FXUS66 KSEW 260617
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
1014 PM PST FRI DEC 25 2009
.SYNOPSIS...HIGH PRESSURE ALOFT WILL PREVAIL OVER THE REGION THROUGH
SUNDAY FOR CONTINUED DRY WEATHER. MEANWHILE...INCREASINGLY STRONG
OFFSHORE FLOW WILL RESULT IN AREAS OF HIGH WINDS OVER PORTIONS OF
THE EAST PUGET SOUND LOWLANDS OVERNIGHT THROUGH SATURDAY. A WEAK...
LOW PRESSURE SYSTEM WILL MOVE OVER THE REGION ON MONDAY FOR THE
POSSIBILITY OF SPOTTY...LIGHT RAIN. EXPECT A MORE ACTIVE PATTERN TO
SET UP ABOUT THE MIDDLE OF NEXT WEEK FOR UNSETTLED WEATHER
CONDITIONS.
&&
.SHORT TERM...
THE SEATTLE /SEA/ TO WENATCHEE /EAT/ PRES GRADIENT WAS -12.6 MB AT 9
PM...INDICATIVE OF A MUCH STRONGER ELY GRADIENT THAN WHAT THE MODELS
PREDICTED. FOR EXAMPLE...THE 12Z RUN OF THE GFS INDICATED THAT THE
SEA-EAT GRADIENT WOULD BE NEAR -6 MB ABOUT THIS TIME WHEREAS THE NAM
PREDICTED NEAR -9 MB. TYPICALLY...AN OFFSHORE GRADIENT OF -13 MB
BETWEEN SEA AND EAT RESULTS IN HIGH WINDS OVER PORTIONS OF THE EAST
PUGET SOUND LOWLANDS. SINCE THE MODELS...DESPITE BEING WOEFULLY WEAK
ON THE GRADIENT...INDICATED THAT THE ELY GRADIENT WILL TIGHTEN
THROUGH SATURDAY MORNING...FELT THAT IT WOULD BE PRUDENT TO ISSUE A
HIGH WIND WARNING FOR THE EAST WIND PRONE AREAS OF THE EAST PUGET
SOUND LOWLANDS. ALSO ISSUED A WIND ADVISORY FOR THE EAST SIDE OF THE
SEATTLE AND TACOMA METRO AREAS FOR LATER TONIGHT.
WILL NEED TO ALSO MONITOR WINDS ON THE COAST AND OVER THE WESTERN
STRAIT OF JUAN DE FUCA AS THERE COULD BE LOCALIZED EASTERLY WIND
GUSTS TO AT LEAST 50 MPH SATURDAY MORNING.
MEANWHILE...A STRONG...UPPER RIDGE...CENTERED OVER WRN CANADA...
WILL PREVAIL OVER THE REGION THRU SUNDAY...ALTHOUGH HEIGHTS/
THICKNESSES WILL GRADUALLY LOWER DURING THE PERIOD. THE RIDGE WILL
ALSO SERVE TO KEEP THE LARGE TROF OFFSHORE AT BAY. INCREASED ELY
FLOW WILL RESULT IN IMPROVED MIXING OVER PORTIONS OF THE INTERIOR
LOWLANDS ON SATURDAY. THUS DAYTIME TEMPS AT SOME LOCALES WILL NEED
TO BE BUMPED UP ABOUT FIVE DEGREES OR SO. IT IS POSSIBLE THAT
SEA-TAC MAY END UP BEING NEAR 50 DEGREES /OR HIGHER/ ON SATURDAY. IN
GENERAL...TEMPS ON SUNDAY SHOULD BE ABOUT THE SAME AS ON SATURDAY.
THE UPPER RIDGE IS EXPECTED TO SHIFT EWD SUNDAY NIGHT AND MON...
THEREBY ALLOWING A WEAK...UPPER TROF TO MOVE OVER THE REGION. AT
THIS TIME...THE MODELS WERE NOT PRODUCING MUCH PRECIP OVER THE CWA
WITH THIS SYSTEM. IN FACT...DUE TO CONTINUED OFFSHORE FLOW...IT IS
LIKELY THAT MOST AREAS WILL REMAIN DRY. THE MAIN IMPACT WILL BE
INCREASED CLOUD COVER.
.LONG TERM FROM THE PREV DISCUSSION...
IT LOOKS LIKE THE BOTTLENECK OF HIGH AMPLITUDE AND SLOW MOVING
WEATHER FEATURES ACROSS THE ERN PAC AND NORTH AMERICA WILL LOOSEN UP
DURING THE EXTENDED PERIOD. THIS SHOULD BRING A RETURN TO A MORE
TYPICAL PROGRESSION OF MID-LATITUDE WEATHER SYSTEMS MOVING ONSHORE
THE PAC NW UNDER THE INFLUENCE OF MORE CONSOLIDATED ZONAL FLOW. THE
FIRST OF THESE SYSTEMS SHOULD AFFECT WRN WA AROUND TUE NIGHT OR
WED...FOLLOWED BY A STRONGER WARM FRONT/COLD FRONT COMBO ON NEW
YEARS EVE AND DAY WHICH COULD BRING MODERATE RAINFALL AMOUNTS. HANER
&&
.AIR QUALITY...WARM AIR ALOFT AND OUR 15-16 HOUR-LONG CLEAR NIGHTS
THIS TIME OF YEAR WERE LEADING TO STRONG SURFACE-BASED TEMPERATURE
INVERSIONS. THE 00Z KUIL SOUNDING SHOWED A TEMPERATURE OF 54 DEGREES
NEAR 1300 FT MSL...SO THERE WAS PLENTY OF WARM AIR ALOFT. LOWERING
500 MB HEIGHTS SATURDAY THRU SUNDAY WILL WEAKEN THE LOW LEVEL
INVERSIONS...WITH DISSIPATION EXPECTED SUNDAY AFTERNOON.
INCREASING ELY PRES GRADIENTS OR OFFSHORE FLOW WILL CONTINUE TO
IMPROVE MIXING OVER THE EAST PUGET SOUND LOWLANDS WHERE LOCALLY HIGH
WINDS WERE OCCURRING. THE HIGH WINDS OVER THIS AREA WILL BECOME MORE
WIDESPREAD OVERNIGHT...THUS THE AIR STAGNATION ADVISORY WAS
CANCELLED FOR THIS ZONE.
IT WAS ONLY ALONG THE I-5 CORRIDOR FROM EVERETT SOUTH TO TACOMA
WHERE POOR VERTICAL AND HORIZONTAL MIXING HAVE LED TO MODERATE AIR
QUALITY READINGS. THIS WAS WHERE AIR QUALITY CONCERNS WILL PERSIST
THRU SATURDAY MORNING. HANER/05
&&
.HYDROLOGY FROM THE PREV DISCUSSION...
THERE IS NO THREAT OF FLOOD-PRODUCING RAINFALL THROUGH NEXT
WEDNESDAY. THE THREAT OF WIDESPREAD FLOODING FROM NEXT THURSDAY
THROUGH THE FIRST WEEKEND OF 2010 IS LOW. A WEATHER SYSTEM AROUND
NEW YEARS EVE OR NEW YEARS DAY COULD BE ACCOMPANIED BY MODERATE
RAINFALL AMOUNTS. THIS COULD BRING RISES ON AREA RIVERS...BUT
FLOOD-PRODUCING RAINFALL AMOUNTS ARE NOT EXPECTED. RIVER LEVELS WILL
ALSO START OUT LOW AFTER AN EXTENDED PERIOD OF DRY WEATHER. FLOODING
ON THE GREEN RIVER IS NOT EXPECTED FOR THE NEXT 10 DAYS. HANER
&&
.AVIATION...LIGHT FLOW ALOFT UNDER A STRONG UPPER RIDGE...GRADUALLY
SWITCHING TO LIGHT SOUTHEAST FLOW TONIGHT AS THE RIDGE MOVES SLOWLY
EAST. AT THE SURFACE STRONG HIGH PRESSURE EXTENDING FROM SOUTHEAST
BRITISH COLUMBIA INTO NORTHERN IDAHO WILL GIVE CONTINUING OFFSHORE
FLOW THROUGH SATURDAY. THE CROSS CASCADE GRADIENT IS ALREADY DOWN TO
-12 MB.
THE COMBINATION OF WARM AIR ALOFT WITH THE RIDGE AND A RELATIVELY
COOL AIR MASS AT THE SURFACE IS PRODUCING A VERY STABLE AIR MASS. A
STRONG INVERSION FROM THE SURFACE TO AROUND 5000 FT WILL CONTINUE
THROUGH TONIGHT.
WITH THE STRONGER CROSS CASCADE GRADIENT EXPECT THE FOG TO BE
CONFINED TO LOCATIONS SOUTH OF SEATTLE DOWN TO ABOUT KELSO OVERNIGHT
INTO THE MORNING HOURS.
KSEA...NORTHEAST WIND 6-8 KT OVERNIGHT. WITH THE STRONG CROSS
CASCADE GRADIENT WINDS COULD COME AROUND TO MORE EASTERLY AFTER 12Z.
THIS SHOULD PREVENT RESTRICTED VISIBILITIES AT THE AIRPORT SATURDAY
MORNING. FELTON
&&
.MARINE...STRONG OFFSHORE FLOW WILL KEEP GALES GOING AT THE WEST
ENTRANCE THROUGH SATURDAY. SMALL CRAFT ADVISORIES CONTINUING FOR THE
THE CENTRAL STRAIT AND THE COAST. SOME OF THESE SMALL CRAFT
ADVISORIES COULD KEEP GOING FOR SEVERAL DAYS.
THE STRENGTH OF OFFSHORE FLOW WILL EASE SOMEWHAT SUNDAY AND MONDAY
AS A SPLITTING FRONT DISSIPATES OVER THE OFFSHORE WATERS. ANOTHER
WEAK FRONT WILL PROBABLY MOVE INTO THE OFFSHORE WATERS FROM THE WEST
ON TUESDAY.
IN ADDITION WESTERLY SWELLS OF 11 FEET OVER THE COASTAL WATERS WILL
GRADUALLY BUILD TO AROUND 15 FT ON SATURDAY NIGHT...THEN GRADUALLY
SUBSIDE SUNDAY THROUGH TUESDAY. EXPECT SMALL CRAFT ADVISORY LEVEL
ROUGH BAR CONDITIONS AT GRAYS HARBOR FOR THE NEXT SEVERAL DAYS.
FELTON
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...A HIGH WIND WARNING IS IN EFFECT FOR THE EAST PUGET SOUND
LOWLANDS FROM MIDNIGHT TONIGHT TIL 4 PM SATURDAY.
A WIND ADVISORY IS IN EFFECT FOR THE EASTERN SEATTLE AND TACOMA
METRO AREAS FROM MIDNIGHT TONIGHT TIL 2 PM SATURDAY.
AN AIR STAGNATION ADVISORY REMAINS IN EFFECT FOR PARTS OF THE
INTERIOR LOWLANDS TIL 10 AM SATURDAY.
PZ...GALE WARNING IN EFFECT FOR THE WEST ENTRANCE TO THE STRAIT OF
JUAN DE FUCA THROUGH SATURDAY AFTERNOON.
SMALL CRAFT ADVISORY IS IN EFFECT FOR THE COASTAL WATERS AND
CENTRAL STRAIT OF JUAN DE FUCA.
$$
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.
