Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 270439
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
935 PM PDT TUE AUG 26 2014

.SYNOPSIS...UPPER LEVEL RIDGE OVER THE AREA TONIGHT WILL WEAKEN AND
SLOWLY MOVE EAST ON WEDNESDAY. AN UPPER LEVEL TROUGH WILL MOVE
THROUGH THE AREA THURSDAY...BRINGING COOLER WEATHER AND AREAS OF
DRIZZLE. THE UPPER LEVEL TROUGH WILL REMAIN OVER THE AREA THROUGH
THE WEEKEND KEEPING THE WEATHER COOL WITH AT LEAST A CHANCE OF
SHOWERS THROUGH SUNDAY.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS STRATUS RIGHT ALONG THE COAST
BUT NOT MAKING ANY EAST PROGRESS YET AS OF 04Z/9PM. BOTH KUIL AND
KHQM ARE STILL CLEAR ON THE 04Z OBSERVATIONS. ONSHORE GRADIENTS HAVE
PICKED UP A TOUCH WITH KUIL-KBLI PEAKING OUT AROUND 1.5 MB WITH
KHQM-KSEA PEAKING AT AROUND 1 MB. WITH THE WEAK ONSHORE GRADIENTS
AND THE LACK OF ANY EASTWARD PENETRATION OF THE STRATUS WILL STAY
WITH THE MOSTLY CLEAR FORECAST TONIGHT FOR MOST OF THE INTERIOR OF
WESTERN WASHINGTON. LOWS WILL CONTINUE TO BE IN THE 50S TO LOWER
60S.

ANOTHER SUNNY DAY FOR MOST OF THE AREA ON WEDNESDAY WITH THE WEAK
ONSHORE GRADIENTS. UPPER LEVEL RIDGE OVER THE AREA TONIGHT WEAKENING
SLIGHTLY AND BEGINNING TO MOVE EAST ON WEDNESDAY. 500 MB HEIGHTS
NEAR 586 DECAMETERS TONIGHT LOWERING DOWN TO THE LOWER 580
DECAMETERS BY 00Z THURSDAY. TEMPERATURES ALOFT ALSO COOL SLIGHTLY
WITH 850 MB TEMPERATURES GOING FROM AROUND PLUS 16 TO 18 C TONIGHT
TO PLUS 14 TO 16 C BY WEDNESDAY AFTERNOON. 00Z MOS GUIDANCE
INDICATING A LITTLE BIT STRONGER PUSH WITH 5 TO 10 DEGREES OF
COOLING FOR THE INTERIOR DEPENDING ON THE MODEL. CURRENT FORECAST IS
MORE IN LINE WITH A WEAKER PUSH WITH ABOUT 5 DEGREES OF COOLING FOR
THE INTERIOR ON WEDNESDAY WITH HIGHS IN THE MID 70S TO MID 80S. WILL
STAY WITH THIS FORECAST THIS EVENING BUT IF THE THERE IS NO CHANGE
IN THE REMAINDER OF THE 00Z MODEL SUITE LOOK FOR THE MAX
TEMPERATURES TO BE RAISED BY A COUPLE OF DEGREES IN THE EARLY
MORNING FORECAST PACKAGE.

UPPER LEVEL RIDGE CONTINUING TO MOVE EAST TONIGHT AND THURSDAY WITH
A WEAK UPPER LEVEL TROUGH MOVING INTO THE AREA THURSDAY AFTERNOON.
500 MB HEIGHTS FALL INTO THE MID 570 DECAMETERS ALONG WITH AN
INCREASE IN LOW LEVEL ONSHORE FLOW. THE TROUGH COULD DEEPEN THE
MARINE LAYER ENOUGH ALONG THE COAST TO PRODUCE SOME AREAS OF
DRIZZLE. WITH THE INCREASE IN CLOUD COVER AND COOLER TEMPERATURES
ALOFT HIGHS ON THURSDAY WILL BE AROUND 10 DEGREES COOLER FOR THE
INTERIOR VERSUS WEDNESDAYS READINGS WITH LOWER TO MID 70S COMMON.
HIGHS ALONG THE COAST WILL BE IN THE 60S.

NOT MUCH GOING ON THURSDAY NIGHT WITH THE UPPER LEVEL TROUGH MOVING
THROUGH THE AREA AND THE NEXT UPPER LEVEL TROUGH STILL OFFSHORE.
SURFACE GRADIENTS REMAIN ONSHORE BUT ARE NOT VERY STRONG INITIALLY
BUT AS THE NEXT TROUGH APPROACHES ONSHORE GRADIENTS WILL INCREASE
LATE THURSDAY NIGHT INTO FRIDAY MORNING RESULTING IN MORE MORNING
CLOUDS FOR THE INTERIOR. LOWS IN THE 50S WILL BE COMMON FRIDAY
MORNING.

ANOTHER UPPER LEVEL TROUGH MOVING INTO THE AREA FRIDAY AFTERNOON
WITH THE 500 MB HEIGHTS BELOW 570 DECAMETERS BY 00Z SATURDAY. THE
TROUGH IS WEAK WITH MOST OF THE ENERGY MOVING BY TO THE NORTH. THE
BEST CHANCE FOR SHOWERS ON FRIDAY WILL BE IN THE NORTH CASCADES.
WITH THE TROUGH MOVING INLAND DURING THE DAY HIGH TEMPERATURES ON
FRIDAY WILL BE A LITTLE BELOW NORMAL FOR THIS TIME OF THE YEAR WITH
LOWER 70S COMMON FOR THE INTERIOR AND LOWER TO MID 60S FOR THE
COAST. CURRENT FORECAST HAS THE TRENDS COVERED. NO UPDATE THIS
EVENING. FELTON

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS. AS THE UPPER LEVEL TROUGH
MOVES THROUGH THE REGION...THE CHANCE OF SHOWERS WILL CONTINUE FROM
FRIDAY NIGHT THROUGH SUNDAY...WITH THE SHOWERS MOST WIDESPREAD ON
SATURDAY. THE GFS AND ECMWF DISAGREE BEGINNING MONDAY ON WHAT WILL
HAPPEN NEXT...THE GFS HAS SHOWERS CONTINUING OFF AND ON LABOR DAY
AND TUESDAY...BUT THE ECMWF SHOWS A TREND OF DRYING OUT WITH AN
UPPER LEVEL RIDGE BUILDING IN. BECAUSE OF THIS...THERE IS QUITE A
BIT OF UNCERTAINTY IN THE FORECAST FROM LABOR DAY ONWARDS. JSMITH

&&

.AVIATION...WEAK HIGH PRES OVER THE PAC NW WILL MAINTAIN A DRY AND
STABLE AIR MASS ACROSS WESTERN WA TONIGHT AND WED. THE FLOW ALOFT IS
WESTERLY. LOW LEVEL STRATUS CLOUDS COVER THE COAST THIS EVENING AND
WILL LIKELY PUSH THROUGH THE STRAIT AND LOWER CHEHALIS VALLEY
OVERNIGHT - IFR CONDITIONS ARE POSSIBLE. LOW CLOUDS WILL BURN BACK
TO THE COAST WED AFTERNOON WITH VFR CONDITIONS EXPECTED. CLOUDS WILL
INCREASE WED NIGHT AS A COLD FRONT APPROACHES FROM THE WEST. 33

KSEA...VFR CONDITIONS WILL PREVAIL WITH LIGHT NORTH WINDS AT THE
SURFACE. 33

&&

.MARINE...HIGH PRES OVER THE NE PAC WILL MAINTAIN ONSHORE FLOW
ACROSS WESTERN WA THROUGH WED. HIGHEST WIND AND WAVES WILL BE FOUND
THROUGH THE CENTRAL AND EAST STRAIT OF JUAN DE FUCA. A WEAK COLD
FRONT WILL CROSS WESTERN WA ON THU...FOLLOWED BY ANOTHER WEAK SYSTEM
FRI NIGHT. THE FLOW WILL REMAIN ONSHORE THROUGH THE WEEKEND. 33

&&

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

&&

$$

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