Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 010334
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
834 PM PDT SUN AUG 31 2014

.SYNOPSIS...A WEAK UPPER LEVEL TROUGH CROSSING THE AREA WILL
MAINTAIN A CHANCE OF SHOWERS OVER THE CASCADES TONIGHT. THE
ASSOCIATED WEAK FRONT WILL BRING A CHANCE OF SHOWERS TO THE NORTH
COAST AND NORTH CASCADES LATE TONIGHT AND MONDAY. MOSTLY CLOUDY
SKIES WILL CONTINUE OVER THE REST OF WESTERN WASHINGTON. RAIN IS
LIKELY ON TUESDAY AS A VIGOROUS UPPER LEVEL TROUGH CROSSES THE
REGION. AN UPPER LEVEL RIDGE OFFSHORE WILL BRING A GRADUAL WARMING
AND DRYING PATTERN FROM WEDNESDAY THROUGH NEXT WEEKEND.

&&

.SHORT TERM...WATER VAPOR IMAGERY SHOWS A WEAK UPPER LEVEL SHORTWAVE
TROUGH ALONG THE B.C. COAST THAT WILL BRUSH PAST THE NORTH PART OF W
WA TONIGHT. THE WILL BRING A CHANCE OF SHOWERS TO THE CASCADES...BUT
MAINLY THE NORTH PART. IR AND VISIBLE SATELLITE IMAGERY SHOW A LOT
OF MID LEVEL CLOUDS JUST OFFSHORE THAT SHOULD SPREAD OVER W WA
OVERNIGHT...ALTHOUGH THE OLYMPICS MAY SHIELD THE PUGET SOUND REGION
A BIT. THE VERY WEAK SURFACE FRONT PRESENTLY ASSOCIATED WITH THE
UPPER TROUGH WILL GLIDE TO THE COAST LATE TONIGHT THEN FALL APART
MONDAY. A LITTLE LIGHT RAIN IS POSSIBLE FOR THE N COAST AND N
INTERIOR MONDAY AS THE FRONT DISSIPATES. THE PUGET SOUND REGION AND
SW INTERIOR WILL PROBABLY BE THE LEAST AFFECTED BY THE DISSIPATING
FRONT AND SO COULD SEE SOME PARTIAL SUNSHINE MONDAY AFTERNOON.

MODELS HAVE BEEN REASONABLY CONSISTENT WITH THE TIMING AND TRACK OF
THE VIGOROUS UPPER LEVEL TROUGH THAT IS EXPECTED TO CROSS THE AREA
ON TUESDAY. THIS TROUGH SHOWS UP NICELY ON WATER VAPOR IMAGERY OVER
THE GULF OF AK THIS EVENING...AND WILL MOVE SE ALONG THE B.C. COAST
THROUGH MONDAY NIGHT...ENDING UP OVER W WA ON TUESDAY. LATEST MODEL
TIMING HAS THE BEST CHANCE OF RAIN TUESDAY AFTERNOON. THE NAM IS THE
DRIEST MODEL WITH ONLY A FEW HUNDREDTHS OF AN INCH OF RAIN...WHILE
THE ECMWF IS WETTEST WITH 2 TO 4 TENTHS OVER THE NORTH. THIS IS A
FAIRLY DRY TRACK FOR THE TROUGH AND IT IS FAST MOVING...SO ACTUAL
RAINFALL ACCUMULATIONS WILL PROBABLY FALL IN-BETWEEN THE TWO MODELS.
THE TROUGH WILL DRAG MUCH COOLER AIR ALOFT OVER THE AREA TUESDAY
NIGHT WITH SNOW LEVELS OVER THE CASCADES DROPPING DOWN TO JUST ABOVE
6000 FEET AFTER MIDNIGHT. THERE COULD STILL BE SOME LIGHT SHOWERS
AROUND FOR A TINY BIT OF MOUNTAIN S.N.O.W.

THE UPPER LEVEL TROUGH WILL MOVE QUICKLY EAST TUESDAY
NIGHT...ALLOWING DRIER N FLOW ALOFT FROM AN OFFSHORE UPPER LEVEL
RIDGE TO MOVE OVER THE AREA. WELL DEVELOPED SW ONSHORE FLOW TUESDAY
NIGHT WILL KEEP THE LOWER AIR MASS MOIST WITH STRATUS THROUGH
WEDNESDAY MORNING. WITH DRY AIR ALOFT THE STRATUS SHOULD CLEAR OUT
BY AFTERNOON. WEDNESDAY SHOULD BE SLIGHTLY WARMER THAN TUESDAY...BUT
THE RESIDUAL COOL AIR MASS FROM THE UPPER TROUGH WILL CAP MAX TEMPS
IN THE 60S TO NEAR 70. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 334 PM AFD...MEDIUM RANGE
MODELS ALL AGREE ON THE ZONAL FLOW OVER THE NORTHEAST PACIFIC
BUCKLING INTO A HIGHER AMPLITUDE PATTERN BY MIDWEEK. THIS PATTERN
WILL BE ANCHORED BY A LARGE SCALE RIDGE THAT WILL BUILD ALONG 140W
TUESDAY AND INTO BRITISH COLUMBIA BY WEDNESDAY NIGHT. MODELS DO
DISAGREE ON THE FLOW DOWNSTREAM OF THE RIDGE ON FRIDAY WITH VARYING
DEGREES OF SPLIT FLOW ALOFT OVER THE NORTHWEST AND A WEAK UPPER
LEVEL LOW OVER THE GREAT BASIN OR CENTRAL ROCKY MOUNTAINS.
NONETHELESS THE LARGE SCALE PATTERN WILL KEEP THE PACIFIC NORTHWEST
DRIER AND WARMER BEGINNING WEDNESDAY AFTERNOON AND PERSISTING
THROUGH THE WEEKEND. MODELS GENERALLY AGREE ON A SYSTEM RIDING OVER
THE TOP RIDGE AND INTO THE NORTHWEST LATE SUNDAY INTO MONDAY
MORNING.

&&

.AVIATION...LIGHT NW FLOW ALOFT OVER THE REGION THROUGH MONDAY.
LIGHT LOW LEVEL ONSHORE FLOW. A WEAK TROUGH WILL DISSIPATE AS IT
REACHES THE COAST MONDAY MORNING. THE AIR MASS IS GENERALLY MOIST
AND STABLE...EXCEPT WEAKLY UNSTABLE MONDAY AFTERNOON. ONLY A SLIGHT
CHANCE OF A FEW SHOWERS...MAINLY N OF SEATTLE ON MONDAY AFTERNOON.
AN MVFR CEILING IS EXPECTED TO FORM OVER SOME PARTS OF THE AREA LATE
TONIGHT AND MONDAY MORNING...LIFTING AND SCATTERING BY 18Z.

KSEA...LIGHT WIND...BECOMING S-SW TO 5 KNOTS LATE TONIGHT AND
MONDAY. MVFR CIGS ARE LIKELY TO FORM AFTER MIDNIGHT...THEN LIFT AND
SCATTER BY 18Z. DTM

&&

.MARINE...A WEAK SYSTEM WILL DISSIPATE AS IT REACHES THE COAST
MONDAY MORNING...GIVING A SLY BREEZE OVER THE COASTAL WATERS.
ANOTHER SYSTEM WILL BRING A BOUT OF STRONG ONSHORE FLOW TUESDAY
NIGHT...THEN EASING ON WEDNESDAY. THE REST OF THE WEEK WILL BE A
MORE SUMMERY WIND REGIME WITH A TYPICAL NW BREEZE EACH DAY. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA
     THROUGH 1 AM PDT MONDAY.

&&

$$

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