Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 191558
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT WED SEP 19 2014

.SYNOPSIS...A WEAK UPPER LEVEL TROUGH OVER THE AREA TODAY WILL KEEP
SKIES MOSTLY CLOUDY WITH A CHANCE OF MORNING SHOWERS. CLOUDS WILL
GRADUALLY DISSIPATE TONIGHT AS A STRONG UPPER LEVEL RIDGE BUILDS
OVER THE PACIFIC NORTHWEST. THE STRONG RIDGE AND LOW LEVEL OFFSHORE
FLOW WILL BRING SUNNY SKIES AND A WARMING TREND THIS WEEKEND. A
WEAKENING FRONT ON MONDAY WILL BRING INCREASING CLOUDS...A FEW
SHOWERS...AND COOLER TEMPERATURES. A LARGE UPPER LEVEL TROUGH WILL
MAINTAIN A WETTER AND COOLER PATTERN ALL NEXT WEEK.

&&

.SHORT TERM...A WEAK SURFACE TROUGH EMBEDDED IN WLY FLOW IS
DISSIPATING ALONG THE N WA COAST THIS MORNING. MODELS SHOW NO
REFLECTION AT THE SURFACE BY THIS AFTERNOON AND NOT MUCH UPPER LEVEL
SUPPORT. RADAR DATA DOES SHOW A FEW RESIDUAL SHOWERS FROM THE N
COAST EWD ACROSS THE STRAIT OF JUAN DE FUCA AND OVER THE N INTERIOR.
THESE SHOWERS WILL DECREASE THIS AFTERNOON. THE REST OF THE AREA
INCLUDING THE PUGET SOUND REGION SHOULD BE DRY TODAY ALTHOUGH CLOUDS
WILL HANG IN THROUGH THE AFTERNOON GIVEN THE MOIST ONSHORE FLOW.
HIGHS WILL AGAIN BE COOL...AROUND 70 FOR THE SOUND AND LOW 70S
ACROSS THE SW INTERIOR.

THE 500 MB RIDGE RAPIDLY AMPLIFIES OVER THE REGION TONIGHT THROUGH
SATURDAY. LOW LEVEL NORTHERLY FLOW DEVELOPS TONIGHT...THEN BECOMES
MORE STRONGLY OFFSHORE SATURDAY AS THERMALLY INDUCED LOW PRESSURE
FORMS JUST OFF THE COAST. DRYING SHOULD TAKE PLACE TONIGHT IN THE
MID/UPPER LEVELS. HOWEVER...A STRONG SUBSIDENCE INVERSION WILL TRAP
SOME AREAS OF LOW CLOUDS AND FOG IN A THIN LAYER BELOW 2 KT
ACCORDING TO NAM-BUFFER SOUNDINGS. ANY FOG/STRATUS SHOULD BURN OFF
RATHER QUICKLY BY MID-LATE MORNING. WITH THE PICK UP IN OFFSHORE
FLOW AND THE DRYING AIR MASS...HIGH TEMPERATURES WILL WARM A GOOD
4-10 DEGREES SATURDAY. THE COAST MAY SEE THE WARMEST TEMPERATURES
GIVEN THE LOCATION OF THE THERMALLY INDUCED TROUGH AHEAD OF THE
STRATUS SURGE.

UPPER LEVEL HIGH PRESSURE WILL START TO SHIFT FURTHER INLAND
SATURDAY NIGHT AND SUNDAY. THIS WILL ALLOW THE THERMALLY INDUCED
TROUGH TO SHIFT INLAND OVER INTERIOR WRN WA. SUNDAY SHOULD THEREFORE
BE THE WARMEST DAY OVER INLAND AREAS. HIGHS IN THE MID 85S WILL
PROBABLY BE COMMON. THE NAM MOS SHOULD AGAIN DO BEST IN THIS
SCENARIO BUT THE GUIDANCE IS STILL LIKELY HANDICAPPED A BIT BY
CLIMATOLOGY. THE FORECAST IS ALREADY TRENDED 2-4 DEGREES ABOVE THE
WARMEST MOS WHICH SEEMS REASONABLE. MERCER

.LONG TERM...NO CHANGES FROM PREVIOUS DISCUSSION...ALL FACTORS ARE
IN PLACE FOR A SW MARINE PUSH SUNDAY NIGHT. STRATUS SURGE UP THE
COAST ON SUNDAY...UPPER LEVEL RIDGE AXIS SHIFTING OVER W MT...AND
WEAKENING COLD FRONT APPROACHING THE COAST. THE SW MARINE PUSH
SUNDAY NIGHT SHOULD PUSH STRATUS UP THROUGH AT LEAST MOST OF PUGET
SOUND BY MONDAY MORNING. EARLIER NAM RUNS WERE LESS EXTENSIVE WITH
THE BOUNDARY LAYER RH MONDAY MORNING...BUT THE LATEST 06Z NAM COVERS
ALL OF W WA. THE NAM TIME-HEIGHT SECTION SHOWS A MARINE LAYER AROUND
4000 FEET DEEP MONDAY MORNING. IF THIS WORKS OUT THEN CLOUD COVER
WILL HAVE TO BE INCREASED FOR MONDAY.

THE MODELS HAVE BEEN FAIRLY CONSISTENT WITH MONDAYS APPROACHING
FRONTAL SYSTEM. MODELS SHOW THE FRONT REMAINING INTACT UNTIL IT
MOVES E OF 130W MONDAY MORNING. AT THAT POINT MODEL CROSS-SECTIONS
SHOW THE UPPER LEVEL TROUGH SHEARING AWAY FROM THE LOW LEVEL
FRONT...LEAVING IT TO STALL ALONG THE COAST. THE MODELS HAVE BEEN
DRY FOR W WA ON MONDAY...BUT WITH A DEEP MARINE LAYER IT IS POSSIBLE
THAT SOME DRIZZLE COULD DEVELOP MONDAY MORNING...CAUSED BY WEAK LIFT
FROM THE UPPER TROUGH AS IT CONTINUES MOVING INLAND.

BOTH GFS AND ECMWF HAVE A LONGWAVE UPPER LEVEL TROUGH DEVELOPING
OFFSHORE OVER THE NE PACIFIC BY TUESDAY WITH SW FLOW ALOFT OVER W
WA. THE TROUGH WILL DOMINATE THE PATTERN ALL NEXT WEEK FOR PERIODS
OF RAIN OR SHOWERS OVER W WA. BOTH MODELS SHOW A COLD FRONT REACHING
W WA TUESDAY NIGHT OR EARLY WEDNESDAY. THIS SYSTEM WILL PROBABLY
SCOOP UP THE REMNANTS OF MONDAYS STALLED FRONT AS IT MOVES INLAND.
KAM

&&

.AVIATION...WESTERLY FLOW ALOFT TODAY AND TONIGHT. AN UPPER LEVEL
RIDGE WILL BUILD TO THE SOUTHEAST OF THE AREA ON SATURDAY TURNING
FLOW ALOFT TO THE SOUTHWEST. AT THE SURFACE...A WEAKENING FRONTAL
SYSTEM OVER THE COASTAL WATERS THIS MORNING WILL DISSIPATE AS IT
MOVES INLAND TODAY. SURFACE HIGH PRESSURE WILL BUILD OVER SOUTHERN
BRITISH COLUMBIA ON SATURDAY GIVING INCREASING NORTHERLY FLOW AT THE
LOW LEVELS. THE AIR MASS IS MOIST AND STABLE.

SATELLITE IMAGERY AND PILOT REPORTS SHOW MULTIPLE CLOUD LAYERS WITH
TOPS OVER 8000 FEET. FROM ABOUT KSEA SOUTHWARD IN THE INTERIOR...THE
LOWER LEVELS BELOW 3000-5000 FT ARE CLEAR. MVFR CEILINGS ARE FOUND
IN THE INTERIOR FROM ABOUT KSEA NORTHWARD WITH LOCAL IFR CONDITIONS
OVER THE KITSAP PENINSULA. CONDITIONS N OF KHQM ALONG THE COAST ARE
LOW IFR IN STRATUS...LIGHT RAIN AND FOG.

CONDITIONS WILL GRADUALLY IMPROVE FROM THE SOUTH AND EAST TODAY WITH
MAINLY VFR CONDITIONS EXPECTED AFTER 22Z. CEILINGS WILL BREAK THIS
EVENING AS FLOW BECOMES MORE N/NE IN THE LOWER LEVELS AND THE AIR
MASS DRIES SOMEWHAT.

THE LATEST NAM12 SOLUTION SHOWS A LOW STRATUS AND FOG LAYER FORMING
BELOW A DEVELOPING LOW LEVEL SUBSIDENCE INVERSION IN THE INTERIOR AS
LOW LEVEL NORTHERLY FLOW INCREASES AND IS PRECEDED BY RADIATIONAL
COOLING. THE NAM12 MOS GUIDANCE AND TIME HEIGHT SECTIONS SHOW IFR
CONDITIONS THROUGH MUCH OF THE INTERIOR FROM KPAE SOUTHWARD 08Z-20Z
SAT...AND LOW IFR VIS IS POSSIBLE IN LOCATIONS WHERE THE STRATUS
INTERSECTS THE GROUND. THIS IS TYPICAL FOR THE EARLY FALL SEASON AS
OFFSHORE FLOW DEVELOPS BEHIND A WEAKENING FRONT. THIS CHANGE WILL BE
REFLECTED IN THE 18Z TERMINAL FORECASTS FOR KPAE...KSEA...KBFI AND
KOLM. THIS STRATUS AND FOG IS ALSO LIKELY TO FORM IN THE CHEHALIS
VALLEY AND IMPACT THE KHQM TERMINAL DURING THE LATE NIGHT AND
MORNING HOURS. ALBRECHT

KSEA...CEILINGS HAVE IMPROVED TO OVER 3000 FT AT THE TERMINAL AND
LOWER CEILINGS ARE LIFTING NORTH OF THE TERMINAL EXPECT CIGS TO
IMPROVE TO 050-060 LATE THIS MORNING WITH CLOUDS BREAKING DURING THE
LATE AFTERNOON OR EVENING HOURS. AS MENTIONED ABOVE...EXPECT LOW
STRATUS WITH CIGS 003-005 TO DEVELOP AROUND AND TO THE NORTH OF THE
TERMINAL ABOUT 09Z AS A SUBSIDENCE INVERSION DEVELOPS OVER THE AREA
AND TRAPPED LOW LEVEL MOISTURE CONDENSES WITH RADIATIONAL COOLING.
THE STRATUS WILL RAPIDLY SPREAD/DEVELOP SOUTHWARD ONCE IT FORMS.
THERE IS ABOUT A 50 PERCENT CHANCE THAT VISIBILITIES AT THE TERMINAL
WILL DROP TO BELOW 1/2SM FG IF THE STRATUS BASES ARE SLIGHTLY LOWER
THAN CURRENTLY EXPECTED. THE STRATUS WILL LIKELY BREAK UP AROUND 20Z
SAT. SOUTH WINDS TODAY 4-8 KT WILL BECOME LIGHT IN THE AFTERNOON
THEN BECOME NORTH 6-8 KT AFTER 05Z THROUGH SAT MORNING. ALBRECHT

&&

.MARINE...A WEAK AND DISSIPATING FRONT OVER THE COASTAL WATERS WILL
MOVE INLAND TODAY. BEHIND THE FRONT...SURFACE HIGH PRESSURE WILL
DEVELOP OVER SOUTHERN BRITISH COLUMBIA AND WILL STRENGTHEN SATURDAY
AND SATURDAY NIGHT. THIS WILL INCREASE LOW LEVEL NORTHERLY FLOW
TONIGHT THROUGH SATURDAY NIGHT.

FLOW WILL BECOME MORE EASTERLY LATE SATURDAY NIGHT INTO SUNDAY AS
THE HIGH OVER BRITISH COLUMBIA MOVES SOUTHEASTWARD. SMALL CRAFT
ADVISORY EAST WINDS ARE LIKELY AT THE EAST ENTRANCE TO THE STRAIT OF
JUAN DE FUCA AND THE ADJACENT COASTAL ZONE LATE SATURDAY NIGHT INTO
SUNDAY MORNING.

A WEAKENING FRONT WILL APPROACH THE COASTAL WATERS LATE SUNDAY INTO
SUNDAY NIGHT. THE APPROACH OF THE FRONT WILL LIKELY INDUCE A
SOUTHERLY SURGE OVER THE COASTAL WATERS SUNDAY INTO SUNDAY NIGHT. A
MUCH STRONGER FRONTAL SYSTEM WELL OFFSHORE WILL BE APPROACHING FROM
THE WEST ON TUESDAY. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.


&&

$$

WWW.WEATHER.GOV/SEATTLE

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