LOWLANDS THE STEADY RAIN HAS
COME TO AN END BUT SATELLITE IMAGERY SHOWS PLENTY OF SHOWER ACTIVITY
OFF THE COAST WHICH WILL MOVE INTO WESTERN WASHINGTON TODAY.
TEMPERATURES ALOFT WILL COOL WITH THE 500 MB TEMPS DROPPING BELOW
-30C. LIFTED INDEXES ALONG THE COAST WILL BE NEAR 0. WILL STAY WITH
THE CHANCE OF THUNDERSTORMS ALONG THE COAST TODAY. FOR THE MOUNTAINS
MOST LOCATIONS GOT 3 TO 6 INCHES OF NEW SNOW OVERNIGHT BUT WITH THE
OROGRAPHIC FLOW REMAINING STRONG TODAY...850 MB WINDS WESTERLY 30 TO
40 KNOTS...AND THE COLDER AIR MASS MOVING IN WILL STAY WITH THE
WINTER STORM WARNING FOR ANOTHER 7 TO 14 INCHES OF NEW SNOW BY THIS
EVENING.

GRADUAL DRYING TREND LATER TONIGHT INTO SATURDAY MORNING WITH A WEAK
UPPER LEVEL RIDGE BUILDING OFF THE COAST AND SURFACE HIGH PRESSURE
BUILDING OVER WESTERN WASHINGTON. A FEW SHOWERS LINGERING AROUND
INTO SATURDAY MORNING...MAINLY IN THE MOUNTAINS...WITH A DRY
SATURDAY AFTERNOON IN STORE FOR THE AREA. SURFACE GRADIENTS GOING
OFFSHORE COMBINED COOL AIR MASS OVER WESTERN WASHINGTON WILL KEEP
DAYTIME HIGHS IN THE LOWER TO MID 40S.

MODELS REMAINING CONSISTENT WITH THEIR INCONSISTENT SOLUTIONS FOR
SATURDAY NIGHT INTO SUNDAY. THE GFS CONTINUES TO BRING A WARM FRONT
UP FROM THE SOUTH WHILE THE ECMWF KEEPS THE WARM FRONT OFFSHORE WITH
AN UPPER LEVEL RIDGE OVER WESTERN WASHINGTON. BOTH MODELS HAVE BEEN
CONSISTENT IN STAYING WITH THEIR SOLUTIONS FOR THE LAST FEW RUNS.
THE NAM MODEL ALSO SHOWS A WEAK WARM FRONT MOVING THROUGH WESTERN
WASHINGTON SATURDAY NIGHT AND SUNDAY MORNING WHILE THE CANADIAN
MODEL KEEPS THE WARM FRONT OFFSHORE AND TO THE NORTH OF THE AREA.
NOTHING IN THE CURRENT SATELLITE IMAGERY TO LATCH ON TO ONE SOLUTION
OVER THE OTHER SO AT THIS POINT WILL HAVE TO STAY WITH THE CHANCE
POPS FOR SATURDAY NIGHT AND SUNDAY. FELTON

.LONG TERM...INCONSISTENCY IN THE MODEL SOLUTIONS CONTINUES INTO THE
EXTENDED PERIOD WITH THE ECMWF BRINGING A WEAKENING COLD FRONT INTO
WESTERN WASHINGTON ON MONDAY WHILE THE GFS HAS SLOWED THIS FEATURE
DOWN TO MONDAY NIGHT. THERE IS A BIT OF AGREEMENT ON TUESDAY WITH A
SOUTHWEST TO NORTHEAST ALIGNED BAROCLINIC BAND SETTING UP OFFSHORE
WITH THE BULK OF THE MOISTURE AIMED AT VANCOUVER ISLAND. THE BAND
EVENTUALLY SAGS SOUTH THROUGH WESTERN WASHINGTON WEDNESDAY INTO
THURSDAY WITH SOME HEAVY PRECIPITATION POSSIBLE ESPECIALLY OVER THE
OLYMPICS AND NORTH CASCADES. FELTON

&&

.HYDROLOGY...A FLOOD WARNING WILL REMAIN IN EFFECT FOR THE SKOKOMISH
RIVER THIS MORNING. THE RIVER FELL BELOW FLOOD STAGE YESTERDAY BUT
THE SOUTH FORK IS STARTING TO TURN UP AND THE RIVER IS CLOSE ENOUGH
TO FLOOD STAGE RIGHT NOW THAT IT WILL NOT TAKE MUCH OF A SURGE TO
PUSH IT BACK TO FLOOD STAGE.

THIS HAS NOT BEEN A TYPICAL FLOOD PRODUCING WEATHER PATTERN FOR
WESTERN WASHINGTON...HOWEVER THE CUMULATIVE EFFECT OF SUCCESSIVE
STORMS HAS DRIVEN A FEW OF THE MOST FLOOD-PRONE RIVERS CLOSE TO
FLOOD STAGE. WHILE THE SNOQUALMIE RIVER NEAR CARNATION AND THE
PUYALLUP NEAR ORTING BEGAN RECEDING LAST NIGHT THE UPSTREAM GAUGES
HAVE TURNED UP AGAIN EARLY THIS MORNING AND THE STEADY RAIN
CONTINUES IN THE MOUNTAINS. THERE IS A CHANCE...NOT A VERY HIGH
ONE...BUT STILL A CHANCE THAT THESE RIVERS COULD APPROACH FLOOD
STAGE AGAIN LATER TODAY. WILL LEAVE THE WATCH UP FOR THE NEXT FEW
HOURS.

A DRIER WEATHER PATTERN WILL PREVAIL SATURDAY THROUGH NEXT
TUESDAY...AND THE THREAT OF FLOODING ACROSS THE ENTIRE FORECAST AREA
WILL EASE. ON TUESDAY A MOIST BAROCLINIC ZONE WILL DEVELOP...
EXTENDING SOUTHWEST FROM VANCOUVER ISLAND INTO THE NORTHEAST PACIFIC
OCEAN. THIS FEATURE WILL PROBABLY SHIFT SLOWLY INTO WESTERN
WASHINGTON WEDNESDAY AND THURSDAY AS IT WEAKENS. THE GFS MODEL HAS
WEAKENED THIS FEATURE SIGNIFICANTLY ON THE 00Z RUN WHILE THE ECMWF
STILL SHOWS A FAIRLY WET SYSTEM REMAINING INTACT AS IT SLOWLY MOVES
THROUGH WESTERN WASHINGTON WEDNESDAY INTO THURSDAY. IN ADDITION TO
THE PRECIPITATION...SNOW LEVELS WILL BE HIGH FOR THIS EVENT. THIS
SCENARIO COULD RENEW A THREAT OF AT LEAST MINOR FLOODING ON SOME OF
THE MORE FLOOD-PRONE RIVERS. FELTON

&&

.AVIATION...STRONG WESTERLY JET WILL REMAIN OVER THE AREA TODAY.
SURFACE WINDS WILL REMAIN QUITE GUSTY AT TIMES OVER WESTERN
WASHINGTON TODAY AS THE GRADIENT BECOMES STRONGLY ONSHORE FOLLOWING
FRONTAL PASSAGE EARLY THIS MORNING. AIR MASS IS MOIST AND
STABLE...BUT WILL BECOME UNSTABLE LATER THIS MORNING AND AFTERNOON
AS THE UPPER LEVEL TROUGH MOVES ONSHORE. CEILINGS ARE GENERALLY VFR
THIS MORNING WITH A FEW POCKETS OF MVFR. ASIDE FROM AREAS OF MVFR IN
AND NEAR SHOWERS MOVING ONSHORE...EXPECTING VFR TO BECOME MORE
DOMINANT AS THE DAY GOES ON.

KSEA...WELL MIXED LOW LEVELS AND INCREASING INSTABILITY SHOULD LEAD
TO MOSTLY VFR CEILINGS TODAY...WITH POCKETS OF MVFR AROUND SHOWERS.
WINDS NEAR 5000 FEET WILL REMAIN WEST TO SOUTHWEST 40 TO 50 KNOTS
BEFORE EASING BY THIS EVENING. INCREASING POST-FRONTAL ONSHORE
GRADIENT WILL LEAD TO GUSTY SOUTH TO SOUTHWEST SURFACE WIND 15 TO 30
KNOTS THROUGH THE AFTERNOON.

&&

.MARINE...FRONT MOVING ONSHORE EARLY THIS MORNING WITH ONSHORE
GRADIENT IN THE PROCESS OF INCREASING. GALE WARNINGS REMAIN IN
EFFECT FOR ALL WATERS AS STRONG ONSHORE FLOW IS EXPECTED TO BRING A
SHARP INCREASE OF WEST TO SOUTHWEST WINDS BY THIS AFTERNOON. LATE
TONIGHT...LARGE SWELL OF 18-20 FEET WILL ARRIVE IN THE COASTAL
WATERS.

HIGH PRESSURE AND OFFSHORE FLOW SETS UP OVER THE WEEKEND. ON SUNDAY
A WEAK WARM FRONT WILL MOVE NORTH OVER WESTERN WASHINGTON. OFFSHORE
FLOW COULD BE STRONG ENOUGH TO BRING GALES TO THE WEST ENTRANCE. FOR
NOW HAVE INDICATED A PERIOD OF EAST WINDS 20-30 KT THERE. MORE
WEATHER SYSTEMS ARRIVE EARLY NEXT WEEK. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING THROUGH THIS EVENING FOR THE NORTH
     AND CENTRAL WASHINGTON CASCADES.
     FLOOD WATCHES AND/OR WARNINGS FOR PORTIONS OF THE FORECAST AREA
PZ...GALE WARNING ALL WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

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

WWW.WEATHER.GOV/SEATTLE

&&