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000
FXUS66 KSEW 170943
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT FRI MAY 17 2013

.SYNOPSIS...A WEAK UPPER TROUGH WILL MOVE INTO WESTERN WASHINGTON
TODAY...PRODUCING SOME SHOWERS AND ALLOWING SUN BREAKS. A STRONGER
TROUGH WILL ARRIVE SATURDAY FOR A SHOWERY COOLER DAY. AN UPPER RIDGE
WILL BUILD OFFSHORE SUNDAY AND MOVE INLAND MONDAY FOR WARMER AND
MOSTLY DRY WEATHER. A DEEP UPPER LOW WILL REACH THE AREA TUESDAY AND
LINGER THROUGH MIDWEEK FOR SHOWERY AND UNSEASONABLY COOL WEATHER.

&&

.SHORT TERM...LIGHT SOUTHWEST FLOW ALOFT CONTINUES OVER WESTERN
WASHINGTON AHEAD OF A WEAK UPPER TROUGH EXTENDING FROM JUST WEST OF
VANCOUVER ISLAND ACROSS THE OFFSHORE WATERS AND INTO NORTHERN
CALIFORNIA. A BAND OF SCATTERED SHOWERS HAS BEEN LIFTING SLOWLY
NORTHWARD ACROSS THE FORECAST AREA OVERNIGHT...WHICH SEEMS TO BE A
COMMA SHAPED FEATURE ASSOCIATED WITH A WEAK CIRCULATION IN THE UPPER
TROUGH OFFSHORE. TEMPERATURES AT 2 AM WERE MAINLY IN THE UPPER 40S
TO MID 50S.

THE OFFSHORE TROUGH WILL WEAKEN AND MOVE INTO THE PACIFIC NORTHWEST
TODAY...AND ONSHORE LOW LEVEL FLOW WILL INCREASE DURING THE DAY. THE
AIR MASS WILL REMAIN FAIRLY MOIST AND BECOME SLIGHTLY UNSTABLE WITH
DAYTIME HEATING. IT IS HARD TO SAY IF ANOTHER ORGANIZED BAND OF
SHOWERS WILL DEVELOP OR MOVE INTO THE FORECAST AREA...BUT THE
COMBINATION OF MOIST ONSHORE FLOW AND INSTABILITY SHOULD ANYWAY
PRODUCE AT LEAST LIGHT SPOTTY SHOWERS. THERE SHOULD ALSO BE SUN
BREAKS IN THE AFTERNOON...THOUGH LESS SUN AND LOWER TEMPERATURES
THAN THURSDAY LOOKS LIKE THE RIGHT FORECAST.

A WEAK FRONT AND ITS UPPER TROUGH WILL APPROACH THE PACIFIC
NORTHWEST FROM OFFSHORE TONIGHT...WEAKENING AND MOVING INLAND ON
SATURDAY. THERE ARE STILL SOME DIFFERENCES IN MODEL TIMING WITH THIS
FEATURE...BUT SATURDAY SHOULD BE A CLOUDY AND COOLER DAY WITH
SHOWERS GIVING MEASURABLE PRECIPITATION TO PRETTY MUCH THE ENTIRE
FORECAST AREA. THE SNOW LEVEL WILL FALL TO AROUND 4500 FT. DRIVING
THE MOUNTAIN HIGHWAYS SHOULD NOT BE A REAL PROBLEM...BUT SEVERAL
INCHES OF SNOW ARE LIKELY OVER THE HIGHER TERRAIN OF THE OLYMPICS
AND CASCADES.

THE UPPER TROUGH WILL MOVE EAST OF THE CASCADES SATURDAY NIGHT...AND
A MINOR UPPER RIDGE WILL BUILD OFFSHORE WHILE LOW LEVEL ONSHORE FLOW
CONTINUES. THIS SHOULD PRODUCE A PUGET SOUND CONVERGENCE ZONE
SATURDAY NIGHT THAT WILL PROBABLY LINGER INTO SUNDAY MORNING. THE
UPPER RIDGE WILL MOVE TO THE PACIFIC NORTHWEST COAST SUNDAY
AFTERNOON THOUGH...AND LIGHT SCATTERED SHOWERS SHOULD BECOME LIMITED
TO THE MOUNTAINS WITH PARTLY SUNNY SKIES OVER THE LOWLANDS. THE
RIDGE WILL MOVE INLAND SUNDAY NIGHT AND ANY SHOWERS LINGERING INTO
THE EVENING SHOULD END.   MCDONNAL

.LONG TERM...THE UPPER RIDGE WILL BE WELL EAST OF THE CASCADES
MONDAY...AND A DEEP UPPER LOW WILL START DIGGING SOUTHEAST TOWARD
THE REGION ACROSS SOUTHEAST ALASKA AND THE NORTHERN BRITISH COLUMBIA
COAST. IT SHOULD BE A DRY AND PARTLY SUNNY DAY FOR WESTERN
WASHINGTON.

THE UPPER LOW WILL DIG INTO WESTERN WASHINGTON OR ITS OFFSHORE
WATERS TUESDAY AFTERNOON OR TUESDAY NIGHT AND LINGER OVER THE
PACIFIC NORTHWEST THROUGH WEDNESDAY NIGHT. THIS FEATURE WILL BRING
SHOWERY AND UNSEASONABLY COOL WEATHER...AND THE SNOW LEVEL WILL
PROBABLY FALL TO AROUND 3000 FT.

THE UPPER LOW WILL PROBABLY START SHIFTING EAST OF THE AREA
THURSDAY...BUT IT LOOKS LIKE WE ARE STARTING TO SEE A TREND IN THE
MODELS THAT KEEPS THE LOW IN THE VICINITY OF THE FORECAST AREA A BIT
LONGER. TIMING THE DEPARTURE OF THESE UPPER LOWS IS ALWAYS UNCERTAIN
IN THE FORECAST DAY 5 TO 7 RANGE...AND THEY HAVE A TENDENCY TO HANG
AROUND LONGER THAN INITIALLY EXPECTED. FOR NOW THE FORECAST FOR
MARGINAL IMPROVEMENT ON THURSDAY LOOKS FINE.   MCDONNAL

&&

.AVIATION...AN UPPER LEVEL TROUGH OVER THE PAC NW WILL MAINTAIN
ISOLATED TO SCATTERED SHOWERS ACROSS WESTERN WA TODAY. THE FLOW
ALOFT IS WESTERLY. MAY SEE POCKETS OF MVFR CIGS THIS
MORNING...OTHERWISE EXPECT VFR CONDITIONS BY THE AFTERNOON. SHOWERS
WILL DECREASE TONIGHT AS THIS TROUGH LIFTS E/NE AND EXITS. 33

KSEA...ISOLATED TO SCATTERED LIGHT SHOWERS WILL REMAIN IN THE
VICINITY TODAY...SOUTHERLY WINDS AT THE SURFACE. 33

&&

.MARINE...ONSHORE FLOW WILL PREVAIL ACROSS WESTERN WA TODAY. PRES
GRADIENTS WILL STRENGTHEN TONIGHT AND WILL LIKELY SEE SMALL CRAFT
ADVISORY WINDS THROUGH THE CENTRAL AND EAST STRAIT OF JUAN DE FUCA.
A COLD FRONT WILL THEN CROSS THE AREA ON SAT FOR INCREASING WINDS
OVER THE WATERS. HIGH PRES OVER THE NE PAC WILL MAINTAIN ONSHORE
FLOW THROUGH THE WEEKEND. ANOTHER FRONT WILL CROSS THE REGION MON
NIGHT OR TUE. 33

&&

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

&&

$$

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