439
FXUS66 KSEW 151529
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
829 AM PDT TUE SEP 15 2026
SYNOPSIS
UPPER LEVEL TROUGH MOVING THROUGH THE AREA TODAY. UPPER LEVEL
RIDGE BUILDING INTO BRITISH COLUMBIA TONIGHT AND WEDNESDAY.
THERMALLY INDUCED SURFACE TROUGH MOVING UP THE COASTLINE
WEDNESDAY NIGHT WILL SHIFT INLAND THURSDAY. UPPER LEVEL RIDGE
OVER THE AREA THURSDAY REMAINING IN PLACE THROUGH SUNDAY WITH
LOW LEVEL ONSHORE FLOW. WEAK SYSTEM APPROACHING MONDAY.
SHORT TERM /TODAY THROUGH THURSDAY/
THE LATEST FORECAST REMAINS ON TRACK WITH NO UPDATES THIS
MORNING. UPPER LEVEL TROUGH OVER SOUTHERN BRITISH COLUMBIA THIS
MORNING MOVING THROUGH WESTERN WASHINGTON LATER THIS MORNING
INTO THE EARLY AFTERNOON HOURS. SKIES REMAINING CLOUDY/MOSTLY
CLOUDY AS THE TROUGH MOVES THROUGH WITH A SLIGHT CHANCE OF
SHOWERS. DRY NORTHERLY FLOW ALOFT BEHIND THE TROUGH WITH SKIES
CLEARING FROM THE NORTH THIS AFTERNOON OVER THE NORTHERN HALF OF
THE AREA. HIGHS BELOW NORMAL, IN THE 60S.
UPPER LEVEL RIDGE BUILDING INTO BRITISH COLUMBIA TONIGHT AND
WEDNESDAY AS THE UPPER LEVEL TROUGH MOVES FURTHER SOUTH. NORTH
NORTHEASTERLY FLOW ALOFT WILL DRY OUT THE AIR MASS CLEARING
SKIES OUT IN THE EVENING. LIGHT FLOW IN THE LOWER LEVELS
OVERNIGHT COMBINED WITH THE RECENT RAINS WILL ALLOW FOG TO FORM
MOSTLY IN THE FOG PRONE AREAS LIKE THE SOUTHWEST INTERIOR. THERE
WILL BE A LITTLE CHILL IN THE AIR WEDNESDAY MORNING WITH LOWS
IN THE MID 40S TO LOWER 50S.
DRY NORTHEASTERLY FLOW ALOFT CONTINUING WEDNESDAY WITH A
THERMALLY INDUCED SURFACE TROUGH MAKING ITS WAY UP THE OREGON
COAST. LOW LEVEL FLOW TURNING OFFSHORE. WHAT LITTLE FOG THAT
DOES FORM WILL DISSIPATE IN THE MORNING WITH THE DRYING AIR
MASS IN THE LOWER LEVELS. UNDER SUNNY SKIES WITH A LITTLE
OFFSHORE FLOW TEMPERATURES WARMING BACK UP TO NEAR NORMAL. HIGHS IN
THE MID 60S TO MID 70S. IN THIS SCENARIO IT IS POSSIBLE THE
WARMEST TEMPERATURES WILL BE ALONG THE COAST.
THERMALLY INDUCED SURFACE TROUGH MOVING UP THE COAST WEDNESDAY
NIGHT SHIFTING INLAND THURSDAY. THE TRANSITION TO ONSHORE FLOW
ALONG THE COAST WILL COOL HIGHS BACK TO NEAR 70. OVER THE
INTERIOR THURSDAY WILL BE THE WARMEST DAY IN THE FORECAST PERIOD
WITH HIGHS IN THE 70S. WARMEST LOCATIONS LIKE THE CASCADE
FOOTHILLS AND SOUTHWEST INTERIOR COULD GET TO 80. LOWS THURSDAY
MORNING IN THE UPPER 40S TO MID 50S.
SEPTEMBER IS OFF TO A COOL START IN WESTERN WASHINGTON. SEATTLE
AVERAGE TEMPERATURE FOR THE FIRST TWO WEEKS OF THE MONTH IS 61.8
DEGREES, 2.7 DEGREES BELOW NORMAL. THIS IS THE COOLEST START TO
SEPTEMBER IN SEATTLE SINCE 2010 ( 61.0 DEGREES ).
FELTON
LONG TERM /THURSDAY NIGHT THROUGH MONDAY/
MODELS IN GOOD AGREEMENT WITH A FLAT UPPER LEVEL RIDGE REMAINING
OVER WESTERN WASHINGTON THROUGH THE WEEKEND. LOW LEVEL ONSHORE
FLOW WILL KEEP TEMPERATURES NEAR NORMAL. HIGHS IN THE MID AND
UPPER 60S ALONG THE COAST AND UPPER 60S TO MID 70S INLAND. LOWS
IN THE UPPER 40S TO MID 50S.
UPPER LEVEL TROUGH MOVING OUT OF THE GULF OF ALASKA FIRST PART
OF NEXT WEEK. OPERATIONAL MODELS ARE MUCH SLOWER WITH THIS
FEATURE VERSUS THE NBM WHICH HAS A SYSTEM NEAR THE COAST
MONDAY. FOR NOW WILL LEAVE THE LOW POPS IN THE FORECAST MONDAY.
WILL NOT BE SURPRISED IF FUTURE RUNS PUSH THIS BACK INTO
TUESDAY. THERE IS GENERAL AGREEMENT THAT A PATTERN CHANGE IS IN
THE OFFING NEXT WEEK. IT IS JUST A MATTER OF WHEN THE WEEKEND
RIDGE BREAKS DOWN.
FELTON
AVIATION
WESTERLY FLOW ALOFT WILL TURN NORTHERLY TONIGHT AS AN UPPER TROUGH
DROPS SOUTHWARD ACROSS THE PACIFIC NORTHWEST. A MIX OF MVFR AND VFR
CEILINGS WILL CONTINUE THROUGH ROUGHLY 20Z. CEILINGS WILL IMPROVE
THIS AFTERNOON WITH CLEARING TONIGHT AND VFR EXPECTED AREAWIDE.
THERE'S ROUGHLY A 30-40% CHANCE OF IFR/LIFR REDEVELOPMENT
OVERNIGHT/WEDNESDAY MORNING IN POCKETS OF THE INTERIOR (AS WINDS
BECOME LIGHT OR CALM OVERNIGHT OUT OF THE NORTH). SOME OF THE LOWER
CEILINGS MAY DROP TO THE GROUND FOR POCKETS OF FOG/MIST.
KSEA...MIX OF MVFR AND VFR CEILINGS LIKELY TO CONTINUE THROUGH 20Z.
CEILINGS WILL LIFT AND SCATTER OUT LATER THIS AFTERNOON AND EVENING.
SURFACE WINDS NORTHERLY 4 TO 7 KNOTS WILL RISE TO 9 TO 13 KNOTS LATE
THIS AFTERNOON. WINDS BECOME LIGHT OR CALM OUT OF THE NORTH
TONIGHT/WEDNESDAY MORNING. THERE IS A 20-30% CHANCE OF IFR/LIFR
CONDITIONS DEVELOPING WEDNESDAY MORNING, BUT AT THIS POINT THE
CEILING PROBABILITIES ARE MORE ELEVATED OVER THE CHANCE OF LOWER
VISIBILITIES.
27/HPR
MARINE
THE FLOW WILL TURN NORTHERLY TODAY THEN OFFSHORE FLOW AT TIMES
WEDNESDAY AND THURSDAY AS THERMALLY INDUCED LOW PRESSURE BRIEFLY
EXPANDS NORTHWARD ALONG THE COAST. ONSHORE FLOW RETURNS FRIDAY AND
INTO THE WEEKEND AS SERIES OF SYSTEMS PASS WELL NORTH OF THE WATERS.
27
FIRE WEATHER
MINIMAL FIRE WEATHER CONCERNS THROUGH NEXT WEEK. A WEAK WEATHER
SYSTEM WILL MOVE THROUGH THE AREA TODAY BRINGING A CHANCE FOR
LIGHT PRECIPITATION. DRIER AND WARMER PATTERN WEDNESDAY INTO
THURSDAY. SURFACE WINDS WILL BRIEFLY TRANSITION OFFSHORE LATE
WEDNESDAY MORNING AND REMAIN OFFSHORE INTO THURSDAY MORNING.
THIS WILL DECREASE MINIMUM RH VALUES TOWARD THE MID 20S TO LOW
30S, ESPECIALLY IN THE NORTH CASCADES AND SOUTHWEST INTERIOR.
MINIMUM RELATIVE HUMIDITY VALUES REBOUND FRIDAY AS ONSHORE FLOW
RETURNS. THE ONSHORE FLOW PATTERN WILL CONTINUE THROUGH THE
WEEKEND.
FELTON
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...NONE.
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