AREA FORECAST DISCUSSION
FXUS66 KSEW 241155
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST TUE NOV 24 2009
.SYNOPSIS...A WEAK COLD FRONT OVER WESTERN WASHINGTON WILL EXIT TO
THE EAST TODAY WHILE A WEAK WARM FRONT ARRIVES FROM THE WEST...AND
MOST AREAS WILL RECEIVE A LITTLE RAIN. A WEAK COLD FRONT WILL SIT
OFF THE COAST TONIGHT AND WEDNESDAY...THEN BRING WETTER WEATHER
WEDNESDAY NIGHT THROUGH THURSDAY NIGHT AS IT STRENGTHENS AND MOVES
INLAND. COOL SHOWERY WEATHER WILL FOLLOW ON FRIDAY.
&&
.SHORT TERM...A WEAK COLD FRONT MOVED INTO WESTERN WASHINGTON LAST
EVENING...WHICH HAS BEEN WEAKENING FURTHER AS IT MOVES SLOWLY
EASTWARD ACROSS THE REGION. LIGHT RAIN CONTINUES TO FALL ACROSS MUCH
OF THE FORECAST AREA...GRADUALLY DECREASING FROM THE WEST.
TEMPERATURES ARE MILD -- MOSTLY IN THE MID 40S -- AND THE SNOW LEVEL
HAS CLIMBED WELL ABOVE 5000 FT.
AN UPPER LEVEL RIDGE CURRENTLY OVER THE OFFSHORE WATERS WILL SHIFT
INTO THE PACIFIC NORTHWEST THIS MORNING AND EAST OF THE CASCADES
LATE TODAY...ALLOWING THE SOUTHEAST END OF ANOTHER WARM FRONT
IMMEDIATELY UPSTREAM OF THE RIDGE TO REACH WESTERN WASHINGTON.
BETWEEN THE DEPARTURE OF THE COLD FRONT THIS MORNING AND THE ARRIVAL
OF THE WARM FRONT...MOST OF WESTERN WASHINGTON WILL GET A LITTLE BIT
OF RAIN TODAY. IN THE INTERIOR LOWLANDS IT WILL AMOUNT TO ONLY A FEW
HUNDREDTHS OF AN INCH. THE HEAVIEST AMOUNTS -- UP TO A HALF INCH --
WILL FALL ALONG THE COAST AND OVER THE OLYMPIC PENINSULA.
THE WARM FRONT WILL MOVE NORTHEAST INTO BRITISH COLUMBIA TONIGHT...
LEAVING A WEAK COLD FRONT OR BAROCLINIC BAND EXTENDING FROM
VANCOUVER ISLAND SOUTHWEST ACROSS THE OFFSHORE WATERS. THE FRONT
WILL MOVE SLOWLY EASTWARD INTO THE FORECAST AREA ON WEDNESDAY. IT
WILL BRING SOME RAIN TO THE AREA DURING THIS PERIOD...ESPECIALLY
ALONG THE COAST...OVER THE OLYMPIC PENINSULA...AND IN THE NORTHWEST
INTERIOR. THE FORECAST WILL CALL FOR A CHANCE OF RAIN FOR THE
WESTERN WASHINGTON INTERIOR LOWLANDS TONIGHT AND WEDNESDAY...MEANING
IT SHOULD REMAIN MOSTLY DRY FROM THE EVERETT AREA SOUTHWARD. AGAIN
THE ONLY AREA THAT WILL RECEIVE SIGNIFICANT PRECIPITATION WILL BE
THE COAST AND OLYMPICS.
THE FORECAST FOR WEDNESDAY NIGHT THROUGH THURSDAY NIGHT...INCLUDING
THANKSGIVING DAY OF COURSE...IS LOOKING MORE INTERESTING -- AND MORE
UNCERTAIN. THE MODELS HAVE SHOWN A VARIETY OF SOLUTIONS FOR THIS
PERIOD OVER THE PAST FEW DAYS...BUT THEY ARE BEGINNING TO AGREE THAT
A FRONTAL WAVE WILL DEVELOP ALONG THE EXISTING BAROCLINIC BAND LATE
WEDNESDAY WHERE IT EXTENDS INTO THE WATERS WELL OFF THE COAST OF
NORTHERN CALIFORNIA.
HOW THIS WILL PLAY OUT IS STILL A BIG QUESTION -- IN FACT THERE IS A
FAIRLY BIG DIFFERENCE EVEN BETWEEN THE 00Z TUE SUITE OF MODELS AND
THE 06Z TUE RUN...WHICH JUST CAME IN OVER THE PAST COUPLE HOURS.
MOST MODEL RUNS HAVE SHOWN VARIOUS TAKES WITHIN THE GENERAL IDEA
THAT IT WILL BE RATHER WET AND MILD...THOUGH THE LATEST EUROPEAN
MODEL SHOWS A DEEP (IE COLD) UPPER LOW FOLLOWING THE FRONTAL WAVE
INTO WESTERN WASHINGTON LATE THURSDAY NIGHT.
AT THIS POINT I THINK WE CAN BE FAIRLY CONFIDENT THAT WEDNESDAY
NIGHT THROUGH THURSDAY NIGHT WILL BE WET...WITH SOMETHING LIKE 3 TO
4 INCHES OF PRECIPITATION FALLING OVER THE OLYMPIC MOUNTAINS AND
MAYBE THE NORTH CASCADES AS THE SNOW LEVEL FALLS FROM 7000 FT TO
4500 FT.
A BIGGER QUESTION IS THE POTENTIAL FOR WIND. SOME MODEL SOLUTIONS
...FOR EXAMPLE THE 00Z TUE GFS...HAVE SHOWN A DEVELOPING SURFACE LOW
WITH THE FRONTAL WAVE TRACKING NORTHEAST ACROSS WESTERN WASHINGTON
THURSDAY AFTERNOON AND EVENING. HOWEVER THE 06Z TUE GFS RUN SHOWS A
MARKEDLY DIFFERENT SCENARIO WITH THE LOW MOVING ACROSS WESTERN
OREGON. AT ANY RATE THERE IS AT LEAST A POSSIBILITY OF A STORMY
THANKSGIVING IN WESTERN WASHINGTON. I KNOW...HARD TO BELIEVE.
MCDONNAL
.LONG TERM...THE BEST BET IS THAT WE WILL HAVE COOL SHOWERY
CONDITIONS ON FRIDAY IN THE POST-FRONTAL AIR MASS. THERE IS NOT A
GREAT DEAL OF CONFIDENCE IN ANY DETAILS BEYOND FRIDAY THOUGH...AS
THE LONGER RANGE MODEL RUNS HAVE BEEN SOMEWHAT INCONSISTENT. BEST
BET IS THAT A DIRTY RIDGE WILL BE OVER THE REGION ALLOWING WEAK
SYSTEMS TO BRING A BIT OF RAIN TO THE AREA AT TIMES. MCDONNAL
&&
.HYDROLOGY...AS THE FORECAST STANDS...THE ONLY SIGNIFICANT THREAT OF
RIVER FLOODING IS FOR THE SKOKOMISH RIVER AS A RESULT OF RAIN THIS
WEDNESDAY NIGHT THROUGH THURSDAY NIGHT. DETAILS FOLLOW.
THE WEAK SYSTEMS AFFECTING THE AREA TODAY AND TONIGHT WILL HAVE
LITTLE OR NO AFFECT ON WESTERN WASHINGTON RIVERS.
PRECIPITATION WILL INCREASE OVER THE OLYMPIC MOUNTAINS WEDNESDAY
AFTERNOON AND...AS MENTIONED ABOVE...THE WEDNESDAY NIGHT THROUGH
THURSDAY NIGHT PERIOD WILL BE RATHER WET. IT LOOKS REASONABLE THAT 3
TO 4 INCHES OF PRECIPITATION WILL FALL OVER THE OLYMPIC MOUNTAINS
AND PERHAPS THE NORTH CASCADES DURING THAT 36 HOUR PERIOD WITH THE
SNOW LEVELS FAIRLY HIGH -- PROBABLY FALLING FROM 7000 FT TO AROUND
4500 FT DURING THE PERIOD. MOST LIKELY THAT AMOUNT OF RAIN WOULD
PRESENT A FLOOD THREAT ONLY FOR THE SKOKOMISH RIVER.
A BIT LESS PRECIPITATION IS EXPECTED OVER THE CENTRAL CASCADES --
LIKELY IN THE 1.5 TO 3 INCH RANGE. AT THE OUTSIDE THAT WOULD PRESENT
A FLOOD THREAT FOR ONLY THE MOST FLOOD-PRONE RIVER REACHES LIKE THE
PUYALLUP RIVER NEAR ORTING.
COOLER SHOWERY WEATHER WILL ARRIVE FRIDAY...AND THE WEATHER LOOKS
LESS WET IN THE EXTENDED PERIOD FOR SATURDAY AND BEYOND.
NO FLOODING IS EXPECTED ON THE GREEN RIVER DURING THE PERIOD.
&&
.AVIATION...A WEAK UPPER LEVEL RIDGE WILL MOVE TO THE WA COAST
AROUND 18Z...AND ACROSS THE CASCADES BY 02Z. LOW AND MID LEVEL
MOISTURE WILL HANG AROUND THE W WA INTERIOR TODAY. EXPECT THE STABLE
AND MOIST AIR MASS TO KEEP MAINLY MVFR CIGS TODAY WITH SOME IFR THIS
MORNING ESP ALONG THE COAST AND CENTRAL SOUND. A WEAK WARM FRONT
WILL BRUSH THE AREA THIS EVENING KEEPING SOME MVFR CIGS AROUND.
KSEA...CIGS WILL BE MAINLY MVFR TODAY...BUT THE IFR CIGS WILL STAY
FOR SOME TIME THIS MORNING. EXPECT SURFACE WINDS TO REMAIN S THROUGH
THIS EVENING. 13
&&
.MARINE...SEAS ARE HIGH ENOUGH FOR SCA ALONG THE COAST AND W
ENTRANCE THIS MORNING. S-SE GRADIENTS RISE LATE THIS AFTERNOON AS
THE NE PACIFIC LOW TAKES OVER AND ANOTHER COLD FRONT APPROACHES
SLOWLY FROM THE NW. SCA WINDS EXPECTED COAST AND ENTRANCES THIS
EVENING THROUGH WED AFTERNOON. WILL STICK WITH THE GFS SOLUTION
BRINGING A LOW ACROSS THE AREA ON THURSDAY...THOUGH THERE IS STILL A
BIT OF DOUBT ABOUT JUST WHAT TRACK THE LOW WILL REALLY TAKE. STRONG
ONSHORE FLOW WILL FOLLOW THE LOW FOR GALES ON THE COAST AND THROUGH
THE STRAIT THU NIGHT.
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST...AND ENTRANCES TO THE STRAIT.
SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS AT GRAYS
HARBOR BAR.
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.