FXUS66 KSEW 021036
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST WED DEC 2 2009
.SYNOPSIS...AN UPPER RIDGE WILL PROVIDE TWO MORE DAYS OF COOL SUNNY
WEATHER WITH AREAS OF NIGHT AND MORNING FOG. A COLD FRONT AND UPPER
TROUGH WILL REACH THE AREA LATE IN THE WEEK FOR A CHANCE OF SHOWERS
FRIDAY AND SATURDAY...POSSIBLY FALLING AS SNOW LATE FRIDAY NIGHT AND
EARLY SATURDAY. THE PATTERN WILL KEEP COOL TEMPERATURES AND A THREAT
OF LIGHT PRECIPITATION IN THE FORECAST SUNDAY AND EARLY NEXT WEEK.
&&
.SHORT TERM...AN UPPER RIDGE IS OVER THE OFFSHORE WATERS...PROVIDING
DRY NORTHWEST FLOW ALOFT ACROSS THE REGION AS IT CONTINUES ITS SLOW
EASTWARD PROGRESSION. A SURFACE HIGH IS SITTING OVER THE CANADIAN
ROCKIES...GIVING WEAK NORTHERLY OFFSHORE FLOW OVER WESTERN
WASHINGTON. AT 3 AM TEMPERATURES RANGED FROM THE UPPER 20S TO THE
30S WITH PATCHY FOG IN SPOTS.
THE UPPER RIDGE WILL SHIFT SLOWLY EASTWARD TODAY AND TONIGHT...
PROGRESSING ALMOST TO THE COAST LATE TONIGHT. THE RIDGE WILL SHIFT
INLAND ON THURSDAY...BUT IT WILL WEAKEN ON ITS WAY IN RESPONSE TO AN
UPPER TROUGH APPROACHING BRITISH COLUMBIA AND AN AMPLIFYING RIDGE
UPSTREAM ALONG 140W. AT THE SAME TIME THE SURFACE HIGH OVER THE
CANADIAN ROCKIES WILL WEAKEN AND DRIFT SOUTH TO THE NORTHERN UNITED
STATES ROCKIES.
IN THE VERY SHORT TERM...FOG WILL BECOME MORE WIDESPREAD THIS
MORNING BEFORE DAYBREAK...BUT IT SHOULD BURN OFF MOST EVERYWHERE BY
LATE MORNING. TONIGHT WILL BE COLD AND CLEAR AGAIN...WITH AREAS OF
FOG FORMING OVERNIGHT. THE FOG COULD BE MORE STUBBORN ON THURSDAY.
THE MOVEMENT OF THE SURFACE WILL MEAN A DECREASING NORTHERLY
COMPONENT TO THE PRESSURE GRADIENT...RESULTING IN WEAKER WINDS
ACROSS THE WESTERN WASHINGTON LOWLANDS. OTHERWISE THURSDAY WILL BE
ANOTHER SUNNY COOL DAY.
THE CHANGES TO THE PATTERN UPSTREAM THAT BEGIN ON THURSDAY WILL
BEGIN TO PLAY OUT IN THE DAYS AHEAD. UNFORTUNATELY THE MODELS --
EVEN AT THIS FAIRLY CLOSE RANGE -- DISAGREE IN IMPORTANT WAYS. THE
GFS MODEL HAS BEEN CONSISTENTLY BRINGING THE TROUGH ACROSS BRITISH
COLUMBIA THURSDAY NIGHT...FORMING IT INTO A CLOSED UPPER LOW OVER
WESTERN MONTANA LATE FRIDAY...THEN SWINGING A COLD FRONT ACROSS
WESTERN WASHINGTON FROM THE NORTH LATE FRIDAY NIGHT. THIS SCENARIO
WOULD BRING LITTLE OR NO PRECIPITATION TO THE FORECAST AREA THROUGH
FRIDAY NIGHT...WITH AN ESSENTIALLY DRY COLD FRONT PASSAGE.
THE EUROPEAN...CANADIAN...AND LAM MODELS HAVE BEEN DIGGING THE
TROUGH MORE AGGRESSIVELY INTO WESTERN WASHINGTON ON FRIDAY...THEN
FORMING IT INTO A CLOSED UPPER LOW OVER WASHINGTON LATE FRIDAY OR
FRIDAY NIGHT. WHILE THESE MODELS HAVE NOT SHOWN MUCH PRECIPITATION
OVER THE FORECAST AREA...THE COOL AIR MASS WITH THE UPPER LOW WOULD
MEAN THAT SHOWERS COULD FALL AS SNOW CLOSE TO SEA LEVEL...ESPECIALLY
FRIDAY NIGHT.
TO COMPLICATE MATTERS...THE MOST RECENT RUNS OF THE EUROPEAN AND NAM
MODELS ARE TRENDING CLOSER TO WHAT THE GFS HAS BEEN SHOWING ALL
ALONG. WE WILL KEEP A CHANCE OF PRECIPITATION IN THE FORECAST FOR
FRIDAY AND FRIDAY NIGHT...MEANING MOST LIKELY IT WILL REMAIN DRY.
MCDONNAL
.LONG TERM...IN THE LONGER RANGE THE MODELS AGREE THAT THE UPPER LOW
WILL MOVE SOMEWHERE TO OUR SOUTH OVER THE WEEKEND...FOLLOWED BY
NORTHERLY FLOW ALOFT. MORE IMPORTANTLY THEY AGREE THAT A STRONG
SURFACE HIGH OF AROUND 1050 MB OVER WESTERN CANADA -- THE REFLECTION
OF AN ARCTIC MASS MOVING SOUTHWARD -- WILL EVENTUALLY PUSH COLDER
AIR INTO WESTERN WASHINGTON. THEY ALSO AGREE THAT THE THREAT OF
PRECIPITATION WILL BE LIMITED...HENCE ONLY CHANCE POPS IN THE
FORECAST.
EARLY NEXT WEEK THE UPPER LOW WILL PROBABLY WEAKEN AND SHIFT
EASTWARD...WITH A WEAK UPPER RIDGE APPROACHING THE WEST COAST OF THE
UNITED STATES. THE MODEL SOLUTIONS DIFFER IMPORTANTLY AGAIN --
ESPECIALLY IF YOU LIVE IN CALIFORNIA WHICH COULD BE IN FOR A BIG
RAIN EVENT -- BUT FOR WESTERN WASHINGTON THEY ALL SUGGEST A COOL
NORTHWEST FLOW ALOFT THROUGH TUESDAY WITH LIMITED THREAT OF
PRECIPITATION. MCDONNAL
&&
.HYDROLOGY...THERE IS NO THREAT OF RIVER FLOODING IN THE FORECAST
AREA FOR THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER.
NONE OF THE MODELS INDICATES SIGNIFICANT...LET ALONE FLOOD-PRODUCING
...PRECIPITATION DURING THE PERIOD. MCDONNAL
&&
.AVIATION...WHILE CLEAR SKIES AND DRY AIR DOMINATES EARLY THIS
MORNING...MOISTURE TRAPPED BELOW A SHALLOW SURFACE-BASED INVERSION
IS CAUSING LOCAL PROBLEMS. ONE AREA OF FOG WITH TOPS AROUND 006 HAS
FORMED OVER VALLEYS TO THE EAST OF THE SEATTLE-EVERETT METRO AREA
AND IS PUSHING SW INTO THE KBFI AND KSEA TERMINALS. OTHER AREAS
SEEING SOME STRATUS AND FOG ARE ALONG THE N SLOPES OF THE OLYMPIC
PENINSULA W OF KCLM. WITH SOLAR INSOLATION AT A MINIMUM THIS TIME OF
YEAR...IT WILL BE QUITE DIFFICULT FOR THE SHALLOW INVERSIONS TO MIX
OUT. SHORT TERM MODELS SHOW VERY LITTLE CHANGE IN THE PATTERN
THROUGH THU.
KSEA...NE WIND 8-11 KT WILL GENERALLY CONTINUE TODAY. FOG HAS FORMED
OVER THE VALLEYS E OF I-5 PER SATELLITE IMAGERY...AND THIS IS
ADVECTING SW INTO THE TERMINAL. WILL LIKELY KEEP AT LEAST VCFG TEMPO
1/2SM FG BKN001 IN THE FORECAST THRU ABOUT 18Z...BUT FORECAST WILL
BE SUBJECT TO CHANGE IN SHORT ORDER AS SATELLITE IMAGERY AND
OBSERVATIONS WARRANT. DRIER AIR IS FILTERING INTO THE AREA FROM
ALOFT AND THE NE...SO AT THIS TIME WILL NOT INDICATE A REPEAT FOR
TONIGHT. ALBRECHT
&&
.MARINE...HIGH PRES OVER SE BRITISH COLUMBIA WILL SAG SLOWLY SE INTO
MONTANA AND EASTERN WA BY THU. EXPECT LIGHT OFFSHORE FLOW TO
DOMINATE. THE WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA WILL
LIKELY HAVE SMALL CRAFT ADVISORY CONDITIONS THIS MORNING...BUT OTHER
AREAS WILL LIKELY REMAIN 20 KT OR LOWER THROUGH THU.
A WEAK FRONT WILL SLIDE S THROUGH THE AREA FRI AS IT DISSIPATES. IT
APPEARS THAT A STRONG 1055+ MB HIGH WILL FORM OVER SE YUKON SAT
MORNING AND STAY AROUND 1050 MB AS IT SAGS INTO CENTRAL BRITISH
COLUMBIA SUN. STRONG MODIFIED ARCTIC OUTFLOW SHOULD DEVELOP FRI
NIGHT OR SAT AND CONTINUE THROUGH THE WEEKEND. NEAR GALE NELY FLOW
SHOULD BE EXPECTED IN TYPICAL OUTFLOW AREAS IN THE NORTHERN INLAND
WATERS...THROUGH THE STRAIT OF JUAN DE FUCA -PARTICULARLY WEST
ENTRANCE- AND IN PORTIONS OF THE COASTAL WATER NEAR GAPS IN THE
TERRAIN. SMALL CRAFT ADVISORY CONDITIONS FOR N/NE FLOW ARE EXPECTED
IN THE OTHER WATERS. ALBRECHT
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY WEST ENTRANCE TO THE STRAIT THIS
MORNING.
$$
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