Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 291020
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PDT WED APR 29 2015

.SYNOPSIS...AN UPPER TROUGH WILL BRING SCATTERED SHOWERS TO THE AREA
THROUGH THIS EVENING. ISOLATED THUNDERSTORMS ARE POSSIBLE WITH THE
PUGET SOUND CONVERGENCE ZONE DURING THE AFTERNOON AND EARLY EVENING.
THURSDAY AND FRIDAY WILL BE A MOSTLY DRY DAYS ACROSS WESTERN
WASHINGTON AS A WEAK SYSTEM MOVES THROUGH BRITISH COLUMBIA. A FEW
SHOWERS MAY STILL LINGER ACROSS THE FAR NORTH. SATURDAY THROUGH
MONDAY SHOULD BE A PERIOD OF WARMER AND SUNNIER WEATHER WITH A WEAK
UPPER RIDGE OVER THE PACIFIC NORTHWEST. UNSETTLED WEATHER WITH
COOLER TEMPERATURES WILL RETURN BY TUESDAY.

&&

.SHORT TERM...RADAR INDICATES CONVERGENCE ENHANCED SHOWERS ARE
LIFTING A BIT FURTHER NORTH...MAINLY ACROSS ADMIRALTY INLET TO
SNOHOMISH AND FAR N KING COUNTY. SLY GRADIENT HAS INCREASED A BIT
THE PAST FEW HOURS WHICH EXPLAINS THE NORTHWARD MOVEMENT. MESO
MODELS ALL AGREE THAT THE CONVERGENCE ZONE WILL DISSIPATE EARLY THIS
MORNING AS THE FLOW WEAKENS. OTHERWISE...JUST A FEW SCATTERED
SHOWERS OVER WRN WA WITH A WEAKLY UNSTABLE ATMOSPHERE IN THE LOWER
LEVELS.

AFTER A LULL IN SHOWERS THIS MORNING...ANOTHER SHORT WAVE WILL PUSH
THROUGH WRN WA THIS AFTERNOON AND EVENING. MODELS AGREE THAT SHOWERS
WILL BE MORE NUMEROUS FROM THE COAST ACROSS THE NRN HALF OF THE
FORECAST AREA. A PUGET SOUND CONVERGENCE ZONE IS EXPECTED TO DEVELOP
ACCORDING TO THE 4KM WRFGFS ALTHOUGH DETAILS AND TIMING DIFFER ON
THE NAM-12. THE WRFGFS IS THE FAVORED MODEL WHICH DEVELOPS A
CONVERGENCE ZONE ACROSS N SEATTLE TO AROUND EVERETT THIS
AFTERNOON...THEN LIFTING A BIT NWD DURING THE EVENING POSSIBLY
CLEARING THE SEATTLE AREA. THE GFS40 LI`S ARE -1 TO 0C AS THE 500 MB
TROUGH AXIS MOVES ACROSS RIGHT AROUND 00Z. THIS IS PERFECT TIMING
AROUND THE TIME OF MAXIMUM DIURNAL WARMING WHICH WILL INCREASE
INSTABILITY. THERE WAS ONE STRIKE NOTED EARLIER IN THE EVENING WELL
OFFSHORE WHICH ALSO BACKS UP THE IDEA OF AN UNSTABLE AIR MASS
APPROACHING. DECIDED TO INCLUDE A CHANCE OF THUNDERSTORMS WITH THE
CONVERGENCE BAND THIS AFTERNOON AND EVENING. ELSEWHERE...JUST
SCATTERED OR ISOLATED SHOWERS CONTINUING THROUGH THE EVENING.

SHOWERS WILL GRADUALLY DIMINISH BEHIND THE TROUGH AXIS OVERNIGHT. A
WEAK CONVERGENCE ZONE MAY LINGER INTO EARLY THURSDAY MORNING BUT
NOTHING SHOULD BE WELL ORGANIZED BY THAT TIME. 500 MB HEIGHTS RISE
INTO THE MID 5600`S WITH LIGHT ONSHORE GRADIENTS. HIGHS SHOULD WARM
AT LEAST A COUPLE DEGREES ALONG WITH PARTLY SUNNY SKIES.

A WEAK SHORT WAVE MOVING ACROSS B.C. THURSDAY AND FRIDAY WILL
MAINTAIN SOME CLOUDS ACROSS THE NRN HALF OF THE FORECAST AREA. THERE
COULD BE A FEW SHOWERS OR VERY LIGHT RAIN ACROSS THE EXTREME N AT
TIMES...BUT MOST THE AREA WILL STAY DRY AND PARTLY SUNNY DURING THE
DAY. MODELS ARE A LITTLE SLOWER TO MOVE THE SYSTEM OUT OF THE AREA
FRIDAY SO HEIGHTS MAY STAY SUPPRESSED WITH WEAK ONSHORE FLOW KEEPING
TEMPERATURES FROM WARNING MUCH. HIGHS WILL PROBABLY STAY IN THE LOW
TO MID 60S AROUND PUGET SOUND FRIDAY.

.LONG TERM...A WEAK HIGH PRESSURE RIDGE BUILDS OVER THE PAC NW
SATURDAY THROUGH MONDAY. THERE WILL BE VARYING DEGREES OF ONSHORE
FLOW AND 500 MB HEIGHTS STAY BELOW 5700M. HIGHS WILL LIKELY STRUGGLE
TO REACH PAST THE UPPER 60S...THOUGH A COUPLE OF SPOTS COULD REACH
ABOVE 70 OVER THE WEEKEND. SURFACE GRADIENTS ARE MORE WLY ORIENTED
WHICH SHOULD KEEP MORNING LOW CLOUDS LIMITED TO THE COAST...LOWER
CHEHALIS...AND STRAIT. OTHERWISE PARTLY TO MOSTLY CLEAR SKIES.

MODELS ARE CONSISTENT WITH BRINGING A CUT OFF LOW INTO THE AREA FROM
THE NW MONDAY NIGHT AND TUESDAY. ONSHORE FLOW WILL INCREASE AND
UPPER HEIGHTS WILL FALL. HIGHS ON TUESDAY WILL COOL BACK TO THE
UPPER 50S OR LOW 60S WITH CLOUDS AND SHOWERS POSSIBLE. DEPENDING ON
THE LOW TRACK THERE COULD ALSO BE SOME THUNDERSTORMS BUT THERE IS
PLENTY OF TIME TO WATCH THE EVOLUTION OF THIS SYSTEM.  MERCER

&&

.AVIATION...AN UPPER LEVEL TROUGH AND WEAK SURFACE FRONT OFFSHORE
WILL MOVE MOVE THROUGH THE AREA THIS AFTERNOON. SOUTHWESTERLY FLOW
ALOFT AHEAD OF THE TROUGH BECOMING NORTHWESTERLY BEHIND THE TROUGH.
AIR MASS STABILIZING THIS MORNING BECOMING SOMEWHAT UNSTABLE LATER
THIS AFTERNOON.

STILL SOME SHOWER ACTIVITY EARLY THIS MORNING WITH THE SHOWERS
MOSTLY CONFINED TO THE CONVERGENCE ZONE FROM NEAR KPAE TO NORTH OF
SNOQUALMIE PASS AND ALONG THE COAST. CEILINGS WITH AND JUST SOUTH OF
THE CONVERGENCE ZONE NEAR 2000 FEET. OVER THE REMAINDER OF THE AREA
MOSTLY JUST SCATTERED CLOUDS AROUND 5000 FEET. CONVERGENCE ZONE WILL
DISSIPATE THIS MORNING WITH THE LOWER CEILINGS HANGING ON UNTIL
14-16Z. SCATTERED CLOUDS CONTINUING OVER THE REMAINDER OF THE AREA.
CEILINGS LOWERING THIS AFTERNOON AS THE UPPER LEVEL TROUGH MOVES
INLAND WITH 3500-5000 FOOT CEILINGS COMMON. ANOTHER CONVERGENCE ZONE
WILL FORM LATE IN THE DAY BUT WILL REMAIN BETWEEN MOUNT VERNON AND
KPAE. THERE IS A POSSIBILITY OF A THUNDERSTORM IN THE CONVERGENCE
ZONE.

KSEA...LOWER CEILINGS LEFT BEHIND BY THE CONVERGENCE ZONE HANGING
TOUGH. THE AREA OF 2000 FOOT CEILINGS IS SMALL...BETWEEN KBFI-KSEA
AND KRNT. JUST SCATTERED CLOUDS TO THE WEST AND SOUTH. WILL HOLD ON
TO THE 2000 FOOT DECK FOR A COUPLE MORE HOURS BEFORE GOING FOR
SCATTERED CLOUDS. CEILINGS NEAR 5000 FEET DEVELOPING AGAIN THIS
AFTERNOON. SOUTHERLY WIND 4 TO 8 KNOTS RISING TO 8 TO 123 KNOTS
LATER THIS AFTERNOON. FELTON

&&

.MARINE...NEXT FRONT MOVING THROUGH THE WATERS THIS AFTERNOON BUT
DOES NOT LOOK TO BE STRONG ENOUGH TO CREATE SMALL CRAFT ADVISORY
WINDS. WESTERLY PUSH THROUGH THE STRAIT BEHIND THE FRONT LATE IN THE
DAY AND A SMALL CRAFT ADVISORY HAS BEEN ISSUED FOR THE CENTRAL AND
EASTERN STRAIT TONIGHT.

HIGH PRES WILL BUILD INTO THE COASTAL WATERS ON THURSDAY AND WILL
GRADUALLY SHIFT OFFSHORE FRIDAY INTO THE WEEKEND. SMALL CRAFT
ADVISORY NORTHWEST WINDS ARE LIKELY LATE AFTERNOON AND EVENING HOURS
OVER THE COASTAL WATERS BEGINNING FRIDAY. FELTON

A SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS REMAINS IN EFFECT
EARLY THIS MORNING DUE TO AN EARLY MORNING EBB CURRENT AND SEAS NEAR
10 FEET. SEAS WILL SUBSIDE BELOW 10 FEET AROUND 6 AM. FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.

&&

$$

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










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