FXUS66 KSEW 261727
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
920 AM PST SAT DEC 26 2009
.SYNOPSIS...HIGH PRESSURE ALOFT WILL PREVAIL OVER THE REGION TODAY
FOR CONTINUED DRY WEATHER. STRONG OFFSHORE FLOW WILL RESULT IN
AREAS OF HIGH WINDS OVER PORTIONS OF THE EAST PUGET SOUND LOWLANDS
INTO THIS AFTERNOON. WEAK LOW PRESSURE WILL MOVE OVER THE REGION
LATE SUNDAY INTO MONDAY FOR A CHANCE OF LIGHT RAIN. A MORE ACTIVE
WEATHER PATTERN IS EXPECTED FROM THE MIDDLE OF NEXT WEEK ONWARD AS A
SERIES OF SYSTEMS AFFECT THE AREA.
&&
.SHORT TERM...CLEAR SKIES THANKS TO A STRONG RIDGE ALOFT JUST TO THE
NE. THIS RIDGE HAS GENERATED A STRONG PRESSURE GRADIENT DIFFERENCE
ACROSS THE CASCADES OVERNIGHT AND THIS REMAINS IN PLACE THIS
MORNING. WE HAVE HAD A COUPLE OF REPORTS OF STRONG WINDS RIGHT UP
IN THE GAPS LIKE NORTH BEND. FURTHER OUT THE WIND LETS UP
SIGNIFICANTLY AND ONLY A FEW LOCATIONS IN THAT ZONE HAVE A BREEZE.
FURTHER WEST IN THE LOCAL SEATTLE ZONE...MOST SITES ARE CALM WITH
THE EXCEPTION OF HILL TOP LOCATIONS LIKE SEATTLE WHICH ARE ABLE TO
TAP INTO THE WINDS A BIT OFF THE SURFACE AS THE PROFILIERS
INDICATE. BUT EVEN THERE GUSTS WOULD ONLY PEAK OUT AROUND 25-30
MPH. SO WITH THAT IN MIND WILL BE PULLING THE LOCAL WIND ADVISORY
AND LEAVING THE HIGH WIND WARNING IN PLACE FOR THE LOCAL GAP WINDS.
TONIGHT THE EASTERLY GRADIENT RELAXES SOMEWHAT WHICH WILL ELIMINATE
THE STRONG GAP WINDS BUT ALSO ALLOW THE NIGHT TIME INVERSION TO SET
UP AGAIN ACROSS A MAJORITY OF THE AREA.
GRADIENTS REMAIN WEAK SUNDAY AND A WEAK FRONT WILL DRIFT NORTH
ACROSS THE AREA SUNDAY NIGHT INTO MONDAY MORNING. HOWEVER...WHATS
LEFT OF THIS FRONT WILL BE VERY WEAK AND WILL LIKELY NOT MIX UP THE
LOW LEVELS MUCH AT ALL. MODEL SOUNDINGS INDICATE THAT THE INVERSION
WILL REMAIN BELOW 1000 FT WITH WINDS GENERALLY AROUND 5 KT OR LESS.
WE CURRENTLY HAVE A LOW CHANCE OF RAIN WITH THIS FRONT FOR MONDAY
AND THAT LOOKS GOOD AT THIS POINT. WITH THE WEAK NATURE OF THE
FRONT I WOULDN'T WANT TO EMPHASIZE IT ANY MORE THAN THAT. CERNIGLIA
.LONG TERM...FROM PREVIOUS DISCUSSION...THE MODELS CONTINUE TO HAVE
DIFFICULTLY WITH THE DETAILS INTO EARLY NEXT WEEK AS THE FLOW ALOFT
ATTEMPTS TO BECOME CONSOLIDATED. THE FIRST IN A SERIES OF SYSTEMS
LOOKS TO AFFECT THE REGION BY TUESDAY NIGHT OR SO. THE GFS AND ECMWF
AGREE ON THIS REASONABLY WELL...WITH THE CANADIAN SOMEWHAT OF AN
OUTLIER SPLITTING THIS SYSTEM AND TAKING MUCH OF ITS ENERGY TO OUR
SOUTH. BY NEW YEARS DAY...THE FLOW OFF THE NORTHEAST PACIFIC FINALLY
APPEARS TO GET ORGANIZED WITH A STRONGER WARM FRONT/COLD FRONT
PRODUCING SOME MODERATE RAINFALL. 27
&&
.AIR QUALITY...STRONG SURFACE-BASED TEMPERATURE INVERSIONS WILL
WEAKEN SOME INTO SUNDAY...THEN STRENGTHEN AGAIN SUNDAY NIGHT INTO
MONDAY OVER A SIGNIFICANT PART OF THE AREA. STRONG EASTERLY
GRADIENTS HAVE IMPROVED MIXING OVER THE EAST PUGET SOUND LOWLANDS
WHILE THE INVERSION HELD ELSEWHERE. THESE WINDS WILL EASE UP THIS
EVENING. SO WHILE WE WILL HAVE A BIT OF MIXING TODAY SOME
AREAS...THE NIGHTTIME INVERSION WILL SET UP AGAIN TONIGHT ACROSS THE
AREA.
&&
.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.
&&
.AVIATION...AN UPPER RIDGE IS CENTERED OVER SOUTHERN BRITISH
COLUMBIA WITH A TROUGH WELL OFF THE COAST. THIS IS GIVING LIGHT
SOUTHEAST FLOW ALOFT. AT THE SURFACE STRONG HIGH PRESSURE EXTENDING
FROM SOUTHEAST BRITISH COLUMBIA INTO NORTHERN IDAHO WILL GIVE
CONTINUING OFFSHORE FLOW THROUGH SUNDAY. CROSS CASCADE GRADIENT
HANGING AROUND -12 TO -13 MB TODAY THEN SLOWLY EASING TONIGHT INTO
SUNDAY. THE RESULTANT DOWNSLOPE EASTERLY WINDS HAVE HELPED DRY THE
LOWER LAYERS OF THE AIR MASS OUT SOMEWHAT OVERNIGHT WITH THIS TREND
CONTINUING TODAY.
HARD TO PICK OUT ANY FOG ON SATELLITE IMAGERY THOUGH KSHN AND KTIW
BOTH REPORT LIFR FOG. AT WORST FOG IS PATCHY IN THE SOUTHWEST
INTERIOR AND SOUTH PUGET SOUND AREA...AND IT SHOULD BURN OFF BY THE
AFTERNOON. SKIES WILL BE CLEAR ELSEWHERE.
KSEA...FOG WILL NOT BE A PROBLEM AT SEATAC TODAY. SKIES WILL BE
CLEAR. THIS MORNING SOME EASTERLY WIND...WITH SOME SOUTHERLY
COMPONENT...HAS BEEN SURFACING AT THE TERMINAL. HAVE UPDATED THE TAF
FOR MORE WIND. MOST INTERESTING THING ABOUT THE WIND IS THE MIXING
IS KEEPING THE TEMPERATURE ABOVE 40 WHILE MOST OTHER PLACES ARE IN
THE 20S.
&&
.MARINE...HIGH PRESSURE EXTENDING FROM SOUTHEAST BRITISH COLUMBIA
INTO NORTHERN IDAHO WILL KEEP THE STRONG OFFSHORE FLOW PATTERN
INTACT TODAY WITH GALES AT THE WEST ENTRANCE TO THE STRAIT AND SMALL
CRAFT ADVISORY WINDS OVER THE COAST AND ALSO IN THE CENTRAL STRAIT.
THE STRENGTH OF OFFSHORE FLOW WILL EASE SOMEWHAT TOMORROW INTO
MONDAY AS A SPLITTING FRONT DISSIPATES OVER THE OFFSHORE WATERS.
ANOTHER WEAK FRONT WILL MOVE INTO THE OFFSHORE WATERS FROM THE WEST
ON TUESDAY. THIS SYSTEM WILL BE STRONGER THAN THE FIRST ONE AND HAS
A CHANCE TO MAKE IT INTO WESTERN WASHINGTON SOMEWHAT INTACT.
WESTERLY SWELLS OF 9-11 FEET OVER THE COASTAL WATERS WILL GRADUALLY
BUILD TO AROUND 14 FT TONIGHT AND REMAIN AROUND 14 FEET ON SUNDAY.
THE SWELL WILL GRADUALLY SUBSIDE SUNDAY NIGHT INTO TUESDAY. EXPECT
SMALL CRAFT ADVISORY LEVEL ROUGH BAR CONDITIONS AT GRAYS HARBOR FOR
THE NEXT SEVERAL DAYS. CHB
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...AIR STAGNATION ADVISORY FOR MOST OF THE WESTERN WASHINGTON
LOWLANDS THROUGH SATURDAY MORNING.
.HIGH WIND WARNING EAST PUGET SOUND LOWLANDS TODAY.
PZ...GALE WARNING WEST ENTRANCE STRAIT.
SMALL CRAFT ADVISORY COAST AND CENTRAL STRAIT.
.SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
$$
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
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