FXUS66 KSEW 301640
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
840 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 RAIN. AN UPPER
LEVEL RIDGE AND SURFACE HIGH BUILDING OVER THE REGION TONIGHT...WILL
KEEP THE AREA DRY THROUGH THURSDAY. ANOTHER WEATHER SYSTEM MAY BRING
SOME RAIN TO THE AREA THURSDAY NIGHT OR FRIDAY.
&&
.SHORT TERM...MODELS SEEM ON TRACK WITH THE APPROACHING COLD
FRONT/UPPER LEVEL TROUGH OVER VANCOUVER ISLAND THIS MORNING. IR
IMAGERY SHOWS THE CLOUD TOPS WARMING THE PAST COUPLE OF HOURS SO THE
SYSTEM MAY BE EVEN WEAKER THAN FORECAST. STILL EXPECT A FEW HOURS OF
LIGHT RAIN OVER MAINLY THE NORTH AND CENTRAL ZONES THIS
AFTERNOON...AND SOME LINGERING SHOWERS OVER THE CASCADES THIS
EVENING. FROPA ON THE N COAST SHOULD BE AROUND 21Z...AND ALONG A
HOQUIAM-SEATTLE LINE AROUND 00Z. S GRADIENTS AHEAD OF THE SYSTEM
HAVE BEEN PRODUCING SMALL CRAFT ADVISORY WINDS OVER THE N INTERIOR
THIS MORNING...BUT EXPECT WINDS TO DROP OFF AND TURN N-NE BEHIND THE
FRONT. GFS SHOWS FAIRLY STRONG N-NE GRADIENTS DEVELOPING TONIGHT AS
A SURFACE HIGH BUILDS OVER SOUTHERN B.C. THIS MAY BE OVERDONE AS
THE GFS OFTEN OVER ESTIMATES THE IMPACT OF COLD AIR CROSSING THE
INTERIOR B.C.
A NICE STRONG UPPER LEVEL RIDGE BUILDS OFFSHORE ALONG 130W TONIGHT
THEN MOVES SLOWLY INLAND THROUGH THURSDAY...FOR AN EXTENDED DRY
PERIOD. ABOVE MENTIONED SURFACE HIGH OVER SOUTHERN B.C. SHOULD
PROVIDE N-NE FLOW THOUGH PUGET SOUND THROUGH THURSDAY...WITH
EASTERLY FLOW DEVELOPING NEAR THE CASCADES WEDNESDAY AND THURSDAY.
FOG AND LOW CLOUDS WILL BE THE MAIN FORECAST PROBLEM. 12Z NAM SHOWS
HIGH BOUNDARY LAYER RH TUESDAY MORNING...SO THERE COULD BE LOTS OF
FOG OR LOW CLOUDS AROUND INTO EARLY AFTERNOON. 12Z NAM BUFR SOUNDING
FOR SEA-TAC SHOWS DRYING AFTER 21Z SO THE AFTERNOON COULD BE SUNNY.
OLYMPIA SOUNDING TAKES LONGER TO CLEAR OUT SO THE SOUTHERN ZONES MAY
NOT CLEAR OUT MONDAY. NAM HAS LOWER BOUNDARY LAYER RH WEDNESDAY AND
THURSDAY...PROBABLY DUE TO INCREASING EASTERLY FLOW AND SUBSIDENCE
SO WILL PROBABLY PLAY THIS AS PATCHY FOG. TEMPS WILL BE PRETTY CLOSE
TO CLIMO...LOWS IN THE 30S...HIGHS IN THE 40S. KAM
.LONG TERM...THURSDAY STILL LOOKS DRY WITH BOTH GFS AND ECMWF
KEEPING THE RIDGE OVER THE AREA...AND MAINTAINING EASTERLY OFFSHORE
LOW LEVEL FLOW FROM THE SURFACE HIGH OVER THE NORTHERN ROCKIES.
MODELS DIVERGE FROM THIS POINT ON...SO THERE IS LOW CONFIDENCE IN
THE FRIDAY-SUNDAY FORECAST. 06Z GFS REMAINS PROGRESSIVE AND
FASTEST...BRINGING ANOTHER SHORTWAVE TROUGH AND COLD FRONT ACROSS
THE AREA THURSDAY NIGHT...THEN BUILDS A GIANT HIGH-AMPLITUDE RIDGE
ALONG 135W THAT RESULTS IN COOL DRY NORTH FLOW ACROSS W WA.
00Z ECMWF IS SLOWEST AND ENDS UP DEVELOPING A CLOSED OFF UPPER LEVEL
LOW OFFSHORE OUT OF THE THURSDAY SHORTWAVE TROUGH. IT THEN HAS THE
LOW LOITERING AROUND JUST OFFSHORE...KEEPING A CHANCE OF RAIN IN THE
FORECAST THROUGH THE WEEKEND. 00Z CANADIAN COMPROMISES BETWEEN THE
TWO IN SPEED...BUT OTHERWISE LEANS TOWARD THE DRIER GFS. WILL HAVE
TO SEE WHAT THE 12Z RUNS SHOW BEFORE MAKING ANY DECISIONS. SOLUTIONS
SEEMS MORE REASONABLE...BUT IF THE MODEL DISAGREEMENT CONTINUES...SO
WILL THE CLIMO CHANCE OF RAIN FORECAST. KAM
&&
.HYDROLOGY...NO HYDROLOGICALLY SIGNIFICANT RAINFALL IS EXPECTED FOR
THE NEXT 10 DAYS...OR THROUGH WEDNESDAY DEC 9TH. A WEAK COLD FRONT
WILL BRING SOME LIGHT RAIN TO THE AREA TODAY. ANOTHER WEAK SYSTEM
MIGHT BRING SOME LIGHT RAIN THURSDAY NIGHT...AND POSSIBLY
THROUGH THE WEEKEND AS WELL. BUT THIS UNCERTAINTY DEPENDS UPON
CURRENTLY INCONSISTENT MODELS. REGARDLESS...THE CURRENT TENDENCY FOR
HIGH AMPLITUDE RIDGING OFFSHORE ALONG 135W OR SO IS NOT A HEAVY RAIN
OR FLOODING PATTERN. KAM
&&
.AVIATION...A COLD FRONT WILL MOVE SOUTHEAST THROUGH THE AREA BY
LATE THIS AFTERNOON. HIGH PRES WILL BUILD OVER CENTRAL BRITISH
COLUMBIA BEHIND THE FRONT. WESTERLY FLOW ALOFT WILL BECOME MORE
NORTHWESTERLY THIS EVENING.
A WIDE VARIETY OF CEILINGS AND VISIBILITIES EXIST THIS MORNING...
FROM IFR CEILINGS WITH LOCAL LIFR VISIBILITY IN FOG AROUND CENTRAL
PUGET SOUND AND ON THE COAST...TO BASICALLY NO CEILINGS IN NORTHERN
AREAS AS JUST HIGH CLOUDS FROM THE APPROACHING FRONT ARRIVE. THE
TREND THROUGH THE DAY SHOULD BE FOR CEILINGS TO CONVERGE ON MVFR
VALUES AS THE FRONT MOVES THROUGH. CLOUDS ASSOCIATED WITH THE FRONT
WILL THICKEN AND LOWER...WHILE FOG AND LOW STRATUS WILL MIX OUT TO
REVEAL SOMEWHAT HIGHER STRATUS.
THE FRONT IS FORECAST TO PASS THROUGH WESTERN WASHINGTON LATE THIS
AFTERNOON OR EARLY THIS EVENING. CEILINGS WILL BRIEFLY IMPROVE...
WITH EVEN BRIEFER CLEARING POSSIBLE...BUT LOW STRATUS SHOULD RETURN
TONIGHT WITH PATCHY FOG.
KSEA...S WIND 8-12 KT THIS MORNING BECOMING SW 10-14 KT AS THE FRONT
PASSES THIS AFTERNOON...THEN SHIFTING TO NLY 5-8 KT BEHIND THE FRONT
BY 03Z OR SO. A PERIOD OF LIGHT RAIN IS LIKELY AROUND THE TIME OF
FRONTAL PASSAGE 21Z-03Z. IFR OR LOW IFR CONDITIONS ARE POSSIBLE
AFTER 10Z AND CONTINUING TUE MORNING. CHB
&&
.MARINE...A COLD FRONT WILL MOVE SOUTHEAST THROUGH THE WATERS THIS
AFTERNOON. SMALL CRAFT ADVISORY STRENGTH SOUTH WINDS ARE ALREADY
OCCURRING AT THE WEST ENTRANCE AND OVER THE NORTHERN INLAND
WATERS...AND WILL LIKELY OCCUR SOON ON THE COAST AND AT THE EAST
ENTRANCE. LATE THIS AFTERNOON A WESTERLY PUSH OF SMALL CRAFT
ADVISORY STRENGTH WINDS DOWN THE STRAIT IS LIKELY.
HIGH PRES WILL BUILD BEHIND THE FRONT OVER CENTRAL BRITISH COLUMBIA
TONIGHT THEN SHIFT SOUTHEAST INTO EASTERN WASHINGTON BY WEDNESDAY.
A LARGE 17 FT SWELL TRAIN CONTINUES TO REACH THE COAST THIS MORNING.
THIS SWELL WILL SLOWLY SUBSIDE TONIGHT THROUGH TUESDAY.
ANOTHER COLD FRONT COULD MOVE SOUTHEAST THROUGH THE WATERS LATER
THURSDAY OR FRIDAY. CHB
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE
PZ...SMALL CRAFT ADVISORY COAST...STRAIT...AND NORTHERN INLAND
WATERS.
SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
$$
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