Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 300413
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT FRI MAY 29 2015

.SYNOPSIS...INCREASING ONSHORE FLOW WILL BRING MARINE STRATUS INTO
PORTIONS OF WESTERN WASHINGTON LATE TONIGHT AND SATURDAY MORNING.
SUNSHINE SHOULD PREVAIL SATURDAY AFTERNOON WITH MODERATING HIGH
TEMPERATURES...THEN WARMING A FEW DEGREES ON SUNDAY. AN UPPER LEVEL
LOW OFF THE CALIFORNIA COAST WILL LIFT NORTHEAST INTO THE AREA
MONDAY AND TUESDAY. SHOWERS AND A CHANCE OF THUNDERSTORMS ARE
EXPECTED ALONG WITH A COOLING TREND. UNSETTLED WEATHER MAY CONTINUE
THROUGH THURSDAY...THEN DRYING OUT TOWARD THE END OF NEXT WEEK.

&&

.SHORT TERM...ONSHORE FLOW IS STRONGER AGAIN TONIGHT WITH OVER +3 MB
THROUGH THE STRAIT AND OTH-SEA GRADIENT UP TO +3.8 MB. STRATUS ALONG
THE COAST SHOULD SPREAD DOWN THE STRAIT OVERNIGHT AND ALSO THROUGH
THE CHEHALIS GAP REACHING SHELTON AND OLYMPIA BEFORE DAWN. GRADIENTS
SHOULD BE STRONG ENOUGH TO SPREAD SHALLOW STRATUS INTO PUGET SOUND
ENTERING THE N SOUND FROM THE ERN STRAIT AND THE S/CENTRAL SOUND
FROM THE SRN WEAK SURGE. MOS IS MIXED ON WHETHER STRATUS REACHES
SEA-TAC BUT A FEW HOURS OF LOW CLOUDS AFTER SUNRISE SEEMS LIKELY.
LOW CLOUDS WILL BURN BACK TO THE COAST BY AFTERNOON WITH FILTERED
SUNSHINE GIVEN LOTS OF HIGH CLOUDS OVER THE AREA. HIGHS WILL COOL
BACK TO THE LOW AND MID 70S AROUND PUGET SOUND AND ONLY IN THE LOW
60S NEAR THE COAST AND STRAIT AT BEST.

MODELS HAVE TRENDED TOWARD A SOMEWHAT STRONGER RIDGE FOR SUNDAY WITH
LIGHTER W-NW ONSHORE FLOW. MORNING STRATUS MAY BE MORE LIMITED AND
PATCHY ACROSS THE INTERIOR WITH HIGH TEMPERATURES WARMING A FEW
DEGREES. MODELS SHOW MOISTURE ADVECTION IN THE MID/UPPER LEVELS WITH
A WARM AND WEAKLY UNSTABLE AIR MASS ALOFT. THERE COULD BE SOME AC
CLOUDS AROUND BUT PROBABLY TOO SHALLOW A LAYER FOR MUCH CONVECTIVE
DEVELOPMENT.

THE AIR MASS ALOFT REMAINS UNSTABLE SUNDAY NIGHT AND MONDAY WITH
SOME THREAT OF THUNDERSTORMS JUST ABOUT ANYWHERE. THE CURRENT
FORECAST HAS THUNDER OVER THE CASCADES WHICH IS FINE FOR THE TIME
BEING BUT ISOLATED THUNDER MAY NEED TO BE EXPANDED OVER THE LOWLANDS
IF MODELS CONTINUE TO LOOK CONVECTIVE. AN UPPER LEVEL OFF THE CA
COAST WILL BE MOVING NE AND WILL BRING DEEPER MOISTURE INTO THE AREA
LATER MONDAY AND MONDAY NIGHT WITH A BETTER CHANCE OF SHOWERS AND
POSSIBLY THUNDERSTORMS.

.LONG TERM...NO CHANGE FROM PREVIOUS DISCUSSION...TUESDAY LOOKS LIKE
THE WETTEST DAY AS THE UPPER LOW IS RIGHT OVER WESTERN WASHINGTON.
POPS ARE LIKELY OR BETTER. HIGH TEMPERATURES WILL BE MAINLY IN THE
LOW 60S. CONDITIONS SLOWLY IMPROVE AS THE WEEK WEARS ON BUT THE
OVERALL PATTERN IS STILL TROUGHY AND THERE WILL BE A CHANCE OF
SHOWERS EACH DAY THROUGH THURSDAY. BY FRIDAY HEIGHTS ARE BUILDING
AND THE SHOWER CHANCE SHOULD BE CONFINED TO THE CASCADES.
TEMPERATURES WILL ALSO RISE AND BY FRIDAY SHOULD BE BACK TO NORMAL
OR EVEN A FEW DEGREES ABOVE NORMAL. BURKE

&&

.AVIATION...AN UPPER LEVEL LOW OVER CENTRAL BRITISH COLUMBIA
COMBINED WITH A RIDGE OVER THE DESERT SW WILL GIVE LIGHT SW FLOW
ALOFT. THE AIR MASS IS STABLE WITH SOME HIGH LEVEL MOISTURE AND
MARINE MOISTURE ALONG THE WASHINGTON COAST.

THE COMBINATION OF HEATING IN THE INTERIOR...CONVECTION EAST OF THE
CASCADES...AND THE UPPER LEVEL LOW TO THE NORTH HAS TIGHTENED
ONSHORE PRES GRADIENTS THIS EVENING. WITH THE INCREASED ONSHORE
FLOW...EXPECT STRATUS TO SURGE FARTHER INLAND OVERNIGHT...AND BE
DEEPER AND A LITTLE MORE PERSISTENT INTO MIDDAY SATURDAY. THE 00Z
NAM12 SHOWS A 5-7 HR PERIOD OF STRATUS IN THE SEATTLE AREA SAT
MORNING WITH BASES AROUND 010 AND TOPS 020...CONSISTENT WITH COASTAL
OBSERVATIONS AND THE STRENGTH OF THE ONSHORE FLOW. STRATUS WILL MOVE
IN EARLIER AROUND THE STRAIT OF JUAN DE FUCA AND THE CHEHALIS GAP.

THE AIR MASS ON SATURDAY OVER THE MOUNTAINS WILL BE MORE STABLE THAN
THE PAST SEVERAL DAYS...AND AFTERNOON AND EVENING CONVECTION IS NOT
EXPECTED IN THE CASCADES. ALBRECHT

KSEA...GOOD VFR CONDITIONS ARE EXPECTED THROUGH TONIGHT WITH JUST
HIGH LEVEL CLOUDS OVER THE AREA. STRATUS WILL MOVE INTO THE TERMINAL
AROUND 12Z AND PERSIST UNTIL ABOUT 19Z. AT THIS TIME IT APPEAR THAT
CIGS WILL SETTLE IN AROUND 007 AND VIS AROUND 5SM BR. EXPECT SW WIND
4-6 KT THROUGH SAT. ALBRECHT

&&

.MARINE...A SURFACE RIDGE WELL OFFSHORE COMBINED WITH LOWER PRES E
OF THE CASCADES WILL GIVE ONSHORE FLOW THROUGH THE WEEKEND. INFLOW
THROUGH THE STRAIT IS STRONGEST TODAY AND IS EXPECTED TO BE SOMEWHAT
WEAKER SAT THROUGH SUN.

WHILE WINDS HAVE BEEN SLOW TO INCREASE THIS EVENING...THE KUIL-KBLI
GRADIENT OF +3.4 MB SUGGESTS THAT GALES WILL DEVELOP THIS EVENING.
WILL MAINTAIN THE GALE WARNING FOR THE CENTRAL AND EASTERN STRAIT OF
JUAN DE FUCA AND THE SMALL CRAFT ADVISORY FOR ADMIRALTY INLET AND
THE NORTHERN INLAND WATERS. WINDS AND SEAS WILL GRADUALLY DIMINISH
DURING THE EARLY MORNING HOURS ON SAT.

AN UPPER LEVEL LOW MOVING IN FROM THE SOUTHWEST WILL INDUCE A
SOUTHWESTERLY MARINE PUSH MONDAY MORNING. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL AND EAST STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY ADMIRALTY INLET AND NORTHERN INLAND
      WATERS.

&&

$$

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