FXUS66 KSEW 252345
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
344 PM PST FRI DEC 25 2009
.SYNOPSIS...HIGH PRESSURE ALOFT OVER THE PACIFIC NORTHWEST WILL
CONTINUE THE CURRENT PATTERN OF COLD FROSTY NIGHTS AND MOSTLY SUNNY
AFTERNOONS THROUGH SATURDAY. STRONG OFFSHORE FLOW WILL BRING BREEZY
EASTERLY WINDS TO THE CASCADES AND FOOTHILLS THROUGH SATURDAY. A
SERIES OF WEAK SYSTEMS WILL START TO AFFECT THE AREA AROUND MONDAY.
A MORE ACTIVE PATTERN OF PACIFIC WEATHER SYSTEMS IS POSSIBLE
STARTING AROUND THE MIDDLE OF NEXT WEEK.
&&
.SHORT TERM...A STRONG UPPER LEVEL RIDGE AXIS WAS SITUATED DIRECTLY
OVER WRN AND CNTRL WA THIS AFTERNOON. IN ASSOCIATION WITH THE
RIDGE...WARM AIR ALOFT AND OUR 15-16 HOUR-LONG CLEAR NIGHTS THIS
TIME OF YEAR ARE LEADING TO STRONG SURFACE-BASED TEMPERATURE
INVERSIONS. THE 12Z KUIL SOUNDING SHOWED A TEMPERATURE OF 56F AT
ONLY 1300 FT MSL...SO THERE IS PLENTY OF WARM AIR ALOFT. THE UPPER
RIDGE WILL START TO LIFT OUT INTO INTERIOR B.C. AND ALBERTA ON SAT
AND SUN...LEADING TO SLOWLY LOWERING HEIGHTS. THIS WILL WEAKEN
LOW-LEVEL INVERSIONS ON SATURDAY WITH DISSIPATION FINALLY EXPECTED
ON SUNDAY AFTERNOON.
STRONG EASTERLY GRADIENTS HAVE PROVIDED SOME HORIZONTAL VENTILATION
OF POLLUTANTS TODAY...PARTICULARLY NEAR THE CASCADES AND OVER THE
NORTH INTERIOR. IT IS ONLY ALONG THE I-5 CORRIDOR FROM EVERETT SOUTH
WHERE POOR VERTICAL AND HORIZONTAL MIXING HAVE LED TO MODERATE AIR
QUALITY READINGS TODAY. THIS IS WHERE AIR QUALITY CONCERNS WILL
PERSIST TONIGHT AND SATURDAY MORNING.
FROM A MORE SENSIBLE STANDPOINT...THE WEATHER PATTERN WILL CONTINUE
TO PROVIDE CLEAR AND COOL WEATHER WITH FROSTY NIGHTS THROUGH SUNDAY
MORNING. A WAVE LIFTING NORTH WELL PAST 130W ON SUNDAY WILL TRY TO
BRING SOME LIGHT RAIN ONSHORE FROM THE SOUTHWEST...BUT THE RAIN WILL
LIKELY DISSIPATE BEFORE REACHING THE FORECAST AREA AS IT ENCOUNTERS
THE DRY AIR IN PLACE OVER WRN WA AND UNFAVORABLE OFFSHORE LOW-LEVEL
FLOW. BASICALLY...CLOUDS SHOULD INCREASE ON SUNDAY...BUT THE VAST
MAJORITY OF WRN WA WILL REMAIN DRY.
A WEAKENING UPPER TROUGH IS FORECAST TO CROSS WRN WA ON MONDAY AND
PRESENTS A HIGHER CHANCE OF BRINGING SOME DRIPS OF RAIN. HANER
.LONG TERM...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
&&
.HYDROLOGY...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 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.
EXPECTING A REPEAT OF LAST NIGHT. THAT WOULD BRING AREAS OF FOG TO
SOUTH PUGET SOUND AND THE SOUTHWEST INTERIOR...WHERE IT HAS NOT
COMPLETELY EVAPORATED YET...AND PATCHY FOG ELSEWHERE.
KSEA...NORTHEAST WIND 6-8 KT THROUGH TONIGHT SHOULD BE STRONG ENOUGH
TO PREVENT A LONG PERIOD OF LIFR CONDITIONS. MOST GUIDANCE INDICATES
CLEAR THROUGH THE NIGHT. THERE WAS BRIEF FOG LAST NIGHT. WILL
PROBABLY HAVE A TEMPO GROUP WITH IFR FOG TOWARD MORNING. CHB
&&
.MARINE...STRONG OFFSHORE FLOW HAS KEPT GALES GOING AT THE WEST
ENTRANCE ALL DAY AND HAVE EXTENDED THE GALE WARNING INTO THIS
EVENING. GRADIENTS IS SLACKENING A BIT AND THERE SHOULD BE A PERIOD
OF JUST 20-30 TONIGHT...HAVE INDICATED THAT 03Z-09Z IN THE GRIDS
WITH GALE FORCE WINDS RESUMING LATE TONIGHT AND CONTINUING THROUGH
THE DAY SATURDAY. ALSO ADDED A SMALL CRAFT ADVISORY FOR THE CENTRAL
STRAIT...AND KEPT THEM GOING ON 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 9 TO 10 FT OVER THE COASTAL WATERS
TODAY WILL GRADUALLY BUILD TO AROUND 15 FT ON SATURDAY NIGHT...THEN
GRADUALLY SUBSIDE SUNDAY THROUGH TUESDAY. EXPECT SMALL CRAFT
ADVISORY LEVEL HAZARDOUS SEAS AND ROUGH BAR CONDITIONS AT GRAYS
HARBOR FOR THE NEXT SEVERAL DAYS. CHB
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...AIR STAGNATION ADVISORY FOR PARTS OF THE WESTERN WASHINGTON
LOWLANDS THROUGH SATURDAY MORNING.
PZ...GALE WARNING UNTIL 6 PM FOR WEST ENTRANCE STRAIT.
SMALL CRAFT ADVISORY COAST AND CENTRAL STRAIT.
SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
$$
WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.
