FXUS66 KSEW 301131
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PST MON NOV 30 2009
.SYNOPSIS...A COLD FRONT WILL SWEEP ACROSS THE AREA FROM THE
NORTHWEST THIS AFTERNOON AND EVENING FOR A CHANCE OF PRECIPITATION.
HIGH PRESSURE WILL MOVE OVER THE REGION OVERNIGHT AND WILL PERSIST
THROUGH WEDNESDAY FOR A PERIOD OF DRY WEATHER.
&&
.SHORT TERM...
AN UPPER TROF AND ASSOCIATED CDFNT TO OUR NW WILL SWEEP ACROSS THE
CWA THIS AFTERNOON AND EVENING. ANTICIPATE PRIMARILY LIGHT PRECIP TO
OCCUR WITH AND BEHIND THE FRONT. THE PRECIP WILL THEN END WITH THE
PASSAGE OF THE UPPER TROF. SATELLITE IMAGERY SHOWED RELATIVELY CLOUD
FREE SKIES BEHIND THE TROF...THUS HAVE WENT WITH A CLEARING TREND
OVERNIGHT. LESS CLOUD COVER AND A COOLER AIR MASS WILL TRANSLATE TO
CHILLY MINIMUMS TONIGHT.
LOW LEVEL CAA WILL RESULT IN A FEW DEGREES OF COOLING ACROSS THE
LOWLANDS ON TUE. IN CONTRAST...WARMING ALOFT DUE TO THE PRESENCE OF
AN AMPLIFIED UPPER RIDGE OVER THE PAC NW WILL MEAN THAT THE HIGHER
ELEVATIONS WILL BE MILDER THAN NORMAL FOR THIS TIME OF THE YEAR. A
DRIER AIR MASS TUE NIGHT WILL RESULT IN EVEN COLDER MIN TEMPS ACROSS
THE LOWLANDS. HOWEVER THERE IS THE POSSIBILITY THAT ELY BREEZES
MIGHT KEEP TEMPS UP AT SOME LOCALES. ALTHOUGH THE INVERSION WILL
STRENGTHEN TUE NIGHT...IT IS UNLIKELY THAT THERE WILL BE WIDESPREAD
FOG DEVELOPMENT...MAINLY DUE TO DRYING IN THE LOWER LEVELS OF THE
AIR MASS.
TEMPS ON WED WILL BE ABOUT THE SAME OR A COUPLE OF DEGREES WARMER
ACROSS THE LOWLANDS. THE MOUNTAINS...HOWEVER...WILL WARM AN
ADDITIONAL 5-10 DEGREES DUE TO CONTINUED WARMING ALOFT. THE
EXCEPTION WILL BE THE CASCADE PASSES WHERE TEMPS WILL ACTUALLY
BECOME COOLER DUE TO CAA FROM THE E. OTHERWISE...ANTICIPATE AN UPPER
RIDGE TO KEEP THE REGION DRY ON WED.
.LONG TERM FROM THE PREV DISCUSSION...
LATEST GFS CONTINUES THE IDEA OF SNEAKING ONE MORE WEAK COLD FRONT
THRU THE AREA LATER THIS WEEK AND THEN AMPLIFYING THE RIDGE JUST
OFFSHORE IN RESPONSE TO A COUPLE OF HUGE STORMS THAT DEVELOP IN
SUCCESSION OUT ARND 170W. THAT SWITCHES THE WEATHER PATTERN TO COLD
AND DRY OVR WRN CANADA AND THE PACNW BY THE WEEKEND. A LITTLE EARLY
TO SOUND THE ALARMS ON THIS BUT FOR THE MOMENT THE GLOBAL MODELS
HAVE BEEN TRENDING IN THE SAME DIRECTION AND THE GFS CERTAINLY
DEVELOPS AWFULLY COLD AIR OVER THE PACNW A WEEK FROM NOW. SOMETHING
TO WATCH. 19
&&
.HYDROLOGY...
18-HOUR PRECIP AMOUNTS ENDING AT 0600 UTC TUE /10 PM TONIGHT/ ARE
EXPECTED TO BE UP TO A QUARTER OF AN INCH. HOWEVER...THE NORTH
CASCADES WILL LIKELY RECEIVE HIGHER AMOUNTS OR UP TO SIX TENTHS OF
AN INCH. THE SNOW LEVEL WILL GENERALLY RANGE FROM 4500 TO 5500 FT
TODAY OVER THE N CASCADES BUT WILL FALL INTO THE 2500 TO 3K FT RANGE
OVERNIGHT. CONSEQUENTLY...RIVER FLOODING WILL NOT BE A CONCERN.
SIGNIFICANT RAINFALL IS NOT EXPECTED THROUGH THIS WEEKEND. THUS THE
POTENTIAL FOR FLOODING WILL CONTINUE TO BE LOW. IN FACT...THE MEDIUM
RANGE SOLUTIONS WERE HINTING AT BELOW NORMAL TEMPS AND PRECIPITATION
FOR THE LATTER PART OF THE WEEK THRU THIS WEEKEND.
NO FLOODING IS EXPECTED ON THE GREEN RIVER THROUGH NEXT WEEKEND.
&&
.AVIATION...A COLD FRONT FROM CENTRAL BRITISH COLUMBIA SOUTHWESTWARD
ACROSS NORTHERN VANCOUVER ISLAND AND INTO THE PACIFIC WILL DROP SE
THROUGH THE AREA BY LATE THIS AFTERNOON. HIGH PRES WILL BUILD OVER
CENTRAL BRITISH COLUMBIA BEHIND THE FRONT.
LOW LEVEL LIFT AHEAD OF THE INCOMING FRONT IN COMBINATION WITH
INCREASING HIGH LEVEL MOISTURE HAS RESULTED IN STRATUS SPREADING
EAST INTO AREAS THAT WERE FOGGING IN EARLY THIS EVENING...FOG MIXED
OUT AND STRATUS SETTLED IN BETWEEN 007 AND 020. EXPECT THE STRATUS
TO CONTINUE THRU THE DAY WITH LIGHT RAIN SPREADING SE AHEAD OF THE
FRONT LATER THIS MORNING THROUGH LATE THIS AFTERNOON. INCREASED FLOW
WITH THE FRONT SHOULD KEEP THE BOUNDARY LAYER SUFFICIENTLY MIXED TO
KEEP CEILINGS FROM FALLING MUCH FROM EARLY THIS MORNING.
CONDITIONS ALOFT DRY BEHIND THE FRONT THIS EVENING. ANY CLEARING
WITH LIGHT OFFSHORE FLOW AND MORE RADIATIONAL COOLING MAY GIVE MORE
VERY LOW STRATUS OR FOG LATE TONIGHT THROUGH MIDDAY TUE AS INDICATED
BY THE NAM12 FORECAST SOUNDINGS AND STATISTICAL GUIDANCE.
KSEA...WIND S 5 KT THIS MORNING INCREASING TO 8-12 KT AHEAD OF THE
FRONT THIS AFTERNOON...THEN SHIFTING TO NLY 5-8 KT BEHIND THE FRONT
THIS EVENING. WIND WILL SHIFT TO LIGHT NE OVERNIGHT THROUGH TUE
MORNING. CIGS WILL FALL TO AROUND 010 ABOUT 12Z THEN LIFT BACK TO
020-025 MIDDAY AHEAD OF THE FRONT. A PERIOD OF -RA IS LIKELY AROUND
THE TIME OF FROPA 21Z-03Z. FOG OR LOW STRATUS FORMING TO THE NE OF
THE TERMINAL LATE TONIGHT WILL LIKELY PUSH SW OVERNIGHT GIVING LOW
IFR CONDITIONS IN CIGS AND POSSIBLY VIS AFTER 10Z AND CONTINUING TUE
MORNING. ALBRECHT
&&
.MARINE...A COLD FRONT FROM CENTRAL BRITISH COLUMBIA SOUTHWESTWARD
ACROSS NORTHERN VANCOUVER ISLAND AND INTO THE PACIFIC WILL DROP SE
THROUGH THE WATERS DURING THE AFTERNOON HOURS. SMALL CRAFT ADVISORY
LEVEL WINDS ARE EXPECTED ALONG THE COAST...THRU THE STRAIT...AND IN
THE NORTHERN INLAND WATERS TODAY.
HIGH PRES WILL BUILD BEHIND THE FRONT OVER CENTRAL BRITISH COLUMBIA
TONIGHT THEN SAG SE INTO E WA BY WED.
A LARGE 17 FT WNW SWELL TRAIN CONTINUES TO DECAY ONSHORE EARLY THIS
MORNING. THIS SWELL WILL SLOWLY SUBSIDE TONIGHT THROUGH TUE.
A STRONG COLD FRONT WILL SWEEP SE THROUGH THE WATERS LATER THU OR
FRI. ALBRECHT
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE
PZ...A SMALL CRAFT ADVISORY /FOR WINDS/ IS IN EFFECT FOR THE
NORTHERN INLAND WATERS AND CENTRAL/EASTERN STRAIT OF JUAN DE
FUCA TIL 3 PM TODAY.
A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE COASTAL WATERS
AND WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA TIL 9 PM
TONIGHT.
A SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS
IS IN EFFECT TIL 3 AM TUE.
$$
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
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