Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 151559
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT MON SEP 15 2014

.SYNOPSIS...A STRONG UPPER LEVEL RIDGE WILL BRING ONE MORE SUNNY AND
WARM DAY BEFORE SHIFTING EAST TONIGHT. AN APPROACHING WEAK UPPER
LEVEL TROUGH WILL TRIGGER ONSHORE FLOW MONDAY NIGHT BRINGING A
COOLER MARINE AIR MASS AND MORE CLOUD COVER TUESDAY. ONSHORE FLOW
WILL KEEP TEMPERATURES NEAR NORMAL WEDNESDAY. AN UPPER LEVEL TROUGH
WILL BRING A CHANCE OF SHOWERS LATE WEDNESDAY THROUGH FRIDAY.

&&

.SHORT TERM...CLEAR SKIES OVER WESTERN WASHINGTON THIS MORNING. AN
UPPER RIDGE REMAINS OVER THE AREA AND SURFACE FLOW IS MODERATELY
OFFSHORE. SKIES WILL BE SUNNY TODAY AND TEMPERATURES WILL RISE INTO
THE 80S AND LOW 90S.

SURFACE FLOW TURNS WEAKLY ONSHORE TONIGHT AND BECOMES STRONGER
TUESDAY AFTERNOON. HARD TO SEE STRATUS MOVING FAR INLAND TUESDAY
MORNING BUT THERE COULD BE SOME HIGHER CLOUDS FROM A WEAK SHORTWAVE
PASSING FROM SOUTHWEST TO NORTHEAST. MESOSCALE MODELS ARE IN
REASONABLY GOOD AGREEMENT...BRINGING SOME CLOUDS BY 12Z. WITH UPPER
RIDGING STILL EVIDENT AND ONSHORE FLOW UNIMPRESSIVE IT SEEMS LIKELY
THE CLOUDS WILL MOSTLY CLEAR TUESDAY AFTERNOON EXCEPT FOR THE COAST.
HIGHS ON TUESDAY WILL CERTAINLY BE LOWER THAN TODAY...MOSTLY 70S.

STRONGER ONSHORE FLOW WILL GIVE MORE CLOUD COVER WEDNESDAY MORNING.
ANOTHER SHORTWAVE SPINNING OUT OF AN UPPER LEVEL LOW OFF THE
NORTHERN CALIFORNIA COAST WILL MOVE TOWARD THE AREA DURING THE DAY
ON WEDNESDAY. PLENTY OF MID AND HIGH LEVEL CLOUD COVER IN ADDITION
TO THE LOW STRATUS WEDNESDAY WILL KEEP HIGHS IN THE MID 60S TO MID
70S. THERE IS A CHANCE OF SHOWERS WEDNESDAY AFTERNOON BUT LATEST
MODEL RUNS TEND TO CONFINE SHOWERS TO THE COAST AND OLYMPICS. BURKE

.LONG TERM...PREVIOUS DISCUSSION...EXTENDED MODELS STILL HAVING
TROUBLE WITH THE TIMING AND STRENGTH OF THE UPPER LEVEL TROUGH
MOVING INTO WESTERN WASHINGTON WEDNESDAY NIGHT AND THURSDAY. THE
GENERAL TREND OF THE MODELS IS TO BE SLOWER AND WEAKER WITH THIS
FEATURE. CURRENT FORECAST OF A CHANCE OF SHOWERS FOR THE MOST PART
LOOKS GOOD AT THIS POINT. FRIDAY WILL BE A RACE BETWEEN HOW FAST THE
NEXT UPPER LEVEL RIDGE CAN BUILD VERSUS A WEAK FRONT MOVING INTO
BRITISH COLUMBIA. THE ECMWF IS A LITTLE STRONGER WITH THE RIDGE
PUSHING THE SYSTEM WELL NORTH OF THE AREA. THE GFS STILL KEEPS SOME
LIGHT PRECIPITATION OVER THE AREA WITH A WEAKER RIDGE. WITH THE LACK
OF CONSENSUS WILL STAY WITH THE CHANCE OF SHOWERS FORECAST. UPPER
LEVEL RIDGE CONTINUING TO BUILD OVER THE AREA ON SATURDAY FOR A DRY
DAY. LOW LEVEL FLOW DOES NOT TURN OFFSHORE KEEPING TEMPERATURES JUST
A FEW DEGREES ABOVE NORMAL. SOME DIFFERENCES IN THE MODELS ON SUNDAY
WITH THE ECMWF SHOWING A STRONGER RIDGE VERSUS THE GFS SOLUTION.
STILL A DRY DAY EITHER WAY. BOTH OF THE MODELS SHOW THE LOW LEVEL
FLOW GOING ONSHORE IN THE AFTERNOON SO WILL KEEP HIGHS IN THE 70S
EVEN WITH THE STRONG RIDGING. FELTON

&&

.CLIMATE...THERE IS A CHANCE THAT SEATTLE WILL HAVE A HIGH IN THE
90S TODAY. THIS WOULD ONLY BE THE 14TH TIME SINCE RECORDS STARTED AT
SEA-TAC IN 1945 WITH A HIGH 90 PLUS ON A DAY IN SEPTEMBER. THERE
HAVE ONLY BEEN TWO SEPTEMBERS WITH MORE THAN ONE 90 PLUS DEGREE
DAY...1988 WITH 98 ON THE 2ND AND 92 ON THE 3RD...AND 1967 WITH 93
ON THE 15TH AND 91 ON THE 16TH. TODAY WILL BE THE 45TH DAY OF THE
YEAR WITH A HIGH 80 PLUS IN SEATTLE. THE RECORD IS 47 DAYS SET IN
1958. FELTON

&&

.AVIATION...A STRONG RIDGE OF HIGH PRESSURE OVER WA TODAY WILL SHIFT
EWD TONIGHT AND TUESDAY. LIGHT SW FLOW ALOFT WILL BECOME MODERATELY
STRONG. LOW LEVEL OFFSHORE FLOW WILL TURN ONSHORE TONIGHT AND
INCREASE ON TUESDAY. ATMOSPHERE DRY AND STABLE. INCREASING LOW LEVEL
MOISTURE ALONG THE COAST TONIGHT. PATCHY LOW LEVEL MOISTURE
DEVELOPING IN THE STRAIT AND SW INTERIOR/S PUGET SOUND TUESDAY
MORNING...DISSIPATING BY 18Z. EXPECT CIGS UNDER 500 FEET WHERE
STRATUS FORMS...MOST LIKELY AFFECTING THE KHQM/KCLM/OLM TERMINALS.
THE KBFI/KSEA WILL BE ON THE EDGE BUT LOW IFR CONDITIONS ARE STILL
POSSIBLE FOR A FEW HOURS TUESDAY MORNING.

KSEA...LIGHT WIND BECOMING NLY TO 10 KT IN THE AFTERNOON. WINDS
BECOMING LIGHT IN THE EVENING...THEN SWLY 4 TO 8 KT TUESDAY MORNING.
THERE IS STILL A CHANCE THAT THE LOW CLOUDS DEVELOPING ON THE COAST
TONIGHT SQUIRT IN THROUGH THE CHEHALIS GAP AND GIVE AREAS OF LOW IFR
CIGS AND VIS AROUND PUGET SOUND TUESDAY MORNING. NEWER MODELS
SUGGEST THE STRATUS COULD END UP JUST SHORT OF KSEA AND WILL MAKE
MINOR ADJUSTMENTS IN THE NEXT TAF ISSUANCE. DTM

&&

.MARINE...THE THERMALLY INDUCED AREA OF LOW PRESSURE OVER THE REGION
WILL SHIFT INLAND TONIGHT. LIGHT OFFSHORE FLOW WILL TURN ONSHORE
TONIGHT AND INCREASE ON TUESDAY. LOW MARINE STRATUS WILL MOVE UP THE
COAST THIS EVENING AND MOVE INTO PORTIONS OF THE INLAND WATERS BY
DAYBREAK TUESDAY. SMALL CRAFT ADVISORY STRENGTH WESTERLIES ARE
LIKELY IN THE STRAIT OF JUAN DE FUCA TUESDAY AFTERNOON AS THE
ONSHORE FLOW INCREASES. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML




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