000
FXUS66 KSEW 130956
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT MON OCT 13 2008
.SYNOPSIS...A WEAK WARM FRONT MOVE ACROSS THE AREA TODAY. A COLD
FRONT WILL SWEEP ACROSS THE AREA EARLY THIS EVENING. EXPECT POST
FRONTAL SHOWERS AND ONSHORE FLOW TONIGHT WITH A PUGET SOUND
CONVERGENCE ZONE. TUESDAY THROUGH EARLY WEDNESDAY WILL BE DRY WITH
WEAK HIGH PRESSURE OVER THE AREA. A WEAK DISTURBANCE WILL BRING
CLOUDS AND A CHANCE OF A LITTLE RAIN TO THE AREA WEDNESDAY AFTERNOON
AND WEDNESDAY NIGHT. WESTERLY FLOW ALOFT WILL BRING MORE SYSTEMS
INTO THE REGION LATER IN THE WEEK.
&&
.SHORT TERM...THE ZONAL INDEX ACROSS THE PACIFIC IS INCREASING WITH
TIME. WITH THAT...THE MODEL SOLUTIONS ARE INCREASINGLY
DIVERGING...PARTICULARLY IN THE EXTENDED RANGE.
A WEAK WARM FRONT IS SPREADING LIGHT RAIN ONTO THE COAST AND INTO
THE NORTH INTERIOR EARLY THIS MORNING. THIS WILL GRADUALLY SPREAD
INLAND DURING THE EARLY MORNING HOURS. THERE MAY BE A BIT OF A BREAK
IN PRECIPITATION TO THE LEE OF THE OLYMPICS AS WARM ADVECTION
DECREASES AND INCREASING WESTERLY FLOW OVER THE TERRAIN INDUCES SOME
LEE SIDE SUBSIDENCE. RAINFALL AMOUNTS WITH THE WARM FRONT WILL BE
QUITE LIGHT...ON THE ORDER OF 0.1-0.15 INCHES COAST AND
MOUNTAINS...AND 0.05 INCHES OR SO IN THE INTERIOR LOWLANDS.
A COLD FRONT NOW NEARING 130W WILL SWEEP ACROSS THE AREA LATE THIS
AFTERNOON AND EVENING. THIS FRONT IS TAPPED INTO A MODERATELY MOIST
SUBTROPICAL PLUME EXTENDING NE FROM N OF HAWAII AND CONTAINING
PRECIPITABLE WATER AMOUNTS 1-1.4 INCHES. QPF FROM A BLEND OF GLOBAL
AND MESOSCALE MODELS GIVES 0.6-0.9 INCHES OF RAIN IN THE CENTRAL
CASCADES AND EAST CENTRAL PUGET SOUND LOWLANDS IN THE WAKE OF THE
COLD FRONT THIS EVENING. MUCH OF THIS PRECIPITATION WILL BE
ASSOCIATED WITH AN ACTIVE POST FRONTAL PUGET SOUND CONVERGENCE ZONE.
CONDITIONS RAPIDLY DRY TO THAT ONLY CLOUDS ARE LEFT TUE MORNING.
MOIST GROUND AND COOLING WILL GIVE PATCHY MORNING FOG TO FAVORED
LOCATIONS TUE MORNING. EXPECT PARTLY SUNNY SKIES TUE AFTERNOON AS
WEAK HIGH PRES BUILDS INTO THE AREA.
RADIATIONAL COOLING AND LIGHT PRES GRADIENTS WILL RESULT IN AN
INCREASE IN COVERAGE AND DURATION OF FOG LATE TUE NIGHT INTO WED
MORNING. BEGINNING WED AFTERNOON MODELS SHOW INCREASING DIVERGENCE
IN SOLUTIONS AS THE GFS BRINGS A WARM FRONT OR WEAK LOW NE FROM THE
OREGON WATERS AND INTO SOUTHERN PORTIONS OF THE AREA WED EVENING.
THE ECMWF AND NAM ARE DRYER AND WEAKER WITH THIS FEATURE. AT THIS
POINT WILL INCREASE CLOUDS WED AFTERNOON AND EVENING AND GIVE A
CHANCE OF RAIN TO THE AREA. LATER SHIFTS CAN REFINE THIS AS WE GET
CLOSER TO THE EVENT AND MODELS CONVERGE. ALBRECHT
.LONG TERM...WE MOVE INTO A MORE ACTIVE PERIOD THU AND BEYOND AS
INCREASING WESTERLY FLOW BRINGS A SERIES OF FRONTS ACROSS THE AREA.
THE PROBLEM IS TIMING. THE GFS IS FASTER IN BRINGING A FRONT INTO
THE AREA SOMETIME THU WHILE THE ECMWF IS SLOWER. AT THIS TIME THE
FORECAST IS A BROADBRUSH WITH CLOUDS AND A CHANCE OF RAIN...LIKELY
POPS ON THE COAST. WILL NOT MAKE ANY CHANGES TO THIS THINKING AT
THIS TIME. ALBRECHT.
&&
.AVIATION...UPPER LEVEL RIDGE WILL BREAK DOWN TODAY AS COLD FRONT
CURRENTLY LOCATED ACROSS SOUTHERN BC TRACKS SE AND INTO W WA. THIS
FRONT WILL BRING AN INCREASE IN RAIN THIS MORNING /MVFR CONDITIONS/
ALONG WITH BREEZY SOUTHERLY FLOW...ESPECIALLY ACROSS THE INTERIOR.
THE COLD FRONT WILL PASS THROUGH THE COAST LATE MORNING...THEN MOVE
THROUGH THE INTERIOR THIS AFTERNOON. SHOWERS ARE EXPECTED BEHIND THE
FRONT WITH ASSOCIATED SHORT WAVE TROUGH ALOFT. MODELS INDICATING
PUGET SOUND CONVERGENCE ZONE FORMING NEAR KPAE/KAWO THIS
EVENING...THAT WILL SAG SOUTH INTO KING CO AS THE NIGHT GOES ON.
SHOWERS WILL BE MAINLY CONFINED TO THE CASCADES DURING THE OVERNIGHT
HOURS. 33
KSEA...CIGS WILL GRADUALLY LOWER THIS MORNING WITH MVFR CONDITIONS
EXPECTED. LIGHT RAIN ALSO POSSIBLE EARLY...WITH AN INCREASE IN
ACTIVITY BY MID MORNING. IN ADDITION...SOUTHERLY WINDS WILL RISE TO
10 TO 20 KNOTS. EXPECT SHOWERS IN THE VICINITY THIS EVENING WITH THE
DEVELOPING PSCZ. 33
&&
.MARINE...BLUSTERY DAY AHEAD AS COLD FRONT OVER SOUTHERN BC SLIDES
SE THROUGH W WA. EXPECTING SMALL CRAFT ADVISORY WINDS ACROSS ALL
WATERS TODAY. NW/W ONSHORE FLOW WILL INCREASE THIS AFTERNOON BEHIND
THE FRONT LEADING TO A STRONG WLY SURGE THROUGH THE STRAIT OF JUAN
DE FUCA. MEANWHILE...SEAS WILL ALSO BUILD TO NEAR 10 FT THIS
EVENING. SCA WINDS WILL CONTINUE THROUGH THE STRAIT AND N INLAND
WATERS THIS EVENING WITH WINDS GRADUALLY WEAKENING EARLY TUE
MORNING. BROAD AREA OF HIGH PRES WILL SHIFT INTO THE COASTAL WATERS
ON TUE WITH LIGHT ONSHORE FLOW EXPECTED. ANOTHER FRONTAL SYSTEM WILL
APPROACH W WA WED AFTERNOON/EVENING. 33
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR ALL WATERS.
$$
WWW.WEATHER.GOV/SEATTLE
FOR THE ENHANCED AFD PLEASE VISIT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML (ALL LOWERCASE)
