237
FXUS66 KSEW 192138
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
238 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/
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
WESTERLY ZONAL FLOW WILL PREVAIL ALOFT TODAY. MARINE STRATUS IS
BEGINNING TO SCATTER AND LIFT, THOUGH PORTIONS OF PUGET SOUND
WILL HANG ONTO IT UNTIL 21Z-22Z. CLEAR CONDITIONS ARE RETURNING
TO THE COASTAL TERMINALS. GIVEN THE WEAK GRADIENT, WINDS ARE
LIGHT AND VARIABLE, WITH SOME CENTRAL PUGET SOUND TERMINALS
BEGINNING TO GO NNW AND WILL REMAIN THAT WAY FOR MOST OF THE
VALID TAF PERIOD. ANOTHER ROUND OF LOW STRATUS CLOUDS LOOKS
LIKELY TOMORROW MORNING, SETTING IN AFTER 12Z AND LASTING
THROUGH SUNDAY MORNING.
KSEA...LIFR CIGS HAVE IMPROVED IN RECENT OBS TO IFR. VISBYS HAVE
ALSO RISEN TO 10SM. CIGS BELOW 1000 FEET WILL LINGER UNTIL 22Z, WITH
SCATTERING AND CLEARING THEREAFTER. WINDS HAVE BEEN LIGHT AND
GENERALLY OUT OF THE NW AT 3 KTS OR LESS. EXPECT LIGHT N WINDS TO
CONTINUE THROUGHOUT THE OVERNIGHT PERIOD. THERE IS STILL A 30%
CHANCE OF IFR CIGS (20% CHANCE LIFR) TOMORROW MORNING, BEGINNING
BETWEEN 12Z-14Z. THE TAF INDICATES A RETURN TO IFR AND LIFR CIGS AND
VISBYS TOMORROW MORNING, AND WILL LIKELY IMPROVE AFTER 19Z-20Z WITH
CONTINUED N TO NNE WINDS 4-6 KTS.
21
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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