814
FXUS66 KSEW 112145
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
245 PM PDT FRI SEP 11 2026
SYNOPSIS
UNSETTLED WEATHER REMAINS ON TRACK TO MOVE INTO WESTERN WASHINGTON
THIS WEEKEND WITH A TROUGH MOVING THROUGH. THIS INCLUDES A SLIGHT
CHANCE OF THUNDERSTORMS SATURDAY AFTERNOON, ALONG WITH A STEADIER
RAIN SATURDAY EVENING INTO SUNDAY. MANY LOCATIONS WILL SEE AT LEAST
HALF AN INCH OF RAIN IN TOTAL, WITH A HIGHER POTENTIAL FOR OVER AN
INCH OF RAIN IN THE CASCADES SOUTH OF INTERSTATE 90. DRIER WEATHER
WILL MOVE IN NEXT WEEK, WITH TEMPERATURES WARMING BACK UP FROM
WEDNESDAY ONWARDS.
SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/
A ZONAL FLOW CONTINUES OVERHEAD THE REGION THIS AFTERNOON WITH
ONSHORE FLOW CONTINUING. CLOUD COVERAGE WILL REMAIN PARTLY SUNNY
THROUGH THE AFTERNOON WITH CLOUD COVERAGE INCREASING TONIGHT INTO
SATURDAY. AN UPPER LEVEL LOW WILL DIVE SOUTHEASTWARD THIS WEEKEND
FROM B.C. AND DEEPEN AS IT COMES ASHORE OVER THE WEEKEND. A SURFACE
TROUGH AND LOW PRESSURE WILL ALSO PROGRESS INLAND THIS WEEKEND AS
WELL. THE ENSEMBLES ARE INDICATING AN ABUNDANCE OF MOISTURE
AVAILABLE WITH THIS SYSTEM COMPARED TO NORMAL FOR THIS TIME IN
SEPTEMBER (AS THE ECMWF SHOWS ANOMALIES FOR PRECIPITABLE WATER AND
INTEGRATED VAPOR TRANSPORT AT 4.0 AND 4.9 STANDARD DEVIATIONS ABOVE
NORMAL). WITH THE ABUNDANT MOISTURE AVAILABLE FOR THIS TIME OF YEAR,
THE REGION WILL SEE SOME BENEFICIAL RAIN THIS WEEKEND.
THE TIMING OF THE PRECIPITATION WILL START SATURDAY, WITH THE COAST
SEEING SOME WRAP AROUND PRECIPITATION TO START THE MORNING. AS THE
LOW PROGRESSES INLAND, MORE WRAP AROUND SHOWERS WILL BEGIN TO FORM
ACROSS THE INTERIOR AND THE CASCADES. THE INITIAL SHOWER ACTIVITY
HAS A 15-20% CHANCE OF THUNDER. WHILE SPC HAS ALL OF WESTERN WA IN A
GENERAL THUNDERSTORM RISK FOR SATURDAY, THE ACTUAL THUNDER MAY BE
MORE CONCENTRATED INTO PORTIONS OF THE SOUTH INTERIOR AND OLYMPICS
WITH UPSLOPE/OROGRAPHIC LIFTING AS THE PRECIPITATION MOVES NORTHWEST.
OTHERWISE, THE PRECIPITATION COVERAGE WILL BECOME MORE STRATIFORM
GOING INTO SATURDAY NIGHT/SUNDAY, AND COVER PRIMARILY THE CASCADES
(SOUTH OF I-90), AND ADJACENT PORTIONS OF THE SOUTH INTERIOR.
MOST LOCATIONS WILL SEE AT LEAST 0.50" OF RAIN WITH THIS SYSTEM FROM
SATURDAY THROUGH SUNDAY EVENING. THE BULK OF THE PRECIPITATION IS
EXPECTED AGAIN IN THE SOUTH CASCADES, WHERE THERE IS A 60-80% CHANCE
OF RAINFALL EXCEEDING 1" AND 1.50". RAIN RATES ARE NOT EXPECTED TO
EXCEED 0.25" TO 0.35" AN HOUR WITH THIS SYSTEM (EXCEPT POTENTIALLY
IN THUNDERSTORMS), WHICH SHOULD KEEP HYDROLOGIC IMPACTS MINIMAL
(EXCEPT FOR SOME PONDING). WINDS THIS WEEKEND WILL REMAIN GENERALLY
LIGHT, BUT COULD GET A LITTLE BREEZY SUNDAY ACROSS PORTIONS OF THE
INTERIOR, AND ALONG THE CASCADE CREST WITH GUSTS UP TO 20 MPH. HIGH
TEMPERATURES WILL REMAIN IN THE UPPER 60S TO AROUND 70, WITH LOWS IN
THE MID 50S.
LONG TERM /MONDAY THROUGH FRIDAY/
ENSEMBLES HAVE THE UPPER LEVEL LOW/TROUGH MOVING OUT OF THE REGION
LATE SUNDAY, WITH NORTHWEST FLOW ALOFT INCREASING. THERE COULD BE A
FEW REMAINING SHOWERS IN THE CASCADES GOING INTO MONDAY MORNING,
OTHERWISE CONDITIONS WILL DRYING OUT THROUGH THE DAY FOR MOST, BUT
WITH MORE CLOUDS THAN SUN KEEPING TEMPERATURES THE SAME. ANOTHER
TROUGH WILL PASS THROUGH EARLY TUESDAY, HOWEVER THE MODELS CAP ANY
MOISTURE WITH THIS TROUGH MOVING THROUGH. WEDNESDAY IS WHERE THE
MODELS HAVE AN UPPER LEVEL HIGH BUILDING OVER THE PACIFIC, WITH A
SPLIT DIFFERENCE AS TO HOW LONG THE HIGH REMAINS OFFSHORE GOING INTO
LATE NEXT WEEK. A COUPLE MODELS EVEN SHOW AN OMEGA BLOCK OVER THE
PACIFIC. REGARDLESS, MOST WILL SEE A WARM UP WITH THE SURFACE
PRESSURE CENTERED OVER SOUTHERN B.C., AND LOWER PRESSURE TO THE
SOUTHWEST OFF THE COAST. HIGHS IN THE LOWLANDS WILL LIKELY CLIMB
BACK INTO THE MID/UPPER 70S (WITH PERHAPS A FEW 80S).
HPR
AVIATION
VFR AT THE TERMINALS TODAY AND SATURDAY. A WEATHER SYSTEM IS
EXPECTED TO IMPACT THE TERMINALS SATURDAY AFTERNOON/EVENING. LATEST
GUIDANCE SUGGESTS PRECIPITATION WILL START IMPACTING THE COAST
AROUND 20Z AND THE INTERIOR TERMINALS AFTER 00Z SATURDAY.
ADDITIONALLY, THERE IS A THREAT FOR ISOLATED THUNDERSTORMS SATURDAY
AFTERNOON/EVENING. HIGHEST CHANCES (20%) WILL BE OVER THE OLYMPICS.
AS FOR THE INTERIOR TERMINALS, THERE IS A 10-15% FROM PAE
SOUTHWARDS. CHANCES FOR THUNDERSTORMS WILL SUBSIDE SATURDAY NIGHT.
SOUTHWESTERLY WINDS THIS AFTERNOON WILL TRANSITION NORTHERLY THIS
EVENING AROUND 4-6 KT. LIGHT NE TO CALM WINDS OVERNIGHT.
KSEA...VFR TODAY AND SATURDAY. PRECIPITATION CHANCES WILL
INCREASE INTO THE AFTERNOON, WITH PRECIPITATION LIKELY IMPACTING
THE TERMINAL AFTER 00Z. THERE IS A SLIGHT CHANCE (10%) FOR
ISOLATED THUNDERSTORMS SATURDAY AFTERNOON/EVENING. SOUTHWESTERLY
WINDS THIS AFTERNOON WILL SHIFT NORTHERLY AFTER 02Z-03Z AT 3-5
KT. NORTHERLY WINDS WILL INCREASE TO 5-7 KT SATURDAY AFTERNOON.
SOUTHWESTERLY WINDS RETURN SATURDAY EVENING AFTER 00Z.
MARINE
HIGH PRESSURE WILL CONTINUE TO WEAKEN TODAY AS A LOW PRESSURE SYSTEM
MOVES ACROSS AREA WATERS TOWARDS OREGON TODAY INTO SATURDAY. BEHIND
THE SYSTEM, HIGH PRESSURE WILL BEGIN TO REBUILD OVER AREA WATERS AND
STRENGTHEN THROUGH THE MIDDLE OF THE WEEK. WESTERLY WINDS WILL
INCREASE THROUGH THE STRAIT OF JUAN DE FUCA WITH THE POTENTIAL TO
REACH SMALL CRAFT THRESHOLDS SATURDAY EVENING AND SUNDAY EVENING.
LATEST PROBABILITIES ARE AROUND 40-55% ON SATURDAY AND 70-85% ON
SUNDAY. SEAS GENERALLY AROUND 4-6 FEET THIS WEEKEND, DECREASING
SLIGHTLY EARLY NEXT WEEK TO 3-5 FEET.
29
FIRE WEATHER
ONSHORE MOISTURE WILL INCREASE THIS WEEKEND AS A TROUGH MOVES ACROSS
WESTERN WASHINGTON. THE INITIAL PRECIPITATION SATURDAY AFTERNOON
WILL BE MORE SHOWERY (WITH A 15-20% CHANCE OF THUNDER), BUT COVERAGE
WILL BECOME MORE STRATIFORM LATE SATURDAY NIGHT INTO SUNDAY,
INCREASING THE CHANCES OF A WETTING RAIN FOR MOST (ESPECIALLY THE
SOUTH CASCADES). AS A RESULT, THE FIRE WEATHER CONCERNS WILL REMAIN
MINIMAL GOING THROUGH THE WEEKEND AND EARLY NEXT WEEK. HIGH PRESSURE
WILL BUILD IN SOUTH B.C. BY MIDWEEK, WITH LOWER PRESSURE OFFSHORE AT
THE SURFACE. THE PATTERN WILL BECOME DRIER WITH WARMER TEMPERATURES,
AND ADDITIONALLY SOME MODELS ARE SHOWING BRIEF BREEZY EAST TO
SOUTHEAST WINDS WEDNESDAY EVENING/THURSDAY MORNING. A FEW CASCADE
LOCATIONS MAY SEE RH VALUES DIP INTO THE 20S, THOUGH THERE
REMAINS SOME UNCERTAINTY IN THE ENSEMBLES AT THIS TIME AS TO HOW
STRONG AND LONG THE HIGH PRESSURE WILL REMAIN LATE NEXT WEEK.
HPR
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...NONE.
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