AREA FORECAST DISCUSSION
FXUS66 KSEW 250435
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PST TUE NOV 24 2009
.SYNOPSIS...A WEAK WARM FRONT WILL REMAIN NORTH OF WASHINGTON
TONIGHT WITH A COLD FRONT STALLED JUST OFF THE NORTH COAST. THE COLD
FRONT WILL PUSH INLAND WEDNESDAY EVENING THEN STALL OVER THE
REGION. A LOW PRESSURE CENTER WILL MOVE UP ALONG THE FRONT LATER
THURSDAY INTO FRIDAY BRINGING MORE RAIN TO THE AREA. ADDITIONAL
FRONTAL SYSTEMS WILL BRUSH THE NORTHWEST THROUGH THE WEEKEND WITH
DRIER WEATHER ON TAP FOR NEXT WEEK.
&&
.SHORT TERM...THE FLOW HAS LIFTED NWD THIS EVENING WITH MAINLY DRY
CONDITIONS NOW ACROSS WRN WA. NOT MUCH SHOWING UP ON RADAR AND IR
SATELLITE IMAGERY SHOWS ALL THE COLD CLOUD TOPS NOW OFF TO THE NW
WITH THE INCOMING FRONTAL BAND. BEING IN THE WARM SECTOR WITH LOW
LEVEL MOISTURE STILL PRESENT AND CLEARING MID/HIGH CLOUDS...FOG IS
BEGINNING TO FORM AROUND THE AREA. GRADIENTS ARE SLOW TO COME UP
TONIGHT SO FOG MAY BE MORE WIDESPREAD THROUGH MOST OF THE NIGHT
UNTIL SLY GRADIENTS AND BREEZY CONDITIONS PICK UP TOWARD DAYBREAK.
SOME SPOTS AROUND WRN WA ESPECIALLY S MAY HAVE SOME BREAKS OF
SUNSHINE WEDNESDAY MORNING AHEAD OF THE FRONT.
THE FRONTAL BAND WILL BRING STEADY RAIN TO THE FAR N COAST BY LATE
WEDNESDAY MORNING...SHIFTING INTO THE OLYMPICS/N INTERIOR AND N
CASCADES DURING THE AFTERNOON. BY WEDNESDAY EVENING...RAIN WILL
SPREAD FURTHER INLAND OVER CENTRAL WASHINGTON. SEE THE LATEST HYDRO
DISCUSSION FOR DETAILS ON POSSIBLE FLOOD CONCERNS.
MODELS HAVE ALL GONE TOWARD A WEAKER DOUBLE SURFACE LOW STRUCTURE
WHICH SWINGS NE TO JUST OFF THE WA/OREGON COAST THURSDAY
NIGHT...THEN MOVES INLAND WITH THE BULK OF SURFACE GRADIENT HEADING
INTO OREGON. THERE MAY BE SOME BREEZY CONDITIONS BEHIND THE LOW OVER
WA FRIDAY MORNING BUT MODELS HAVE DEFINITELY TRENDED AWAY FROM A
DEEPER CONSOLIDATED SYSTEM HEADING INTO WA WHICH WOULD HAVE RESULTED
IN MORE OF A WIND CONCERN.
POST LOW FLOW WILL QUITE STRONG AND MOIST ON FRIDAY WITH SNOW LEVELS
FALLING TO 2500-3000 FEET. THIS SHOULD PRODUCE SOME PRETTY GOOD
OROGRAPHIC SNOWFALL IN THE MOUNTAINS AND PASSES. A TRAILING SHORT
WAVE BRINGS ADDITIONAL MOISTURE IN NW FLOW LATE FRIDAY AFTERNOON AND
FRIDAY NIGHT. THIS COULD BE ENOUGH TO PRODUCE ADVISORY AMOUNTS OF
SNOW IN THE MOUNTAINS AFFECTING TRAVEL IN THE PASSES THE DAY AFTER
THANKSGIVING. MERCER
.LONG TERM...PREVIOUS DISCUSSION FOLLOWS...THROUGH THE WEEKEND WE
ARE EXPECTING TO HAVE A BROAD LONGWAVE RIDGE BUILDING OVER THE
REGION...HOWEVER THERE LOOKS AS THOUGH THERE WILL BE ABUNDANT
MOISTURE MOVING THROUGH THE RIDGE AND ACROSS WASHINGTON. SO
ALTHOUGH IT WILL BE MILD WITH LIFTING FREEZING LEVELS...I CAN NOT
REMOVE THE THREAT OF RAIN FROM THE FORECAST FOR THE WEEKEND AS A
RESULT OF ALL THE MOISTURE.
BEYOND THAT...THE LONGER RANGE MODELS ARE BEGINNING TO HINT AT A
PATTERN CHANGE BEGINNING WITH A SYSTEM DIVING SE ACROSS THE
NORTHWEST MONDAY NIGHT. IF THIS ACTUALLY OCCURS IT WOULD USHER IN A
PERIOD OF DRY WEATHER BY THE MIDDLE NEXT WEEK WITH SOME SUNSHINE.
BEING THAT MODELS AT THAT RANGE ARE SUSPICIOUS AT BEST WITH
TIMING...I LEFT THE SLIGHT CHANCE OF RAIN IN THE FORECAST FOR MONDAY
NIGHT AND TUESDAY. IF THIS TREND CONTINUES TOMORROW...I WILL LIKELY
MAKE SIGNIFICANT CHANGES TO THIS PERIOD OF THE FORECAST. CERNIGLIA
&&
.HYDROLOGY...THE LATEST 18Z/00Z GFS/NAM MODEL RUNS ARE CONTINUING
THE IDEA OF STALLING THE FRONTAL BOUNDARY OVER WRN WA FROM WEDNESDAY
AFTERNOON THROUGH FRIDAY MORNING. MODELS HAVE TRENDED TOWARD
SHIFTING THE HEAVIEST RAIN FROM THE OLYMPICS AND FAR N CASCADES TO
THE SW PORTION OF WA NEWD TO THE S PORTION OF THE N CASCADES. THIS
WOULD RESULT IN 24 HOUR RAINFALL AMOUNTS OF MAYBE 2-3 INCHES OVER
THE OLYMPICS/FAR N CASCADES WITH 3-4 INCHES FOR THE FAR S PORTION OF
THE N CASCADES. MUCH LESS PRECIP IS EXPECTED OVER THE SRN PORTION OF
THE CENTRAL CASCADES. HOWEVER...IT IS VERY DIFFICULT TO TELL IF THIS
SHIFT IN THE FRONTAL BOUNDARY WILL OCCUR DUE TO HIGH UNCERTAINTY OF
THE EVOLUTION OF THE LOW OVER THE PACIFIC TRACKING TOWARD THE PAC
NW. DIFFERENCES IN THE STRENGTH AND TRACK OF THE THE LOW WILL
DETERMINE WHERE THE MOISTURE PLUME SETS UP AND FOR HOW LONG. THERE
IS ENOUGH CONCERN AT THIS TIME TO WARRANT A FLOOD OUTLOOK...SEE THE
LATEST ESF FOR FURTHER INFORMATION.
THIS PATTERN IS NOT GOOD FOR GENERATING SIGNIFICANT RAINS IN THE
GREEN RIVER BASIN HOWEVER SO NO FLOODING IS EXPECTED ON THE GREEN
RIVER DURING THE PERIOD. MERCER
&&
.AVIATION...WEAK FLOW TONIGHT. THE LOWER AIR MASS VERY MOIST AND
STABLE WITH WIDESPREAD IFR/MVFR. LITTLE CHANGE THROUGH THE NIGHT...
BUT AFTER 12Z THE FRONTAL SYSTEM OFFSHORE WILL GRADUALLY APPROACH
THE AREA...WITH S WINDS EXPECTED TO SLOWLY INCREASE. CEILINGS WILL
RISE TO MVFR WEDNESDAY.
KSEA...LIKELY TO BE IFR OR LOWER THROUGH THE NIGHT WITH CALM WIND.
WILL SHOW SOME IMPROVEMENT AFTER 12Z...WITH SOME SOUTHERLY WIND...
BUT KEEP YOUR FINGERS CROSSED. CHB
&&
.MARINE...KEPT GALES ON THE COAST AND SOME SITES INLAND HAVE BEEN
RESPONDING TO A SLOW MOVING FRONT OFFSHORE. HOWEVER NOT REALLY
EXPECTING A LOT OF WIND INLAND UNTIL WEDNESDAY AFTERNOON AT THE
EARLIEST. GRADIENTS ON THE COAST BEGIN WEAKENING IN THE AFTERNOON.
LATEST NAM BRINGS A SURFACE LOW INTO THE SOUTH WASHINGTON COAST FROM
THE SOUTHWEST AND INLAND AS A TROUGH ON FRIDAY. STRONG ONSHORE FLOW
BEHIND THE LOW. THERE IS A CHANCE THIS WILL BE THE FIRST REAL WIND
OVER THE INLAND AREAS...AND IT COULD BRING GALES ALL WATERS. CHB
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD 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
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.