FXUS66 KSEW 032336
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
335 PM PST THU DEC 3 2009
.SYNOPSIS...AN UPPER LEVEL RIDGE WILL KEEP WESTERN WASHINGTON COOL
AND DRY TONIGHT...ALTHOUGH HIGH CLOUDS FROM AN APPROACHING UPPER
LEVEL TROUGH WILL CROSS THE AREA. AN UPPER LEVEL TROUGH MOVING SOUTH
FROM BRITISH COLUMBIA WILL PUSH A WEAK COLD FRONT ACROSS THE AREA
FRIDAY AFTERNOON AND EVENING FOR A CHANCE OF SHOWERS. ANOTHER UPPER
LEVEL TROUGH WILL PUSH A MODIFIED ARCTIC FRONT ACROSS THE AREA
SATURDAY NIGHT FOR A CHANCE OF SNOW SHOWERS. AN UPPER LEVEL RIDGE
JUST OFFSHORE WILL MAINTAIN COLD NORTH TO NORTHEAST FLOW ALOFT OVER
THE AREA EARLY NEXT WEEK. A CHANGE TO WESTERLY FLOW ALOFT AROUND MID
WEEK WILL BRING A CHANCE OF RAIN AND ALLOW TEMPERATURES TO WARM.
&&
.SHORT TERM...HIGH CLOUDS AHEAD OF THE UPPER LEVEL TROUGH MOVING
INLAND OVER B.C. WILL REMAIN OVER THE AREA TONIGHT. AIR MASS REMAINS
COLD WITH HIGHS ONLY IN THE LOWER TO MID 40S THIS AFTERNOON. THE
HIGH CLOUDS MAY LIMIT RADIATIONAL COOLING OVERNIGHT...BUT WITH
DEWPOINTS IN THE 20S TO LOWER 30S...FRIDAY MORNING LOWS WILL BE
MAINLY IN THE 20S.
MODELS HAVE REACHED GOOD AGREEMENT WITH THE OVERALL EVOLUTION OF THE
PATTERN THE NEXT FEW DAYS. THE UPPER LEVEL LOW OVER B.C. MOVES SE
OVER THE EASTERN WA/ID AREA ON FRIDAY. THIS PATH PUSHES A WEAK
PRETTY MUCH DRY COLD FRONT SOUTH ACROSS W WA IN THE AFTERNOON. THE
DRYING TREND WITH THE MODELS MEANS THAT THE CHANCE OF SHOWERS FRIDAY
AFTERNOON WILL BE RESTRICTED TO THE NORTH CASCADES AND THE NORTHERN
LOWLANDS CLOSE TO THE CASCADES. WILL KEEP A CHANCE OF SHOWERS OVER
THE CASCADES INTO THE EVENING. THE COLD DRY AIR MASS MOVING SOUTH
ACROSS THE AREA FRIDAY NIGHT COMBINED WITH CLEARING...WILL PRODUCE
LOWS IN THE 20S...BUT IT WOULD NOT BE SURPRISING TO SEE A FEW
READINGS IN THE UPPER TEENS IN THE COLDEST SPOTS.
A HIGH AMPLITUDE UPPER LEVEL RIDGE SETS UP JUST W OF THE B.C. COAST
ON FRIDAY...WHICH WILL KEEP THE AREA IN COOL N-NE FLOW ALOFT INTO
EARLY NEXT WEEK. MODELS BRING AN UPPER LEVEL TROUGH S ACROSS THE
AREA SATURDAY NIGHT. THIS IS A DRY TRAJECTORY COMING DOWN FROM B.C.
SO POPS WITH THE MODIFIED ARCTIC FRONT WILL BE LOW...BUT NOT ZERO.
THE AIR MASS IS COLD ENOUGH SO THAT EVEN THOUGH THERE COULD BE A
RAIN/SNOW MIX...IT WILL PROBABLY JUST BE SNOW SHOWERS. ACCUMULATIONS
WILL BE NEGLIGIBLE...AND THERE IS A CHANCE THIS WILL END UP BEING A
DRY FRONT AND TROUGH. THE SURGE OF COLD AIR BEHIND THE FRONT SHOULD
PRODUCE SOME PRETTY COLD LOWS SUNDAY AND MONDAY MORNINGS...20S FOR
SURE AND POSSIBLY UPPER TEENS IN SOME SHELTERED SPOTS. HIGHS SUNDAY
WON'T ESCAPE THE 30S. KAM
.LONG TERM...THE HIGH AMPLITUDE RIDGE W OF THE B.C. COAST WILL
REMAIN IN PLACE THROUGH WEDNESDAY. CONTINUING N FLOW ALOFT AND
OFFSHORE FLOW KEEP THE AIR MASS COLD AND DRY. SHOULD SEE A LITTLE
MODERATION IN TEMPS BY WEDNESDAY...BUT IT WILL STILL BE COLDER THAN
NORMAL. BOTH GFS/ECMWF CONTINUE TO SHOW W FLOW ALOFT UNDERCUTTING
THE UPPER LEVEL RIDGE STARTING TUESDAY...BUT WITH THE WESTERLIES
AIMED AT CA THIS TIME. ECMWF HAS KEPT ALL PRECIP S OF W WA...BUT THE
12Z GFS RUN HAD RAIN REACHING W WA LATER WEDNESDAY. NEWEST 18Z GFS
IS DRY...SO GIVEN THE MODEL INCONSISTENCY CONFIDENCE IN PRECIP IS
LOW. THERE IS A CHANCE OF RAIN IN THE FORECAST BASED MAINLY ON THE
12Z GFS AND CLIMO. MORE CERTAIN CHANGE WILL BE A MODERATION IN
TEMPERATURE. KAM
&&
.HYDROLOGY...THERE IS NO THREAT OF RIVER FLOODING IN THE FORECAST
AREA FOR THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER.
TWO DRY UPPER LEVEL TROUGHS/WEAK SURFACE FRONTS WILL CROSS THE
AREA...FRIDAY AFTERNOON AND AGAIN SATURDAY EVENING. LITTLE IF ANY
PRECIP IS EXPECTED FROM EITHER OF THESE SYSTEMS. IN THE LONG
TERM...WARMER AND POTENTIALLY WETTER WEST FLOW ALOFT RETURNS AROUND
MID NEXT WEEK. SHOULD SEEM SOME RAIN OVER THE AREA BY LATE NEXT
WEEK...BUT THE MODELS DEPICT FASTER WESTERLY FLOW WITH FRONTAL
SYSTEMS PERIODICALLY CROSSING THE AREA...WHICH IS TYPICALLY NOT A
FLOODING PATTERN. KAM
&&
.AVIATION...AN UPPER LEVEL TROUGH WILL SLIDE SE ACROSS THE REGION
FRIDAY INCREASING THE NLY FLOW TONIGHT AND FRIDAY ALOFT...AND AT THE
SURFACE ON FRIDAY. THIS WILL BE ASSOCIATED WITH PLENTY OF MID AND
HIGH CLOUDS. A PERSISTENT AREA OF LOW LEVEL MOISTURE OVER THE
EXTREME SOUTH SOUND SHOULD PERSIST THROUGH FRIDAY MORNING BEFORE
DISSIPATING WITH THE INCREASING N WINDS FRIDAY AFTERNOON. CERNIGLIA
KSEA...CLEAR AOB 120 WITH LIGHT...MAINLY NORTHEAST WINDS. WIND
EXPECTED TO BECOME LIGHT SLY 12Z-16Z THEN SNAP AROUND TO THE N EARLY
IN THE AFTERNOON.
ONLY CONCERN AT THIS POINT...THROUGH FRIDAY NIGHT...IS THE LOW
STRATUS/FOG S OF KTIW POSSIBLY SPREADING SLOWLY N EARLY FRIDAY
MORNING AND REACHING THE TERMINAL BEFORE THE N WIND KICKS IN. AT
THIS POINT WILL BET AGAINST THAT HAPPENING AND I WILL NOT INCLUDE IT
IN THE TAF. CERNIGLIA
&&
.MARINE...ALL QUIET...FOR NOW. THE ONLY CURRENT ISSUE IS THE 10 TO
11 FT SWELL OVER THE COASTAL WATERS WHICH WILL SUBSIDE EARLY
FRIDAY.
A WEAK PACIFIC FRONT WILL MOVE SE ACROSS THE AREA LATE TONIGHT WITH
INCREASING N-NW WINDS IN ITS WAKE. LIKELY TO REACH SCA STRENGTH
FRIDAY AFTERNOON OVER THE COAST AND PUGET SOUND IN ITS WAKE.
ARCTIC HIGH PRES WILL BUILD S ACROSS B.C SATURDAY...FORCING A
MODIFIED ARCTIC BOUNDARY S ACROSS WESTERN WASHINGTON SATURDAY NIGHT.
OFFSHORE GRADIENTS WILL BECOME STRONG BEHIND THIS FRONT LATE
SATURDAY NIGHT INTO SUNDAY RESULTING DECENT NE WINDS. THERE IS A
POTENTIAL FOR GALE FORCE NE WINDS OVER PARTS OF THE NORTHERN
INTERIOR WATERS NORTH OF WHIDBEY ISLAND AND THE ENTRANCES TO THE
STRAIT OF JUAN DE FUCA. AT THIS POINT WE HAVE 20 TO 30 KT IN THE
FORECAST BUT WE COULD HAVE A 6-12 HR PERIOD OF LOW END GALES.
BEYOND THAT...THE EARLY PART OF THE COMING WEEK WILL SEE EASING
OFFSHORE GRADIENTS FOR LIGHTER WINDS. CERNIGLIA
CERNIGLIA
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY HAZARDOUS SEAS COASTAL WATERS AND WEST
ENTRANCE.
SMALL CRAFT ADVISORY ROUGH BAR CONDITIONS GRAYS HARBOR BAR.
$$
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