Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 202140
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
240 PM PDT SAT SEP 20 2014

.SYNOPSIS...AN UPPER RIDGE OVER THE PACIFIC NORTHWEST WILL SHIFT
EAST TO MONTANA THROUGH SUNDAY. LOW LEVEL OFFSHORE FLOW WILL SWITCH
TO ONSHORE FLOW SUNDAY. A WEAK FRONT WILL REACH THE AREA BY MONDAY
NIGHT AND A STRONGER FRONT WILL REACH THE AREA TUESDAY NIGHT AND
WEDNESDAY. A WEAK UPPER TROUGH WILL LINGER OVER THE REGION THURSDAY
AND FRIDAY BUT MAY GIVE WAY TO HIGH PRESSURE FOR NEXT WEEKEND.

&&

.SHORT TERM...THERMALLY INDUCED LOW PRESSURE EXTENDS FROM SW OREGON
TO THE NORTHERN OREGON COASTAL WATERS THIS AFTERNOON WITH AN UPPER
RIDGE OVER THE REGION. TONIGHT WILL BE A MILD CLEAR NIGHT. A SURGE
OF LOW MARINE STRATUS AND FOG WILL MOVE NORTH ALONG THE COAST LATER
TONIGHT AND SUNDAY MORNING AS PRESSURE GRADIENTS TURN ONSHORE AND
THE THERMAL LOW PRESSURE MOVES INLAND. THE COAST SHOULD SEE A RAPID
INCREASE IN LOW CLOUDS AND FOG SUNDAY MORNING WHILE THE INTERIOR
SHOULD REMAIN WARM AND SUNNY. CLOUDS WILL INCREASE AHEAD OF A WEAK
SYSTEM SUNDAY NIGHT AND THERE IS A CHANCE OF A LITTLE RAIN OR
SHOWERS...MAINLY COAST AND MOUNTAINS...AS THE WEAK FRONT BREAKS UP
OVER THE AREA.

.LONG TERM...TUESDAY NIGHT AND WEDNESDAY IS THE PERIOD OF TIME WHEN
WESTERN WA IS LIKELY TO HAVE PERIOD OF RAIN AS A STRONGER FRONTAL
SYSTEM MOVES SLOWLY THROUGH THE REGION. THERE MIGHT BE TWO SURGES OF
FAIRLY STRONG SLY FLOW ALOFT...ONE TUESDAY EVENING...AND ANOTHER
LATER WED OR WED NIGHT AND TUE-THU DO NOT LOOK LIKE A GOOD TIME TO
BE CAMPING OR HIKING...ESPECIALLY IN THE OLYMPICS. THERE WILL BE A
BROAD UPPER TROUGH CENTERED AROUND 140W TUESDAY. GFS HEIGHT FALLS
ASSOCIATED WITH THE FRONTAL SYSTEM STRETCH AND PETER OUT IN THE GFS
WHICH IS NOT NEARLY AS WET AS THE ECMWF. BOTH MODELS SHOW THE
HEAVIEST RAIN COAST AND OLYMPICS. AFTER A WET PERIOD...THE MAIN
UPPER TROUGH IS LIKELY TO DROP SOUTH THU AND FRI INTO CALIF/NV. THE
GFS AND EURO SOLUTIONS DIVERGE LATE IN THE WEEK...THE EURO KEEPS A
WEAKENING UPPER TROUGH OVER THE WEST WITH A CHANCE OF SHOWERS WHILE
THE GFS IS FASTER WITH AN APPROACHING UPPER RIDGE WITH A BETTER
PROSPECT FOR DRY WEATHER BY NEXT WEEKEND.

&&

.AVIATION...AN UPPER RIDGE OVER THE AREA WILL SHIFT INLAND TONIGHT
GIVING INCREASING SOUTHWEST FLOW ALOFT. AT THE SURFACE...HIGH
PRESSURE OVER SOUTHEASTERN BRITISH COLUMBIA COMBINED WITH A
THERMALLY INDUCED TROUGH OF LOW PRESSURE OVER THE COASTAL WATERS
WILL GIVE NORTHEAST TO EAST FLOW TONIGHT. THE SURFACE HIGH TO THE NE
OF THE AREA AND THE THERMAL TROUGH ALONG THE COAST WILL SHIFT
EASTWARD EARLY SUNDAY AND MARINE AIR ALONG THE SOUTHERN OREGON COAST
WILL SURGE NORTHWARD TO THE WASHINGTON COASTAL ZONES DURING THE
EARLY MORNING HOURS. THE AIR MASS IS STABLE AND DRY. LOW LEVEL
MOISTURE WILL INCREASE FROM THE SOUTH OVER THE COASTAL ZONES AFTER
12Z. THE INLAND AREAS WILL REMAIN CLEAR THROUGH SUNDAY WITH SHALLOW
FOG EARLY SUNDAY CONFINED TO THE MOST FOG PRONE RIVER VALLEYS
12Z-16Z. ALBRECHT

KSEA...SKIES WILL BE CLEAR THROUGH SUNDAY WITH GOOD VISIBILITY.
EXPECT NORTH WIND 10-15 KT THIS AFTERNOON TO BECOME NE 6-8 KT THIS
EVENING AND EASTERLY AFTER 09Z. WIND WILL SHIFT TO SW 4-6 KT ABOUT
15Z SUN. ALBRECHT

&&

.MARINE...HIGH PRES OVER SOUTHEASTERN BRITISH COLUMBIA COMBINED WITH
A THERMALLY INDUCED TROUGH OF LOW PRES ALONG THE COAST WILL GIVE
NORTHEAST TO EAST WINDS THROUGH TONIGHT. CHANNELED NORTHERLY FLOW
OVER THE PUGET SOUND AND HOOD CANAL WATERS ARE STRONG ENOUGH TO
MAINTAIN A SMALL CRAFT ADVISORY THERE UNTIL 8 PM THIS EVENING.

EASTERLY WINDS ARE EXPECTED TO INCREASE TO SMALL CRAFT ADVISORY
CRITERIA LATER THIS EVENING THROUGH SUNDAY MORNING AT THE WEST
ENTRANCE TO THE STRAIT OF JUAN DE FUCA AND THE ADJACENT NEARSHORE
COASTAL ZONE FROM CAPE FLATTERY TO JAMES ISLAND. A SMALL CRAFT
ADVISORY HAS BEEN ISSUED FOR THOSE ZONES STARTING AT 11 PM THIS
EVENING.

THE THERMAL TROUGH WILL SHIFT INLAND SUNDAY MORNING AND EAST OF THE
CASCADES SUNDAY AFTERNOON AS A WEAK FRONT MOVES INTO THE OFFSHORE
WATERS. THIS WILL INDUCE A MARINE PUSH THAT WILL RAPIDLY SURGE
NORTHWARD THROUGH THE COASTAL WATERS AROUND 5 AM SUNDAY. SMALL CRAFT
ADVISORY SOUTH WINDS ARE EXPECTED FOR A FEW HOURS STARTING AT THE
INITIATION OF THE SURGE OVER THE COASTAL WATERS AND WILL BE
ACCOMPANIED BY AREAS OF FOG AND VISIBILITIES LESS THAN 1 STATUTE
MILE. SMALL CRAFT ADVISORY WEST WINDS ARE LIKELY TO DEVELOP OVER
PORTIONS OF THE STRAIT OF JUAN DE FUCA DURING THE AFTERNOON AND
EVENING HOURS.

A WEAK FRONT WILL CROSS THE WATERS MONDAY. A STRONGER FRONTAL SYSTEM
WILL MOVE THROUGH THE COASTAL WATERS TUESDAY OR EARLY WEDNESDAY.
GALE OR NEAR GALE SOUTHEAST TO SOUTH WINDS ARE POSSIBLE OVER THE
COASTAL WATERS AHEAD OF THIS SECOND STRONGER FRONT AND SMALL CRAFT
ADVISORY CONDITIONS ARE POSSIBLE OVER PORTIONS OF THE INLAND WATERS.
ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY PUGET SOUND AND HOOD CANAL THROUGH 8 PM
      PDT THIS EVENING.
     SMALL CRAFT ADVISORY WEST ENTRANCE TO THE STRAIT OF JUAN DE
      FUCA AND COASTAL WATERS CAPE FLATTERY TO JAMES ISLAND OUT 10 NM
      STARTING AT 11 PM PDT THIS EVENING.
     SMALL CRAFT ADVISORY ALL COASTAL ZONES STARTING 5 AM PDT
      SUNDAY.

&&

$$

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