Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 180959
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT SAT APR 18 2015

.SYNOPSIS...HIGH PRESSURE AT THE SURFACE AND ALOFT IS CENTERED OVER
THE COASTAL WATERS. THE HIGH PRESSURE WILL GIVE DRY WEATHER TO THE
AREA THROUGH MONDAY AND TEMPERATURES WILL AVERAGE ABOVE NORMAL FOR
MID TO LATE APRIL. THE PATTERN IS EXPECTED TO CHANGE TO A CLOUDIER
AND COOLER ONE ON TUESDAY WITH A CHANCE OF SHOWERS AS A TROUGH OF
LOW PRESSURE ALOFT APPROACHES THE COAST AND LOW LEVEL FLOW BECOMES
ONSHORE.

&&

.SHORT TERM...A RIDGE OF HIGH PRESSURE AT THE SURFACE AND ALOFT IS
CENTERED AROUND 130W EARLY THIS MORNING. SKIES ACROSS THE REGION ARE
MOSTLY CLEAR WITH EXCEPTION TO A SMALL AREA TO THE SOUTHEAST OF THE
OLYMPICS AND A SMALL STRIP ON THE COAST...WHERE THERE ARE SOME
PATCHES OF LOW CLOUDS AND POSSIBLE FOG.

RUN AFTER RUN OF THE HRRR SHOWS THE DEVELOPMENT OF A RATHER LARGE
AREA OF FOG AND STRATUS ALONG THE IMMEDIATE WASHINGTON COAST AND
THROUGH THE STRAIT OF JUAN DE FUCA. BUT MULTISPECTRAL SATELLITE
IMAGES SO LITTLE OR NO FOG FORMING AT ALL. AS A RESULT...THE
FORECAST FOR THIS MORNING WILL BE LIMITED TO PATCHY LOW CLOUDS AND
FOG ALONG THE COAST AND THROUGH THE CHEHALIS GAP INTO THE NORMALLY
FOGGIER LOCATIONS OF THE SOUTH INTERIOR. FOG AND LOW CLOUDS THAT DO
DEVELOP WILL BE QUITE SHALLOW AND SHOULD BE GONE BY 10 AM.

THE OVERALL PATTERN TODAY THROUGH SUNDAY WILL CHANGE LITTLE.
TEMPERATURES ALOFT WARM 1-2C TODAY AND SUNDAY. EXPECT SURFACE
TEMPERATURES TO WARM SIMILARLY WITH HIGHS TODAY IN THE MID 60S
TO MID 70S IN THE INTERIOR AND HIGHS ON SUNDAY ANOTHER 1-3 DEGREES
WARMER. CLEAR SKIES AND DRY CONDITIONS WILL ALLOW TEMPERATURES
TONIGHT TO FALL INTO THE UPPER 30S TO MID 40S.

THE OFFSHORE UPPER RIDGE WILL MOVE INLAND ON MONDAY...AND MONDAY
SHOULD BE THE WARMEST DAY OF THIS WARM SPELL. AREAS FROM EVERETT
SOUTHWARD SHOULD SEE LOWER TO MID 70S ON MONDAY WITH PLENTY OF
SUNSHINE. ALBRECHT

.LONG TERM...THE UPPER RIDGE SHIFTS EAST OF THE CASCADES MONDAY
NIGHT AS A FRONTAL SYSTEM MOVES INTO THE OFFSHORE WATERS. IT APPEARS
LIKELY THAT A GOOD MARINE PUSH WILL BEGIN MONDAY NIGHT OR TUESDAY.

THE 06Z GFS AND 00Z ECMWF ARE A SLIGHTLY SLOWER IN MAKING THE
TRANSITION TO ONSHORE FLOW...AND THAT MAKES THE CLOUD AND
TEMPERATURE FORECAST FOR TUESDAY CHALLENGING. LATE APRIL MARINE
PUSHES AFTER A WARM SPELL CAN BE QUITE DRASTIC HERE IN WESTERN
WASHINGTON WITH 24 HOUR TEMPERATURE FALLS OF 15 TO 20 DEGREES NOT
OUT OF THE QUESTION. THE LATEST WRFGFS FROM THE UNIVERSITY OF
WASHINGTON SHOWS HIGHS IN SEATTLE ON TUESDAY AROUND 70 WHILE THE
NAM12 SHOWS HIGHS IN THE MID TO UPPER 50S. THE CURRENT FORECAST OF
MID 60S WILL BE RETAINED FOR NOW. SHOWER CHANCES DO NOT APPEAR LARGE
WITH THE FRONT OR THE PASSING UPPER TROUGH...GUIDANCE VALUES IN THE
CHANCE CATEGORY ARE SIMILAR TO THE CURRENT FORECAST.

AFTER TUESDAY...THE MODEL RUN-TO-RUN CONSISTENCY IS NOT GREAT WITH
THE 06Z GFS SHOWING WEAK SHORT WAVE RIDGING ON WED WHILE THE ECMWF
SHOWS A STRONG PUSH TUE NIGHT THROUGH THU FOLLOWED BY THE APPROACH
OF A DEEP UPPER TROUGH LATE IN THE WEEK. NO SIGNIFICANT CHANGES
WERE MADE TO THE LONG TERM FORECAST EARLY THIS MORNING. ALBRECHT

&&

.AVIATION...AN UPPER LEVEL RIDGE OVER THE PAC NW WILL LEAD TO DRY
AND STABLE WEATHER ACROSS WESTERN WA TODAY. THE FLOW ALOFT IS
NORTHWESTERLY. VFR CONDITIONS ARE EXPECTED...ALTHOUGH MAY SEE PATCHY
SHALLOW FOG THIS MORNING IN THE SOUTH SOUND AND COAST. OTHERWISE
SKIES WILL REMAIN CLEAR. 33

KSEA...VFR CONDITIONS TODAY. EXPECT N WINDS TO 10 KT. 33

&&

.MARINE...LOW PRESSURE ALONG THE COAST WILL MAINTAIN NORTHERLY FLOW
ACROSS WESTERN WA TODAY. MODELS CONTINUE TO SHOW NW WINDS OF 15 TO
25 KT OVER THE COASTAL WATERS AND STRAIT OF GEORGIA - SMALL CRAFT
ADVISORIES REMAIN IN EFFECT. EXPECT EASTERLY WINDS THROUGH THE
STRAIT OF JUAN DE FUCA...BECOMING WEST 10 TO 20 KT THIS AFTERNOON.
NORTHERLY FLOW WILL CONTINUE THROUGH SUNDAY AND MONDAY. ONSHORE FLOW
WILL INCREASE ON TUESDAY AS A COLD FRONT MOVES INLAND. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS...NORTHERN INLAND
      WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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