FXUS66 KSEW 251720
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
920 AM PST WED NOV 25 2009
.SYNOPSIS...A FRONTAL BAND JUST OFFSHORE WILL SHIFT EASTWARD INTO
WESTERN WASHINGTON THIS AFTERNOON AND EVENING...THEN REMAIN STALLED
OVER THE AREA BRINGING WET WEATHER AND MILD TEMPERATURES TONIGHT
THROUGH THANKSGIVING DAY AND INTO THURSDAY NIGHT. FRIDAY WILL BE
WET...COOL...AND BREEZY WITH SNOW IN THE MOUNTAINS. A WEAK FRONT
WILL BRUSH THE AREA THIS WEEKEND...WITH A FRONT MOVING INLAND ON
MONDAY.
&&
.SHORT TERM...IT WAS NICE TO SEE A SUNRISE FOR A CHANGE...THANKS TO
THE REGION BEING IN THE WARM SECTOR OF THIS SYSTEM AND THE MAIN
CLOUD SHIELD LINGERING JUST OFFSHORE. THIS BAND WILL SHIFT SLOWLY
INLAND...THICKENING THE CLOUDS AND DRIVING THE RAIN INLAND. IT HAS
BEEN RAINING WAY OUT ON THE NW TIP OF THE OLYMPIC PENINSULA THIS
MORNING AND BY EVENING IT WILL BE RAINING NEARLY EVERYWHERE EXCEPT
THE FAR SE CORNER OF OUR FORECAST AREA. WITH THE MILD AIR MASS...IT
WILL BE A WARMER THAN NORMAL DAY TODAY.
TONIGHT THE BAROCLINIC BAND WILL SHIFT OVER THE AREA THE LINGER WELL
INTO THURSDAY GENERATING FAIRLY STEADY LIGHT TO MODERATE RAINS.
THERE IS PLENTY OF MOISTURE WITH THIS SYSTEM WITH SOMEWHAT OF A
TROPICAL CONNECTION...HOWEVER THIS SYSTEM LACKS SIGNIFICANT UPPER
LEVEL DYNAMICS AND THE STRONG LOW TO MID LEVEL FLOW NECESSARY FOR
STRONG OROGRAPHIC ENHANCEMENT. THUS...THE LIGHT TO MODERATE
RAINFALL RATES.
THEN THURSDAY EVENING A WAVE MOVING NE ALONG THE FRONT WILL REACH
THE COAST NEAR ASTORIA AND CONTINUING MOVING NE. THIS WILL PROVIDE
SOME ENHANCEMENT TO THE RAINFALL OVER THE REGION EARLY THURSDAY
NIGHT THEN KICK UP A SW BREEZE EARLY FRIDAY MORNING ON THE BACK SIDE
OF THE SYSTEM. THE SYSTEM LOOKS TO BE A BIT MORE STRETCHED OUT AND
THUS LESS DYNAMIC WHICH BODES WELL FOR LESS WIND LIKE WE HAVE BEEN
GOING WITH BUT IT ALSO IS LOOKING LESS LIKELY TO EXTEND THE MODERATE
RAINS AS LONG AS EARLIER THOUGHT. THIS WOULD BE GOOD FOR OUR RIVER
SITUATION. THE FREEING LEVELS WILL BE DROPPING THURSDAY NIGHT INTO
FRIDAY SO THE PRECIPITATION OVER THE MOUNTAINS WILL CHANGE TO SNOW
BEFORE TAPERING OFF DURING THE DAY FRIDAY.
BY THE TIME WE ROLL INTO LATER FRIDAY...A DRIER NW FLOW DEVELOPS
ACROSS WASHINGTON IN THE WAKE OF THE DEPARTING SYSTEM FOR A BIT OF A
BREAK IN THE ACTION. CERNIGLIA
.LONG TERM...FROM PREVIOUS DISCUSSION...THE MOST RECENT MODEL RUNS
ARE IN ROUGH AGREEMENT FOR THE EXTENDED FORECAST PERIOD SATURDAY
THROUGH TUESDAY. THEY SHOW AN UPPER RIDGE BUILDING OVER THE REGION
AGAIN ON SATURDAY...WITH A WARM FRONT BRUSHING THE PACIFIC NORTHWEST
AS IT HEADS INTO BRITISH COLUMBIA SATURDAY THROUGH SUNDAY...AGAIN
WITH A BAROCLINIC BAND EXTENDING OFF TO THE SOUTHWEST. THEY ALSO
AGREE THAT AN UPPER TROUGH WILL DRIVE THE FRONT THROUGH THE REGION
ON MONDAY...WITH THE UPPER RIDGE BUILDING BEHIND IT AGAIN ON
TUESDAY. OUR CURRENT FORECAST DOES NOT REFLECT THE MONDAY
SYSTEM...BUT THIS DOES NOT WARRANT A REVISION TO THE DAY 6 FORECAST.
MCDONNAL
&&
.HYDROLOGY...WET WEATHER OVER WESTERN WASHINGTON FROM LATER TODAY
THROUGH MIDNIGHT THURSDAY...COMBINED WITH HIGH SNOW LEVELS...POSES A
THREAT OF MINOR FLOODING ON THE MOST FLOOD PRONE RIVER REACHES IN
WESTERN WASHINGTON. THAT INCLUDES THE SKOKOMISH RIVER IN MASON
COUNTY...THE SNOQUALMIE RIVER NEAR CARNATION IN KING COUNTY...THE
PUYALLUP RIVER NEAR ORTING IN PIERCE COUNTY...AND THE LOWER REACH OF
THE CHEHALIS RIVER IN THURSTON AND GRAYS HARBOR COUNTIES.
THE FRONT OFFSHORE WILL MOVE INLAND THIS AFTERNOON AND EVENING THEN
STALL OVER THE AREA THROUGH THANKSGIVING AND INTO THURSDAY NIGHT
BEFORE MOVING EAST. PRECIPITATION TOTALS FOR THE PERIOD WILL
PROBABLY AMOUNT TO 2 TO 4 INCHES OVER THE OLYMPIC PENINSULA WITH 1.5
TO 3 INCHES OVER THE CASCADES. THAT IS NOT TYPICALLY ENOUGH TO POSE
A REAL FLOOD THREAT...ESPECIALLY OVER A 36 HOUR PERIOD AND WITH SOME
LULLS IN PRECIPITATION LIKELY. HOWEVER RIVERS ARE RUNNING HIGHER
THAN NORMAL FOR LATE NOVEMBER DUE TO ABOVE NORMAL PRECIPITATION OVER
THE PAST MONTH. THE LATEST MODEL RUNS ARE HINTING AT A BIT LESS
RAIN AND THE RIVER MODELS ARE RUNNING NOW USING THE LATEST INPUT.
ADDED TO THIS IS THE FACT THAT THE SKOKOMISH OBVIOUSLY FLOODS QUITE
EASILY. THE SNOQUALMIE NEAR CARNATION AND THE PUYALLUP NEAR ORTING
ARE TWO OF THE MOST FLOOD SPOTS ON RIVERS FLOWING OUT OF THE
CASCADES. THE LOWER REACH OF THE CHEHALIS...WHICH DRAINS A LARGE
AREA OF THE SOUTHWEST INTERIOR OF WASHINGTON...HAS BEEN RUNNING
QUITE HIGH FOR MORE THAN A WEEK.
INPUT TO THE RIVER BASINS WILL DECREASE SOMETIME THURSDAY NIGHT AS
THE FRONTAL BAND MOVES EAST AND COOLER AIR MOVING INLAND DROPS THE
SNOW LEVELS. THERE IS NO INDICATION OF A WEATHER PATTERN THAT
RESULTS IN FLOOD-PRODUCING RAINFALL FROM BEYOND FRIDAY.
NO FLOODING IS EXPECTED ON THE GREEN RIVER DURING THE PERIOD.
&&
.AVIATION...UPPER LEVEL RIDGE AXIS OVER WA/ID BORDER CONTINUES TO
SLIDE EAST AS A UPPER LEVEL TROUGH OVER THE GULF OF ALASKA SLOWLY
SLIDES TOWARD THE REGION. FRONTAL BOUNDARY IS STALLED
OFFSHORE...WITH LIGHT RAIN PENETRATING THE COAST AND NORTHWEST
INTERIOR. VFR CONDITIONS REMAIN FOR THE INTERIOR...ESPECIALLY SOUTH.
AS THE FRONT MOVES IN AND WEAKENS...CIGS WILL GENERALLY LOWER THIS
AFTERNOON TO 3-5 KFT AROUND 21Z FOR THE INTERIOR AS RAIN DEVELOPS.
CIGS ARE ALREADY LOWER FOR THE COAST AND STRAIT. FRONTAL BOUNDARY
WILL WEAKEN AND REMAIN STATIONARY OVER THE AREA OVERNIGHT. CEILINGS
WILL CONTINUE TO GRADUALLY LOWER TOWARD AT LEAST IFR EARLY THURSDAY
MORNING. A NEW LOW PRES WILL DEVELOP ALONG THE STALLED FRONTAL
BOUNDARY THURSDAY AND PASS THROUGH WESTERN WASHINGTON AROUND 00Z
FRIDAY. SURFACE WINDS SHOULD GENERALLY REMAIN LIGHT SOUTHERLY
THROUGH THIS EVENING BECOMING OFFSHORE OVERNIGHT. WIND SPEEDS
TOMORROW ALSO LOOK LIGHT BUT EXACT SURFACE LOW POSITIONING REMAINS
SOMEWHAT IN QUESTION SO SURFACE FLOW MAY BE NORTHERLY.
KSEA...PATCHY FOG REMAINS AROUND THE TERMINAL THIS
MORNING...OTHERWISE VFR CONDITIONS PERSIST. LIGHT S WINDS TODAY WITH
CEILINGS GRADUALLY LOWERING AFTER 21Z AS RAIN DEVELOPS. DAMICO
&&
.MARINE...SOUTHEASTERLY GRADIENTS WILL SLOWLY WEAKEN TODAY AS A
FRONTAL BOUNDARY STALLS OVER WESTERN WASHINGTON. WINDS WILL
GENERALLY BE LIGHT OVERNIGHT WILL WEAK PRESSURE GRADIENTS.
A NEW LOW WILL FORM ALONG THE FRONTAL BOUNDARY PASSING THROUGH
WASHINGTON LATE THURSDAY EVENING. CURRENT MODELS HAVE IT TRACKING
THROUGH THE CHEHALIS GAP ON A NORTHEAST PATH AND NOT DEEPENING ALL
THAT MUCH. GOOD WESTERLY PUSH SHOULD DEVELOP BY FRIDAY
MORNING...GENERALLY EASING THROUGH THE DAY ON FRIDAY. DAMICO
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD POTENTIAL OUTLOOK IN EFFECT.
PZ...SMALL CRAFT ADVISORY...ENTRANCES TO THE STRAIT...ADMIRALTY
INLET... AND NORTH INLAND WATERS. SMALL CRAFT ADVISORY FOR
ROUGH GRAYS HARBOR BAR.
$$
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