FXUS66 KSEW 261121
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
320 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...THE UPPER LEVEL RIDGE AXIS HAS SHIFTED EAST OF THE
CASCADES OVERNIGHT. THE BROAD TROUGH WELL OFFSHORE IS STARTING TO
UNDERCUT THE RIDGE...AND SOME OF THE HIGH CLOUDINESS ASSOCIATED WITH
THE SPLITTING SYSTEM OFFSHORE WILL START TO FILTER INTO THE AREA
TONIGHT. A STRONG CROSS CASCADE GRADIENT CONTINUES...-13 MB SEATTLE
TO WENATCHEE AT 10Z. THE 06Z RUN OF THE NAM12 IS FINALLY CATCHING ON
TO THE STRENGTH OF THE GRADIENT THAT EARLIER RUNS OF BOTH THE GFS
AND NAM MISSED. THE GRADIENT SHOULD GRADUALLY RELAX LATE THIS
AFTERNOON THROUGH THE EVENING. WITH GRADIENTS LIKELY MAXED OUT THIS
MORNING...WILL LEAVE CURRENT HIGH WIND WARNINGS AND ADVISORIES FOR
THE CASCADE FOOTHILLS AND EAST LOWLANDS AS IS AND LET DAYSHIFT
DECIDE WHEN TO DROP THEM. STEEP SHALLOW INVERSIONS REMAIN ACROSS
MUCH OF THE INTERIOR LOWLANDS THIS MORNING...AS EVIDENCED BY THE
KSEW PROFILER SHOWING A 15 DEGREE FAHRENHEIT JUMP FROM THE SURFACE
TO 1200 FEET. MORNING FOG FORMATION IS BEING OFFSET BY THE STRONG
OFFSHORE GRADIENT...LIMITING IT TO PATCHY EXCEPT FOR PORTIONS OF THE
SOUTHWEST INTERIOR WHERE AREAL COVERAGE LOOKS MORE LIKELY.
BY SUNDAY...UPPER LEVEL RIDGING WILL HAVE SHIFTED INTO CENTRAL
ALBERTA. AN UPPER LEVEL TROUGH MOVING IN SOUTHWEST OREGON WILL
ATTEMPT TO SPREAD SOME MOISTURE TOWARD THE SOUTHWEST CORNER OF THE
STATE. THE RIDGING OVER WESTERN CANADA WILL EXERT ENOUGH INFLUENCE
TO WEAKEN AND SPLIT THIS SYSTEM CONSIDERABLY. BOTH THE NAM AND GFS
STILL TRY TO SPREAD SOME PRECIP TO NEAR THE OREGON BORDER BY SUNDAY
AFTERNOON. GIVEN THE WEAKENED NATURE OF THE SYSTEM AND CONTINUED
OFFSHORE FLOW...MOST PLACES SHOULD NOT SEE MUCH MORE THAN AN
INCREASE IN CLOUDS FROM THIS. MINIMAL 20 POPS WERE PUT IN THE
FORECAST FOR THE COAST AND SOUTHWEST INTERIOR SUNDAY AFTERNOON.
ON MONDAY...WESTERN WASHINGTON WILL BE UNDER A BAGGY UPPER LEVEL
TROUGH AS THE RIDGING TO OUR EAST DISSIPATES. THERE WILL NOT BE A
LOT OF LIFT TO GET PRECIP GOING. IN ADDITION...THE LOW LEVELS MAY
STILL BE RATHER DRY THANKS IN PART TO CONTINUED OFFSHORE GRADIENTS.
THE END RESULT IS PLENTY OF CLOUDS BUT LOW RAIN CHANCES.
.LONG TERM...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 THROUGH SUNDAY AS LOWERING HEIGHTS ALLOW TEMPERATURES TO COOL
ALOFT. STRONG EASTERLY GRADIENTS HAVE IMPROVED MIXING...ESPECIALLY
OVER THE EAST PUGET SOUND LOWLANDS. CONTINUED POOR MIXING OVER THE
REMAINDER OF THE LOWLANDS HAS LED TO MODERATE AIR QUALITY READINGS.
THIS IS WHERE AIR QUALITY CONCERNS WILL PERSIST THROUGH THE MORNING.
&&
.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...LIGHT FLOW ALOFT UNDER A STRONG UPPER RIDGE...GRADUALLY
SWITCHING TO LIGHT SOUTHEAST FLOW THIS AFTERNOON 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 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.
SOME PATCHY FOG AROUND THE PUGET SOUND AND LAKE WASHINGTON EARLY
THIS MORNING. THE MAIN AREA OF REDUCED VISIBILITIES WILL BE FROM
KOLM TO NEAR KCLS WITH VISIBILITIES BELOW 1 MILE AND CIGS BELOW 200
FEET THIS MORNING. WITH THE AIR MASS DRYING EXPECT THESE AREAS TO
BREAK OUT OF THE FOG THIS AFTERNOON.
KSEA...DEW POINT AT 10Z 7 DEGREES LESS THAN LAST NIGHT. ALTHOUGH THE
WINDS ARE LIGHT THE DRYING FROM THE DOWNSLOPE EASTERLIES HAS REACHED
KSEA. WILL STAY WITH THE NO FOG AT THE TERMINAL FORECAST. WILL KEEP
WINDS OUT OF THE EAST ALL DAY BUT LESS THAN 10 KTS. FELTON
&&
.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 WINDS OVER THE COASTAL WATERS 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 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. FELTON
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...A HIGH WIND WARNING REMAINS IN EFFECT FOR THE EAST PUGET SOUND
LOWLANDS UNTIL 4 PM TODAY.
A WIND ADVISORY REMAINS IN EFFECT FOR THE EASTERN SEATTLE AND
TACOMA METRO AREAS UNTIL 4 PM TODAY.
AN AIR STAGNATION ADVISORY REMAINS IN EFFECT FOR PARTS OF THE
INTERIOR LOWLANDS UNTIL 10 AM TODAY.
PZ...GALE WARNING WEST ENTRANCE STRAIT.
SMALL CRAFT ADVISORY COAST AND CENTRAL STRAIT.
SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
$$
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