Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221606
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
905 AM PDT MON SEP 22 2014

.SYNOPSIS...A WEAK FRONT OFFSHORE WILL MOVE TO NEAR THE WASHINGTON
COASTLINE AND STALL TODAY...THEN MOVE INLAND LATE TONIGHT AND
TUESDAY MORNING. A STRONGER FRONTAL SYSTEM WILL IMPACT THE REGION
TUESDAY NIGHT AND WEDNESDAY BRINGING WET AND UNSETTLED CONDITIONS OR
TYPICAL FALL WEATHER. GRADUAL DRYING WILL OCCUR LATE IN THE WEEK AS
AN UPPER TROUGH OVER THE AREA SHIFTS INLAND AND IS REPLACED WITH
WEAK HIGH PRESSURE ALOFT.

&&

.SHORT TERM...MORNING SATELLITE IMAGES AND SURFACE DATA SHOW A
WEAKENING COLD FRONT SITTING OVER THE WASHINGTON OFFSHORE WATERS.
THE UPPER PORTIONS OF THE FRONT ARE SHEARING EASTWARD ACROSS WESTERN
WASHINGTON THIS MORNING AND WILL PUSH EAST OF THE AREA ABOUT NOON.
MID LEVEL LIFT...MOISTURE AND INSTABILITY ABOVE THE MARINE LAYER AND
ASSOCIATED WITH THIS FEATURE HAVE TRIGGERED SOME THUNDERSTORMS OVER
THE CASCADES...OVER THE EASTERN OLYMPIC PENINSULA...AND NORTH OF
BELLINGHAM. SHOWER ACTIVITY CONTINUES TO DEVELOP SOMEWHAT ON A LINE
FROM THE STRAIT OF GEORGIA SSE TO AROUND MT RAINIER. OTHER SHOWERS
OVER THE COASTAL WATERS PRODUCED SOME LIGHTNING STRIKES THERE AROUND
6 AM. FORECASTS WERE UPDATED TO INCLUDE A CHANCE OF THUNDERSTORMS
FOR THIS MORNING ACROSS THE AREA. EXPECT SHOWERS TO COME TO AN END
ACROSS THE INTERIOR DURING THE AFTERNOON HOURS AS THE UPPER LEVEL
INSTABILITY SHIFTS WELL EAST OF THE AREA AND THE LOW LEVEL FRONT
SLOWS AND NEARLY STALLS OVER THE COASTAL WATERS.

THE 12Z NAM AND A QUICK LOOK AT THE INCOMING 12Z GFS SHOWS A SMALL
SHORT WAVE TROUGH LIFTING NE ACROSS THE AREA FROM ABOUT MIDNIGHT
TONIGHT TIL ABOUT 7 AM TUE. THE MID LEVEL AIR MASS WILL REMAIN
SOMEWHAT UNSTABLE OVERNIGHT AND THE NAM SHOWS RATHER IMPRESSIVE
LIFT AS THIS FEATURE MOVES THROUGH. WILL UPDATE THE FORECAST FOR
LATER TONIGHT TO INCLUDE A CHANCE OF SHOWERS THROUGHOUT WESTERN
WASHINGTON. AT THIS TIME...MID LEVEL INSTABILITY APPEARS MARGINAL
ENOUGH TO KEEP TSTMS OUT OF THE FORECAST LATE TONIGHT.

YET ANOTHER BATCH OF SHOWERS BLOSSOM OVER THE AREA TUESDAY AFTERNOON
AS ANOTHER SHORT WAVE TROUGH WITH ITS UPPER LEVEL DIFFLUENCE AND
SOME INSTABILITY SHOOTS OUT AHEAD OF A DEVELOPING WARM FRONT FROM
THE CENTRAL OREGON COAST INTO AN OCCLUDING 972 MB LOW THAT WILL BE
MOVING TO 46N 138W. WILL ALSO UPDATE THE FORECAST FOR TUE TO INCLUDE
SHOWERS IN THE AFTERNOON HOURS. THERE IS AGAIN A MARGINAL CHANCE OF
TSTM ACTIVITY WITH THIS SHORT WAVE TROUGH.

THE MAIN COLD FRONT ASSOCIATED WITH THE OFFSHORE LOW WILL MOVE
ACROSS THE AREA TUE NIGHT. THE 12Z GFS SHOWS NEARLY AN INCH OF RAIN
WITH THIS FRONT TUE NIGHT...AND THE FRONT WILL BE PRECEDED BY WINDY
CONDITIONS ALONG THE COAST. SHOWERS WILL TAPER OFF FROM THE WEST
BEHIND THE FRONT ON WEDNESDAY.

WITH THE INCREASE IN CLOUDS AND THE PRECIPITATION...THE AROUND 80
DEGREE TEMPERATURES THAT WE EXPERIENCED ON SUNDAY WILL BECOME AN
INCREASINGLY DISTANT MEMORY AS WE MOVE INTO THE FALL SEASON. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...AN UPPER
TROUGH OVER THE NORTHEAST PACIFIC ON THURSDAY WILL SHIFT ONSHORE AND
WEAKEN ON FRIDAY. THERE SHOULD BE A GRADUALLY DECREASING CHANCE OF
SHOWERS DURING THIS PERIOD. FOR THE WEEKEND...A WEAK UPPER RIDGE
WILL MOVE OVER THE AREA FOR DRY AND SUNNIER WEATHER WITH A SLIGHT
WARMING TREND. SCHNEIDER

&&

.AVIATION...A WEAK UPPER TROUGH OVER WRN WA THIS MORNING WILL LIFT
NE THIS AFTERNOON. THE PARENT ROUGH WILL REMAIN OFFSHORE BUT THE
ASSOCIATED FRONT WILL MOVE THROUGH THE AREA TUE NIGHT AND WED. S-SW
FLOW ALOFT THROUGH TUE AND UNSTABLE ABOVE THE MARINE LAYER. ONSHORE
FLOW TODAY WILL MAINTAIN AREAS OF MVFR MARINE CLOUDS THROUGH ABOUT
18Z BEFORE BREAKING UP. MULTIPLE CLOUD LAYERS THIS AFTERNOON BUT
MOSTLY VFR CIGS EXPECTED. THE INSTABILITY ALOFT IS PRODUCING
ISOLATED STRIKES THIS MORNING BUT THIS ACTIVITY WILL LIFT NE OF THE
AREA OVER THE NEXT COUPLE HOURS. ISOLATED STRIKES COULD DEVELOP
AGAIN ACROSS WRN WA LATE TONIGHT AS ANOTHER WEAK TROUGH LIFTS ACROSS
AHEAD OF THE MAIN FRONT TUE NIGHT. THE AIR MASS WILL REMAIN MOIST
THROUGH TUE.

KSEA...MORNING MVFR STRATUS W/ CIGS 1500-2500 FT...SCATTERING AND
LIFTING AFTER 18Z. MULTIPLE CLOUD LAYERS THIS AFTERNOON BUT MOSTLY
VFR EXPECTED. THE SLIGHT RISK OF THUNDER SHOULD END BY LATE THIS
MORNING. ANOTHER RISK OF THUNDER LATE TONIGHT/EARLY TUE IS TOO
MINIMAL TO INDICATE IN TAFS AT THIS TIME. THE SOUTHERLY WINDS 4 TO 8
KNOTS. FELTON/MERCER

&&

.MARINE...THE FORECAST WAS UPDATED TO INCLUDE A CHANCE OF
THUNDERSTORMS OVER THE WATERS FOR THIS MORNING. THE FRONT GENERATING
THE INSTABILITY IS OVER THE COASTAL WATERS AND WILL DISSIPATE THIS
AFTERNOON. SMALL CRAFT WINDS OVER THE COASTAL WATERS ARE EXPECTED
TODAY BEHIND THE FILLING 1010MB SURFACE LOW.

A STRONGER FRONTAL SYSTEM...ASSOCIATED WITH A DEVELOPING SURFACE LOW
WELL OFFSHORE...WILL MOVE INTO THE OFFSHORE WATERS TUESDAY AND INTO
THE COASTAL WATERS ON TUESDAY NIGHT. SOUTH TO SOUTHEAST WINDS AHEAD
OF THE FRONT STILL EXPECTED TO BE IN THE HIGH END OF SMALL
CRAFT...30 KNOTS. SMALL CRAFT ADVISORY SOUTHEAST WINDS ARE LIKELY IN
NW-SE CHANNELED WATERS SUCH AS THE NORTHERN INLAND WATERS AND THE
EAST ENTRANCE TO THE STRAIT AS WELL.

WINDS WILL EASE LATE WEDNESDAY INTO THE END OF THE WEEK AS A 980 MB
LOW NEAR 46N 135W SLOWLY FILLS AND DRIFTS NORTH. MERCER/FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS THIS AFTERNOON.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
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