333
FXUS66 KSEW 200320 CCA
AFDSEW
AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
820 PM PDT SAT SEP 19 2026
SYNOPSIS
WEAK, TRANSIENT HIGH PRESSURE ALOFT BUILDS INTO
SUNDAY, BRINGING A SLIGHT WARM UP TO THE REGION. ONSHORE FLOW AT
THE LOWER LEVELS MAINTAINS MORNING CLOUDS AND DRY CONDITIONS
THROUGH THE FIRST HALF OF THE WEEK. PROGRESSIVELY COOLER
CONDITIONS ARRIVE NEXT WEEK AS AN UPPER TROUGH BUILDS OFFSHORE.
RAIN CHANCES RETURNING AROUND THE MIDDLE OF THE WEEK WITH A
PASSING SYSTEM, FOLLOWED BY ANOTHER DISTURBANCE ON FRIDAY.
SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/
UPDATED THE FORECAST
TO ADD PATCHY/AREAS OF FOG OVERNIGHT. 33
PREVIOUS DISCUSSION...EXTENSIVE MARINE STRATUS IS DISSIPATING
THIS AFTERNOON, HAVING HELD ON FOR THE MORNING AND EARLY
AFTERNOON. A WEAK SURFACE GRADIENT WILL CONTINUE, WITH LIGHT
SURFACE WINDS EXPECTED OVERNIGHT. CLOSER TO THE CASCADE CREST
AND THROUGH PORTIONS OF THE ADJACENT VALLEYS AND FOOTHILLS, A
LIGHT NE WIND WILL BE PRESENT THROUGHOUT THE NIGHT AND INTO
SUNDAY MORNING.
LITTLE CHANGE IN THE PATTERN TONIGHT AND SUNDAY. ANOTHER ROUND OF
LATE NIGHT AND MORNING LOW CLOUDS WITH PATCHY FOG OVER THE INTERIOR
WHILE THE COAST STAYS MOSTLY CLOUDY. EXPECT THAT THE STRATUS WILL
LINGER THROUGH THE MORNING, BREAKING OUT AFTER 12PM LOCAL TIME.
SUNSHINE IN THE AFTERNOON OVER THE INTERIOR WILL PUT HIGHS IN THE
MID 60S TO MID 70S. HIGHS ALONG THE COAST IN THE MID 60S. GIVEN THE
LIGHT OFFSHORE FLOW IN THE CASCADES TONIGHT INTO SUNDAY, EXPECT THAT
THE AFTERNOON RELATIVE HUMIDITY WILL BE ON THE DRIER SIDE IN THE
CASCADES AND FOOTHILLS LOCATIONS, DOWN TO AROUND 30%.
A FEW CHANGES MONDAY BUT THE FORECAST WILL REMAIN THE SAME. UPPER
LEVEL RIDGE MOVES OFF TO THE EAST WHILE A WEAK SYSTEM MOVES INLAND
WELL NORTH OF THE AREA. WEAK LOW LEVEL ONSHORE FLOW IN THE LOWER
LEVELS MAINTAINING THE MORNING CLOUDS AND AFTERNOON SUNSHINE
PATTERN.
LONG TERM /TUESDAY THROUGH SATURDAY/
ENSEMBLE GUIDANCE
REMAINS IN FAIRLY GOOD AGREEMENT FOR THE MIDWEEK TIME PERIOD AS
THE NEXT UPPER LEVEL TROUGH REMAINS POSITIONED OFFSHORE. A
PASSING DISTURBANCE ON WEDNESDAY WILL BRING SOME RAIN CHANCES,
BUT WITH IT WEAKENING AS IT CLIPS THE REGION, IT SHOULD BE
RATHER LIGHT. ANOTHER DISTURBANCE LATER IN THE BRINGS A
REINFORCING ROUND OF RAIN CHANCES AROUND FRIDAY. OTHERWISE,
TEMPERATURES REMAIN RATHER COOL WITH HIGHS ONLY REACHING THE
LOWER TO MID 60S LATE IN THE WEEK. WHILE ENSEMBLES BEGIN TO
DEPICT A LARGER RANGE OF POSSIBILITIES BY THE END OF THE WEEK,
THERE IS PRETTY GOOD AGREEMENT THAT THE POTENTIAL FOR
SIGNIFICANT WEATHER IMPACTS REMAIN QUITE LOW THROUGH THE WEEK
AHEAD.
AVIATION
VFR ACROSS THE TERMINALS THIS EVENING, WITH THE LATEST
SATELLITE IMAGERY SHOWING THE STRATUS WELL OFFSHORE AT THIS TIME.
WESTERLY ZONAL FLOW WILL PREVAIL ALOFT TODAY. ANOTHER ROUND OF LOW
STRATUS CLOUDS LOOKS LIKELY TOMORROW MORNING, SETTING IN AFTER 12Z
AND LASTING THROUGH SUNDAY MORNING. LIGHT NORTHERLY SURFACE FLOW TO
CALM WINDS INTO THE OVERNIGHT HOURS. INCREASING NORTHERLIES IN THE
AFTERNOON AROUND 4-6 KT.
KSEA...VFR CONDITIONS WILL CONTINUE OVERNIGHT. EXPECT LIGHT N WINDS
TO CONTINUE THROUGHOUT THE OVERNIGHT PERIOD. CHANCES FOR IFR CIGS
CONTINUE TO TREND UPWARDS, WITH CHANCES NOW AT 40% FOR IFR (30%
CHANCE LIFR) AFTER 13Z. VFR CONDITIONS RETURN IN THE AFTERNOON WITH
CLOUDS SCATTERING AFTER 18Z-20Z WITH CONTINUED N TO NNE WINDS 4-6
KTS.
21/29
MARINE
HIGH PRESSURE CONTINUES OVER THE WATERS THIS WEEKEND
INTO THE START OF THE WEEK. STRONGER ONSHORE FLOW THROUGH THE
STRAIT OF JUAN DE FUCA LATE MONDAY BRING AROUND A 50-60% CHANCE
OF ADVISORY STRENGTH WINDS, REPRESENTING THE HIGHEST CHANCES OF
THE NEXT SEVERAL DAYS FOR MEETING SMALL CRAFT ADVISORY
THRESHOLDS. OTHERWISE, EXPECT SEAS TO BUILD OVER THE COASTAL
WATERS LATE MONDAY AS A LONGER-PERIOD NORTHWEST SWELL ARRIVES
INTO THE REGION. THIS WILL BRING SEAS FROM AROUND 4 TO 5 FT LATE
SUNDAY UP TO AROUND 7 TO 9 FT OVER THE COASTAL WATERS EARLY
NEXT WEEK.
FIRE WEATHER
WHILE GENERAL LOW-LEVEL ONSHORE FLOW WILL
MAINTAIN CLOUD COVER AND HIGHER HUMIDITIES ACROSS THE REGION,
EXPECT TO SEE SOME LIGHT NORTH OR NORTHEAST WINDS NEAR THE
CASCADES TONIGHT THROUGH SUNDAY, WITH SUNDAY AFTERNOON RH VALUES
APPROACHING THE 30-35% RANGE. GIVEN THE OVERALL PATTERN,
CONCERNS REMAIN MINIMAL THROUGH MUCH OF THE WEEK AHEAD, WITH
COOLER TEMPERATURES AND RAIN CHANCES RETURNING BY THE MIDDLE OF
NEXT WEEK.
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...NONE.
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