FXUS66 KSEW 251153
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 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...A LARGE UPPER RIDGE EXTENDS NORTH-NORTHEAST FROM
NORTHERN CALIFORNIA ACROSS EASTERN WASHINGTON AND THE CANADIAN
ROCKIES INTO NORTHERN ALBERTA. WE HAVE SOUTHWEST FLOW ALOFT ACROSS
WESTERN WASHINGTON...AND A BAROCLINIC ZONE OR FRONTAL BAND EXTENDS
FROM BRITISH COLUMBIA SOUTHWEST ACROSS VANCOUVER ISLAND AND THE
OFFSHORE WATERS. THE AIR MASS OVER THE FORECAST AREA IS VERY MILD
WITH 500 MB HEIGHTS AROUND 5700 METERS...THE SNOW LEVEL AROUND 8000
FT...AND TEMPERATURES MAINLY IN THE 40S AT 2 AM. THE FRONT OFFSHORE
IS ALREADY BRINGING SOME RAIN TO THE NORTH COAST...WHILE SOME BREAKS
IN THE CLOUD COVER HAVE OPENED FROM THE PUGET SOUND AREA SOUTHWARD.
THERE IS ALSO PLENTY OF FOG AROUND IN THE MOIST STABLE AIR MASS
UNDER THE RIDGE.
THE UPPER RIDGE WILL MOVE SLOWLY EASTWARD TODAY AND THE FRONTAL BAND
WILL SHIFT INLAND...BRINGING AN END TO THE BRIEF DRY SPELL AT LEAST
ALONG THE COAST AND OVER THE OLYMPIC PENINSULA. RAIN WILL SPREAD
INLAND TO THE INTERIOR LOWLANDS THIS EVENING...AND THE FRONT WILL
REMAIN MORE OR LESS STALLED OVER WESTERN WASHINGTON TONIGHT...
THANKSGIVING DAY...AND INTO THURSDAY NIGHT.
VARIOUS MODELS AND MODEL RUNS DIFFER ON DETAILS OF LOCATION AND
TIMING...BUT THEY AGREE BROADLY THAT A FRONTAL WAVE WILL BEGIN TO
DEVELOP ALONG THE BAROCLINIC BAND LATE TODAY WELL WEST OF SOUTHERN
OREGON OR NORTHERN CALIFORNIA. THE LOW WILL DEEPEN AS IT HEADS
NORTHEAST...MAKING LANDFALL SOMEWHERE ALONG THE PACIFIC NORTHWEST
COAST THURSDAY NIGHT AND CONTINUING QUICKLY INLAND.
THE GFS SHOWS THE LOW AT 1003 MB CROSSING THE NORTHWEST TIP OF THE
OLYMPIC PENINSULA AND TRACKING ACROSS THE SOUTH PART OF VANCOUVER
ISLAND. THE NAM12 SOLUTIONS HAVE GENERALLY SHOWN THE LOW DEEPENING
LESS AND FOLLOWING A TRACK FURTHER SOUTH...ESPECIALLY THE 06Z WED
RUN WHICH SHOWS A 1008 MB LOW MAKING MOVING INLAND SOUTH OF ASTORIA.
THE GFS SOLUTION WOULD BE THE WINDIER SCENARIO...BUT NONE OF THE
MODELS SUGGESTS A BIG POTENTIAL FOR STRONG WINDS.
THE FRONT WILL BRING FAIRLY SIGNIFICANT PRECIPITATION TOTALS TO
WESTERN WASHINGTON AS IT REMAINS STALLED OVER THE AREA FROM THIS
AFTERNOON INTO THURSDAY NIGHT. DURING THE ROUGHLY 36 HOUR PERIOD
THAT THE FRONT IS OVER THE AREA -- FROM MIDDAY TODAY TO MIDNIGHT
THURSDAY NIGHT -- PRECIPITATION TOTALS OF 2 TO 4 INCHES ARE LIKELY
OVER THE OLYMPIC PENINSULA WITH AMOUNTS OF 1.5 TO 3 INCHES LIKELY
OVER THE CASCADES. THE GFS AND UW WRF-GFS SHOW THE HEAVIER AMOUNTS
OVER THE NORTH CASCADES...WHILE THE NAM12 AND UW GFS-ETA SHOW THE
HEAVIER AMOUNTS OVER THE SOUTH CASCADES. THE SNOW LEVEL WILL BE NEAR
8000 FT TODAY AND GRADUALLY FALL TO 5500 FT THURSDAY NIGHT. THIS
AMOUNT OF RAIN POSES A THREAT OF MINOR FLOODING ON ONLY THE
MOST-FLOOD PRONE RIVER REACHES IN OUR FORECAST AREA...AS EXPLAINED
IN GREATER DETAIL IN THE HYDROLOGY SECTION BELOW.
THE FRONTAL BAND WILL MOVE EAST THURSDAY NIGHT AND A COLDER AIR MASS
WILL MOVE INTO WESTERN WASHINGTON. A VIGOROUS FRONTAL SYSTEM WILL
MOVE THROUGH THE AREA FROM THE WEST OR NORTHWEST FRIDAY AND FRIDAY
NIGHT. FRIDAY WILL PROBABLY BE WET...COOL...AND BREEZY OR WINDY.
THIS SYSTEM COULD ALSO BRING SIGNIFICANT SNOWFALL TO THE MOUNTAINS
AS THE SNOW LEVEL FALLS TO OR BELOW HIGHWAY PASS LEVELS...WHICH
COULD OBVIOUSLY HAVE A BIG IMPACT ON HOLIDAY TRAVELERS. MCDONNAL
.LONG TERM...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 MIDDAY 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.
AS MENTIONED ABOVE 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.
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...STILL SOME FOG AND LIFR CEILINGS AROUND BUT CONDITIONS
ARE SLOWLY IMPROVING. SOUTHERLY FLOW IS INCREASING IN RESPONSE TO
APPROACHING FRONT. AIR MASS IS MOIST AND STABLE BUT IS GETTING
STIRRED UP. CEILINGS WILL RISE TO MVFR OR BETTER...THERE COULD EVEN
BE SOME PERIODS WITHOUT CEILINGS...BUT AS FRONT APPROACHES THE
CLOUDS SHOULD CONVERGE AROUND 3-5K FT. RAIN WILL DEVELOP ON THE
COAST THIS MORNING AND SPREAD INLAND DURING THE AFTERNOON.
KSEA...HAS MANAGED TO SCATTER OUT BUT HAS A ZERO DEW POINT SPREAD.
EXPECT A FEW MORE HOURS OF IN AND OUT FOG...WITH A TREND TOWARD MVFR
OR BETTER. SOUTH WIND 5-8 KT. CHB
&&
.MARINE...GALE ON THE COAST HAS NOT REALLY GOTTEN GOING. MODELS HAVE
A BRIEF PERIOD OF STRONGER WINDS LATER THIS MORNING...BUT THEN
DIMINISH THE WINDS AFTER THE FRONT PASSES. KEPT THE GALE WARNING IN
EFFECT UNTIL 10 AM. GRADIENTS REMAIN LIGHT TONIGHT AND THEN TURN
OFFSHORE AS A LOW DEVELOPS SOUTHWEST OF WASHINGTON AND APPROACHES
THE STATE ON THURSDAY.
THE LOW WILL PASS THROUGH WASHINGTON THURSDAY NIGHT...SAYS THE
GFS...OR MOVE INTO OREGON...SAYS THE NAM. GENERALLY FAVOR THE GFS
BUT IN EITHER CASE THE FLOW TURNS ONSHORE ON FRIDAY...PRETTY
STRONGLY IN THE CASE OF THE GFS. HAVE INDICATED GALES IN THE STRAIT
AND GENERALLY BREEZY CONDITIONS ELSEWHERE ON FRIDAY. CHB
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD POTENTIAL OUTLOOK IN EFFECT.
PZ...GALE WARNING COAST.
.SMALL CRAFT ADVISORY...ENTRANCES TO THE STRAIT...ADMIRALTY
INLET... AND NORTH INLAND WATERS.
.SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
$$
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
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