Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 131102
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
402 AM PDT MON APR 13 2015

.SYNOPSIS...A STRONG COLD FRONT OFFSHORE WILL SWEEP ACROSS THE
REGION LATER TODAY. THIS FRONT WILL USHER IN A COLD AIR MASS AND
BRING MORE SNOW TO THE MOUNTAINS. THE LOW PRESSURE SYSTEM ALOFT
WILL MOVE OVER THE AREA TONIGHT AND THEN EXIT TUESDAY AFTERNOON.
HIGH PRESSURE ALOFT WILL RETURN TO THE REGION TONIGHT AND REMAIN
OVER THE AREA ON WEDNESDAY.

&&

.SHORT TERM...
THE LEADING EDGE OF THE RAIN SHIELD ASSOCIATED WITH A STRONG COLD
FRONT OFFSHORE WAS OVER THE COASTAL WATERS AT THIS TIME. THE RAIN
WILL OVERSPREAD THE WESTERN AND FAR NORTHERN PORTION OF THE CWA
THIS MORNING AND THE REST OF THE CWA THIS AFTERNOON. THE FRONT
WILL MOVE QUICKLY ACROSS THE CWA THIS AFTERNOON. ANTICIPATE GUSTY
WINDS WITH THE FRONTAL PASSAGE ALONG WITH A SHORT PERIOD OF
LOCALLY HEAVY PRECIP.

A COLD /TEMPERATURES NEAR -30 DEGREES CELSIUS AT 500 MB/ AND
UNSTABLE AIR MASS WILL ARRIVE BEHIND THE FRONT. THIS...CANNOT
RULE OUT THE POSSIBILITY OF ISOLATED TSTMS WITH THE FRONT AS IT
MOVES ONSHORE. HOWEVER...THE BEST CHANCE OF TSTMS WILL BE IN THE
WAKE OF THE FRONT OR AS THE VERY COLD POOL ALOFT MOVES OVER THE
REGION...WHICH WILL BE TONIGHT. THE FORECAST INDICATED A MENTION
OF ISOLATED TSTMS ALONG THE COAST WESTWARD AND WITHIN THE PUGET
SOUND CONVERGENCE ZONE /PSCZ/...WHICH WILL DEVELOP BEHIND THE
FRONT. ALTHOUGH TSTMS ARE IN THE FORECAST FOR THESE AREAS...IT
DOES NOT MEAN THAT THERE IS A ZERO THREAT OF TSTMS ELSEWHERE.
ANTICIPATE A LARGE QUANTITY OF SMALL HAIL OR GRAUPEL TO ACCOMPANY
ANY TSTMS OVER THE LOWLANDS. ALSO...CANNOT RULE OUT THE
POSSIBILITY OF A RAIN/SNOW MIX WITHIN THE PSCZ TUE MORNING.

CONCERNING SNOW...A WINTER WEATHER ADVISORY WAS ISSUED FOR THE
MOUNTAINS. THE SNOW WILL PROBABLY BE THE HEAVIEST WITH THE FRONT.
HOWEVER...LOCALLY HEAVY SNOW SHOWERS OVERNIGHT WILL JUST PILE IT
UP. THE HEAVIEST SNOW WILL LIKELY FALL WITHIN THE CONVERGENCE ZONE
THAT WILL DEVELOP NEAR THE KING/SNOHOMISH COUNTY BORDER AND THEN
DRIFT INTO NORTHERN KING COUNTY BY TUE MORNING. THIS MEANS THAT
UNLIKE LAST TIME...THE HEAVIEST SNOW WILL LIKELY FALL ON
SNOQUALMIE PASS INSTEAD OF STEVENS PASS. THERE IS A CHANCE THAT
SNOWFALL AMOUNTS WILL BE HIGHER THAN FORECAST...ESPECIALLY WHERE
TSTMS OCCUR.

GRADUALLY IMPROVING CONDITIONS ARE ANTICIPATED ON TUE AS THE UPPER
TROF EXITS THE REGION AND AN UPPER RIDGE FOLLOWS ON ITS HEELS. THE
UPPER RIDGE WILL PREVAIL OVER THE REGION ON WED BUT IT WILL
PROBABLY BE SOMEWHAT DIRTY /CONTAINING QUITE A BIT OF MID AND/OR
HIGH CLOUDS/...RESULTING IN FILTERED SUNSHINE. TEMPS SHOULD CLIMB
TO NEAR NORMAL ON WED.

.LONG TERM...
CONFIDENCE IN THIS PERIOD WAS STILL NOT VERY HIGH DUE TO LACK OF
AGREEMENT AND CONSISTENCY IN THE MEDIUM RANGE SOLUTIONS. DECIDED
TO LEAVE THE INHERITED FORECAST INTACT. THE ONLY CHANGES MADE WERE
TO MATCH UP BETTER WITH ADJACENT WEATHER FORECAST OFFICES.

&&

.AVIATION...A STRONG COLD FRONT ASSOCIATED WITH A DEEPENING LOW NEAR
50N 135W WILL MOVE RAPIDLY THROUGH THE COASTAL WATERS THIS MORNING
AND SWEEP THROUGH THE INTERIOR OF WESTERN WASHINGTON EARLY THIS
AFTERNOON. THE AIR MASS WILL BE STABLE WITH INCREASING MOISTURE
UNTIL FRONTAL PASSAGE WHEN IT WILL BECOME MOIST AND UNSTABLE.
STRENGTHENING SW FLOW ALOFT WILL TURN WESTERLY THIS AFTERNOON AND
NORTHWESTERLY TONIGHT.

P6SM SCT-BKN050 BKN-OVC120 EARLY THIS MORNING WILL LOWER TO BKN035
OVC050 THIS MORNING AS THE FRONT APPROACHES THE AREA. RAIN WILL
SPREAD ONTO THE COAST EARLY THIS MORNING AND WILL SPREAD INLAND BY
ABOUT 15Z. DESPITE THE LIGHT RAIN...CONDITIONS ARE EXPECTED TO
REMAIN IN THE VFR CATEGORY IN ADVANCE OF THE FRONT. EXPECT CIGS TO
FALL TO BKN010-030 WITH ONSHORE FLOW BEHIND THE FRONT FOR A SHORT
WHILE. CLOUDS AND POST FRONTAL SHOWERS WILL THEN BECOME CONFINED TO
THE TERRAIN...COASTAL AREAS...AND A PUGET SOUND CONVERGENCE ZONE IN
THE KSEA-KPAE AREA LATE THIS AFTERNOON AND TONIGHT. THE AIR MASS
WILL BECOME QUITE COLD AND UNSTABLE ALOFT BEHIND THE FRONT...AND
THERE IS A CHANCE OF TSTMS WITH HEAVIER SHOWERS ON THE COAST AND
WITH THE PUGET SOUND CONVERGENCE ZONE IN THE INTERIOR. ALBRECHT

KSEA...EXPECT CIGS BKN100 TO GRADUALLY THICKEN AND LOWER TO OVC050
16Z-17Z AND TO OVC035 BY 19Z-20Z AS THE FRONT MOVES THROUGH. -RA
WILL BEGIN ABOUT 16Z. A PUGET SOUND CONVERGENCE ZONE WILL QUICKLY
FORM BEHIND THE FRONT EARLY THIS AFTERNOON TO THE NORTH OF THE
TERMINAL. IT IS EXPECTED TO SLIDE SOUTH TO THE KBFI-KSEA AREA BY
THIS EVENING. CIGS MAY LOWER TO BKN010 22Z ONWARD IN THE CONVERGENCE
ZONE. THERE IS A RISK OF TSTMS WITH THE CONVERGENCE ZONE 22Z THIS
AFTERNOON THROUGH TONIGHT. LIGHT WINDS EARLY THIS MORNING WILL
BECOME SOUTH 8-10 KT BY 1Z THEN S TO SW 12-14KT WITH GUSTS TO 25 KT
THIS AFTERNOON AND EVENING. ALBRECHT

&&

.MARINE...A 992 MB LOW NEAR 50N 135W WILL DEEPEN TO 980 MB MIDDAY
TODAY AS IT CURLS NE TO HAIDA GWAII. THE ASSOCIATED STRONG COLD
FRONT WILL MOVE RAPIDLY THROUGH THE COASTAL WATERS LATE THIS MORNING
AND THROUGH THE INLAND WATERS SHORTLY AFTER NOON. EXPECT WINDS TO
RAPIDLY RISE FROM THE S FOR A FEW HOURS IN ADVANCE OF THE FRONT THEN
SHIFT TO WLY BEHIND THE FRONT AND DECREASE. THE STRONGEST W WINDS
THIS AFTERNOON AND TONIGHT WILL BE OVER THE CENTRAL AND EASTERN
STRAIT OF JUAN DE FUCA. POST FRONTAL SMALL CRAFT ADVISORY SW WINDS
WILL AFFECT THE NORTHERN INLAND WATERS AND PUGET SOUND AND HOOD
CANAL. A GALE WARNING IS IN EFFECT FOR THE COASTAL WATERS THOUGH
EARLY THIS AFTERNOON. A GALE WARNING IS IN EFFECT FOR LATE THIS
MORNING THROUGH TONIGHT FOR THE CENTRAL AND EASTERN STRAIT OF JUAN
DE FUCA. SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE REMAINING
WATERS.

STRONG LOW LEVEL ONSHORE FLOW THIS AFTERNOON AND TONIGHT WILL SLOWLY
RELAX ON TUESDAY. EXPECT MOST WATERS TO SEE SMALL CRAFT ADVISORY
WEST TO SOUTHWEST FLOW IN THE ONSHORE FLOW. THERE IS A RISK OF
THUNDERSTORM ACTIVITY OVER THE COASTAL WATERS AND THE CENTRAL PUGET
SOUND CONVERGENCE ZONE AREA THIS AFTERNOON THROUGH TONIGHT.

HIGH PRES WILL BUILD INTO THE WATERS FOR TUE NIGHT INTO THU. ANOTHER
FRONT IS EXPECTED TO RIDE BY TO THE NORTH OF THE COASTAL WATERS WED
AND THU WITH THE RIDGE OF HIGH PRES REMAINING OVER THE INTERIOR OF
WESTERN WA. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORIES ARE IN EFFECT FOR THE OLYMPICS AND
     CASCADES BEGINNING LATE THIS MORNING AND CONTINUING THROUGH AT
     LEAST THIS EVENING.

PZ...GALE WARNINGS ARE IN EFFECT FOR MUCH OF THE COASTAL WATERS
     AND THE CENTRAL/EASTERN STRAIT OF JUAN DE FUCA.

     SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE REMAINING WATERS.

     SMALL CRAFT ADVISORY FOR ROUGH BAR IN EFFECT TIL 4 PM TUESDAY
     FOR THE GRAYS HARBOR BAR.

&&

$$

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