Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201536
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
840 AM PDT SAT SEP 20 2014

.SYNOPSIS...AN UPPER RIDGE OVER THE PACIFIC NORTHWEST WILL SHIFT
EAST TO MONTANA LATE SUNDAY. LOW LEVEL OFFSHORE FLOW WILL INCREASE
TODAY AND THEN SWITCH TO ONSHORE FLOW SUNDAY AFTERNOON. TEMPERATURES
WILL PEAK ON THE COAST TODAY AND AND IN THE INTERIOR ON SUNDAY. A
WEAKENING FRONT WILL ARRIVE MONDAY AND A STRONGER FRONT WILL REACH
THE AREA WEDNESDAY.

&&

.SHORT TERM...AT 830AM LOW CLOUDS AND FOG ARE IN THE STRAIT AND
BANKED UP AGAINST THE NORTH SLOPES OF THE OLYMPICS. THERE ARE LOW
CLOUDS IN PARTS OF THE SAN JUANS AND SRN VANCOUVER ISLAND...AND FROM
AROUND WHIDBEY ISLAND EAST THROUGH THE LOWLANDS OF SNOHOMISH COUNTY
AND NEAR I5 IN SKAGIT COUNTY. THE CLOUDS AND FOG EXTEND SOUTH
THROUGH THE METRO AREA. THE COAST...THE MOUNTAINS...AND MUCH OF SW
WA HAVE SUNNY SKIES AND MOST AREAS WILL BE SUNNY BY MIDDAY. OFFSHORE
FLOW WILL INCREASE TODAY AS THERMALLY INDUCED LOW PRESSURE EXPANDS
NORTH UP THE COAST. HIGHS WILL BE ABOVE NORMAL AND TOP OUT NEAR 80
ON THE COAST AND FROM SEATTLE SOUTH...WHILE HIGHS IN THE STRAIT MAY
STRUGGLE TO HIT 70. AFTER A MILD NIGHT SUNDAY WILL BE WARM FOR MOST
OF WRN WA...EXCEPT ON THE COAST WHERE ONSHORE FLOW AND MARINE
STRATUS WILL DEVELOP. MARINE AIR WILL PUSH INTO THE INTERIOR OF WRN
WA SUNDAY NIGHT GIVING A MOSTLY CLOUDY DAY FOR MONDAY...WITH THE
MORNING LOW CLOUDS FOLLOWED BY INCREASING MID AND HIGH CLOUDS FROM A
WEAKENING FRONT APPROACHING THE AREA. THAT WEAKENING FRONT WILL MOVE
THROUGH THE AREA MONDAY NIGHT FOR A CHANCE OF SHOWERS.

.LONG TERM...EXTENDED MODELS IN GOOD AGREEMENT ON THE 00Z RUN. BOTH
THE GFS AND ECMWF STAYING WITH THE IDEA OF A BREAK ON TUESDAY WITH
THE NEXT SYSTEM APPROACHING THE AREA LATE IN THE DAY. THIS WILL
INDUCE A LITTLE LOW LEVEL OFFSHORE FLOW PUSHING HIGHS INTO THE LOWER
TO MID 70S OVER THE INTERIOR WITH A LITTLE COOLER TEMPERATURES ALONG
THE COAST. LARGE UPPER LEVEL TROUGH SETTING UP OFF THE COAST TUESDAY
NIGHT AND REMAINING OFF THE COAST FOR MOST OF THE REMAINDER OF THE
EXTENDED PERIOD. MUCH STRONGER AND MORE ORGANIZED SYSTEM SPINNING
OUT OF THE TROUGH MOVING THROUGH WESTERN WASHINGTON ON WEDNESDAY
WITH THE PRECIPITATION OUT AHEAD OF THE SYSTEM MOVING INTO THE AREA
TUESDAY NIGHT. WITH THE TROUGH REMAINING OFFSHORE CHANCE OF RAIN
WILL STAY IN THE FORECAST ON THURSDAY. BOTH MODELS HAVE THE TROUGH
MOVING INLAND ON FRIDAY WHICH WOULD LOWER THE POPS. WILL WAIT FOR A
COUPLE OF MORE SIMILAR SOLUTIONS BEFORE TAKING THE CHANCE OF RAIN
OUT OF THE FRIDAY FORECAST. FELTON

&&

.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 NORTHERLY FLOW TODAY AND NORTHEAST TO EAST FLOW TONIGHT.
THE AIR MASS IS STABLE AND DRY EXCEPT FOR AREAS OF LOW LEVEL
MOISTURE TRAPPED BELOW A SHALLOW LOW LEVEL INVERSION IN THE INTERIOR
THIS MORNING.

LOW STRATUS WITH BASES AROUND FLIGHT LEVEL 005 AND TOPS AROUND 015
COVER THE PUGET SOUND LOWLANDS FROM ABOUT KBVS SOUTHWARD TO NEAR
KOLM. STRATUS ALSO COVERS THE STRAIT OF JUAN DE FUCA AND THE NORTH
END OF THE OLYMPIC PENINSULA. FORECAST MODELS AND A LOCAL
RADIATION-BASED TOOL SHOW THE STRATUS RAPIDLY ERODING AROUND
18Z...WHICH WILL LEAVE CLEAR SKIES ACROSS THE AREA. A STRONG
SUBSIDENCE INVERSION IS CENTERED AROUND 2000 FEET THIS MORNING. THIS
INVERSION IS FORECAST TO CONTINUE TO STRENGTHEN AND BECOME SHALLOWER
THIS AFTERNOON THROUGH SUNDAY MORNING. THE INVERSION WILL BE SHALLOW
ENOUGH ON SUNDAY MORNING THAT FOG WILL BE LIMITED TO MOST FAVORED
RIVER VALLEYS AND WILL BE NO MORE THAN 100-200 FT DEEP. ALBRECHT

KSEA...LOW STRATUS AT THE TERMINAL WILL RAPIDLY MIX OUT SHORTLY
AFTER 18Z LEAVING CLEAR SKIES AT THE TERMINAL BY 19Z. CLEAR
CONDITIONS WILL CONTINUE THROUGH SUNDAY. EXPECT NORTH WINDS 9-13 KT
THROUGH 02Z THIS EVENING TO BECOME NE 4-6 KT THIS EVENING AND E 5-8
KT EARLY SUN MORNING. ALBRECHT

&&

.MARINE...HIGH PRES OVER SOUTHEASTERN BRITISH COLUMBIA COMBINED WITH
A THERMALLY INDUCED TROUGH OF LOW PRES ALONG THE COAST WILL GIVE
NORTH TO NORTHEAST FLOW TO THE WATERS TODAY. CHANNELED NORTHERLY
FLOW OVER THE PUGET SOUND AND HOOD CANAL WATERS ARE STRONG ENOUGH TO
WARRANT A SMALL CRAFT ADVISORY THERE UNTIL 03Z THIS EVENING. A 10 FT
SWELL OVER THE COASTAL WATERS PROMPTED THE ISSUANCE OF A SMALL CRAFT
ADVISORY FOR HAZARDOUS SEAS THERE. THE SWELL WILL SUBSIDE TO BELOW
10 FT BY 11 AM THIS MORNING AND WILL CONTINUE TO SUBSIDE TONIGHT.
EASTERLY FLOW WILL LIKELY INCREASE TO SMALL CRAFT ADVISORY CRITERIA
LATE TONIGHT AND SUNDAY MORNING OVER THE WEST ENTRANCE OF THE STRAIT
OF JUAN DE FUCA AND THE ADJACENT NEARSHORE COASTAL ZONE FROM CAPE
FLATTERY TO JAMES ISLAND. WILL WAIT TO SEE LATEST 12Z FORECAST
MODELS BEFORE ISSUING THE ADVISORY THERE.

THE THERMAL TROUGH WILL SHIFT INLAND ON SUNDAY AND EAST OF THE
CASCADES SUNDAY EVENING AS A WEAK FRONT MOVES INTO THE OFFSHORE
WATERS. THIS WILL INDUCE A MARINE PUSH DURING THE DAY SUNDAY THAT
WILL TURN WINDS TO THE SOUTH OVER THE COASTAL WATERS AND RESULT IN
AREAS OF FOG SPREADING NORTHWARD ALONG THE COAST. THE MARINE AIR
WILL PUSH INLAND SUNDAY AFTERNOON AND SUNDAY NIGHT AND WILL LIKELY
RESULT IN THE DEVELOPMENT OF SMALL CRAFT ADVISORY LEVEL WEST WINDS
THROUGH PORTIONS OF THE STRAIT OF JUAN DE FUCA.

A WEAK FRONT WILL CROSS THE WATERS MONDAY. A STRONGER FRONTAL SYSTEM
WILL MOVE THROUGH THE AREA TUESDAY AND WEDNESDAY. THIS SYSTEM COULD
BRING GALE FORCE SOUTHERLY WINDS TO THE COASTAL WATERS AND SMALL
CRAFT ADVISORY WINDS TO PARTS OF THE INLAND WATERS. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS COASTAL WATERS UNTIL
      11 AM PDT TODAY.
     SMALL CRAFT ADVISORY PUGET SOUND AND HOOD CANAL THROUGH
      8 PM PDT THIS EVENING.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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








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