Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 030952
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
252 AM PDT TUE MAY 3 2016

.SYNOPSIS...STRONG ONSHORE FLOW WILL DEVELOP TODAY FOR MUCH COOLER
WEATHER COMPARED TO YESTERDAY. MEANWHILE...A LOW PRESSURE SYSTEM
ALONG OR OFF THE COAST WILL BRING SOMEWHAT UNSETTLED CONDITIONS TO
THE AREA THROUGH THURSDAY. MAX TEMPERATURES WILL BE NEAR NORMAL
WEDNESDAY AND THURSDAY. HIGH PRESSURE ALOFT WILL RETURN ON FRIDAY.

&&

.SHORT TERM...
THERE WERE SCATTERED SHOWERS OVER THE WESTERN HALF OF THE
FORECAST AREA EARLY THIS MORNING. MEANWHILE...TEMPS WILL BE MUCH
COOLER TODAY...BY ABOUT 15 TO 20 DEGREES AT MOST PLACES...AFTER
THE RECORD WARMTH EXPERIENCED ON MONDAY.

IN THE VERY NEAR TERM...EXPECT THE ONSHORE PRESSURE GRADIENT TO
STRENGTHEN DURING THE DAY TODAY FOR WELCOME RELIEF FROM THE VERY
RECENT HOT SPELL. A DEEPENING MARINE LAYER COMBINED WITH
STRENGTHENING ONSHORE FLOW WILL LIKELY KILL THE THREAT OF TSTMS
OVER MUCH OF THE AREA TODAY. THE EXCEPTION WILL BE THE CASCADES
WHERE THERE WILL BE A SLIGHT RISK OF TSTMS THIS AFTERNOON AND
EARLY EVENING.

THE PACIFIC NORTHWEST WILL BE UNDER THE INFLUENCE OF AN UPPER
LEVEL TROF CENTERED OFFSHORE THROUGH WEDNESDAY. THE UPPER TROF
AXIS WILL SHIFT OVER THE WEST COAST ON THURSDAY...HOWEVER THIS
SYSTEM WILL HAVE BECOME SPLIT BY THEN. EXPECT MUCH OF THE ENERGY
TO DIVE SOUTH...CARVING A CLOSED UPPER LEVEL LOW OFF THE CA COAST.
THE MAIN IMPACT THIS SYSTEM WILL HAVE ON OUR WEATHER WILL BE
COOLER TEMPS AND SOMEWHAT UNSETTLED CONDITIONS. MAX TEMPS ACROSS
THE CWA WILL BE NEAR NORMAL ON WEDNESDAY AND THURSDAY.

.LONG TERM...
AN UPPER LEVEL RIDGE WILL BUILD OVER THE AREA ON FRIDAY FOR
SUNNIER AND WARMER CONDITIONS. BEYOND FRIDAY...THE LATEST GFS
SOLUTION WAS COMING MORE IN LINE WITH THE ECMWF SOLUTION OF
DIGGING AN UPPER TROF OVER WESTERN CANADA LATE THIS WEEKEND.
CONSEQUENTLY...THE GFS WAS FLATTER WITH THE RIDGE ON SATURDAY.
THE DEEPENING TROF WILL SERVE TO INITIATE AN ONSHORE PUSH ON
SATURDAY.

TEMPS ARE ANTICIPATED TO START OFF ABOVE NORMAL AT THE BEGINNING
OF THIS PERIOD BUT LOWER TO CLOSER TO NORMAL THE END OF THE
PERIOD. THE FORECAST TEMPS REFLECTED A COMPROMISE BETWEEN THE
WARMER GFS AND THE COOLER ECMWF SOLUTIONS. IN OTHER WORDS...THERE
IS A CHANCE THAT THE FORECAST TEMPS ARE TOO WARM FOR SUNDAY AND
MONDAY.

AT THIS TIME...KEPT THE FORECAST DRY DURING THIS PERIOD BUT
CANNOT RULE OUT THE POSSIBILITY OF A FEW SHOWERS OVER THE NORTH
CASCADES EARLY NEXT WEEK.

&&

.AVIATION...SOUTHERLY FLOW ALOFT OVER WESTERN WASHINGTON EARLY
THIS MORNING...BETWEEN THE UPPER HIGH OVER THE NORTHERN ROCKY
MOUNTAINS AND OFFSHORE UPPER TROUGH ALONG 135W. THE AIR MASS IS
SOMEWHAT MOIST ABOVE 10000 FT WITH WEAK INSTABILITY...PRODUCING
ISOLATED SHOWERS. SOUTHERLY FLOW ALOFT WITH WEAK INSTABILITY AND
INCREASING MOISTURE WILL CONTINUE THROUGH TONIGHT. IN ADDITION LOW
LEVEL ONSHORE FLOW WILL INCREASE.

SCATTERED TO BROKEN ALTOCUMULUS ACROSS THE FORECAST AREA EARLY
THIS MORNING WITH CEILINGS MOSTLY AT OR ABOVE 10000 FT. THERE ARE
ISOLATED SHOWERS AND SCATTERED VIRGA FALLING FROM THESE CLOUDS.
MARINE STRATUS HAS MOVED NORTH UP THE COAST AND INTO THE LOWER
CHEHALIS VALLEY WITH WITH LOW-END MVFR AND LOCAL IFR CONDITIONS.
CONDITIONS SHOULD LOWER ACROSS THE FORECAST AREA TONIGHT AS
MOISTURE INCREASES ALL LEVEL.

KSEA...SOUTH WIND 3-8 KT...BECOMING SOUTHWEST 6-12 KT THIS
AFTERNOON...THEN NORTHWEST AND NORTH TONIGHT. CEILING SHOULD
MOSTLY REMAIN AT OR ABOVE 10000 FT TODAY...THEN GRADUALLY FALL
TONIGHT. MCDONNAL

&&

.MARINE...ONSHORE FLOW WILL INCREASE TODAY AND THIS EVENING AS
THE THERMAL TROUGH MOVES EAST OF THE CASCADES AND HIGH PRESSURE
BUILDS OVER THE OFFSHORE WATERS. THERE IS A GOOD CHANCE OF GALE
WESTERLIES IN THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA THIS
AFTERNOON AND TONIGHT...WITH SMALL CRAFT ADVISORY WINDS LIKELY
OVER ALL OTHER MARINE ZONES EXCEPT PUGET SOUND AND HOOD CANAL. THE
GALE WATCH IS BEING UPGRADED TO A GALE WARNING. ONSHORE FLOW WILL
CONTINUE WEDNESDAY THROUGH FRIDAY. MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING IN EFFECT FROM NOON TODAY TIL 3 AM WEDNESDAY
 FOR THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

  SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE REMAINING
 WATERS...EXCEPT THE PUGET SOUND/HOOD CANAL...BEGINNING
 THIS AFTERNOON.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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


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