568
FXUS66 KSEW 131618
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
918 AM PDT THU AUG 13 2026
.SYNOPSIS
WEAK UPPER LEVEL TROUGH OVER THE AREA THROUGH FRIDAY NIGHT. AIR
MASS SOMEWHAT UNSTABLE DURING THE AFTERNOON AND EVENING HOURS
OVER THE HIGHER TERRAIN. WEAK UPPER LEVEL RIDGE OVER THE
WEEKEND WILL BE REPLACED BY ANOTHER TROUGH MONDAY. THE TROUGH
WILL HANG OVER THE AREA THROUGH MOST OF NEXT WEEK. ONSHORE FLOW
IN THE LOWER LEVELS WILL KEEP HIGH TEMPERATURES NEAR NORMAL
THROUGH THE PERIOD.
SHORT TERM /TODAY THROUGH SATURDAY/
NO SIGNIFICANT CHANGES MADE TO THE FORECAST THIS MORNING. THE
PREVIOUS DISCUSSION CAN BE FOUND BELOW ALONG WITH UPDATES TO THE
AVIATION SECTION:
SATELLITE IMAGERY SHOWS STRATUS ALONG THE COAST MOVING THROUGH
THE LOWER CHEHALIS VALLEY AND DOWN THE STRAIT OF JUAN DE FUCA
EARLY THIS MORNING. CLOUDLESS SKIES OVER THE REMAINDER OF THE
AREA WITH A LAYER OF SMOKE ALOFT.
UPPER LEVEL TROUGH OVER THE AREA TODAY WILL REMAIN IN PLACE
THROUGH FRIDAY NIGHT. AIR MASS BECOMING UNSTABLE OVER THE HIGHER
TERRAIN IN THE AFTERNOON AND EARLY EVENING HOURS BOTH TODAY AND
FRIDAY. MODEL SOUNDINGS HAVE LIFTED INDEXES BELOW 0 OVER THE
CASCADES AND OLYMPICS WITH CAPE VALUES UP TO 1000 J/KG AT 00Z
THIS AFTERNOON AND AGAIN FRIDAY AFTERNOON. THE FIRE WEATHER
WATCH FOR THE OLYMPICS AND CASCADES FOR LATER TODAY WILL BE
UPGRADED TO A RED FLAG WARNING WITH THIS FORECAST PACKAGE. WHAT
CONVECTION THAT DOES DEVELOP HAS A HIGH PROBABILITY OF NOT
PRODUCING WETTING RAINS AND CREATING GUSTY AND ERRATIC OUTFLOW
WINDS NEAR THE STORM. IN THE LOWLANDS LOW LEVEL ONSHORE FLOW
WITH A SHALLOW MARINE LAYER JUST GETTING EAST OF PUGET SOUND
AFTER SUNRISE BOTH MORNINGS. THE LAYER WILL RETREAT BACK TO THE
COASTLINE BY MIDDAY. HIGHS BOTH DAYS IN THE 70S AND LOWER 80S
INLAND, MID AND UPPER 60S ALONG THE COAST. LOWS 50S AND LOWER
60S.
UPPER LEVEL TROUGH DISSIPATING FRIDAY NIGHT WITH AN UPPER LEVEL
RIDGE BACK BUILDING INTO WESTERN WASHINGTON FROM THE SOUTHEAST
SATURDAY. STILL SLIGHTLY UNSTABLE OVER THE CASCADES LATE IN THE
DAY FOR A SLIGHT CHANCE OF CONVECTION. IN THE LOWLANDS LITTLE
CHANGE WITH THE LOW LEVEL ONSHORE FLOW PATTERN REMAINING INTACT.
SHALLOW MARINE LAYER MOVING INLAND IN THE LATE NIGHT AND
MORNING HOURS WILL DISSIPATE OVER ALL BUT THE COASTLINE BY
MIDDAY. LOWS IN THE 50S AND LOWER 60S. LITTLE CHANGE IN THE
HIGHS WITH 70S AND LOWER 80S INLAND AND MID AND UPPER 60S COAST.
SMOKE ALOFT REMAINING OVER THE AREA AT LEAST THROUGH FRIDAY. A
LITTLE BIT OF MIXING WITH THE DAYTIME HEATING WILL GET SOME OF
THE SMOKE DOWN TOWARDS THE SURFACE BUT THE LOW LEVEL ONSHORE
FLOW WILL PREVENT THE SMOKE FROM GETTING TOO BAD AT THE SURFACE
THE NEXT FEW DAYS. FELTON
LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/
MODELS IN GOOD AGREEMENT WITH THE UPPER LEVEL RIDGE REMAINING
OVER THE AREA THROUGH SUNDAY NIGHT BEFORE GETTING REPLACED BY
YET ANOTHER UPPER LEVEL TROUGH MONDAY. UPPER LEVEL TROUGH
REMAINING OVER THE AREA THROUGH WEDNESDAY. LOW LEVEL ONSHORE
FLOW THROUGH THE PERIOD INCREASING MIDWEEK. AIR MASS OVER THE
HIGHER TERRAIN ONCE AGAIN BECOMING UNSTABLE LATE IN THE DAY
UNDER THE TROUGH FOR AT LEAST A SLIGHT CHANCE OF CONVECTION
MONDAY THROUGH WEDNESDAY. LITTLE CHANGE IN THE TEMPERATURES.
HIGHS 70S AND LOWER 80S INLAND WITH MID AND UPPER 60S COAST.
LOWS IN THE 50S AND LOWER 60S. INCREASING ONSHORE FLOW IN THE
MIDDLE OF NEXT WEEK COOLING HIGHS INTO THE MID 60S TO MID 70S.
FELTON
AVIATION
UPPER LEVEL TROUGHING OVERHEAD WILL KEEP WEST TO SOUTHWESTERLY FLOW
ALOFT. IFR TO LIFR STRATUS WITH POCKETS OF FOG REMAINS IN PLACE THIS
MORNING ACROSS PORTIONS OF THE INTERIOR AND THE COAST. AS
TEMPERATURES INCREASE THIS MORNING THE FOG WILL BURN OFF AND LIFT
AROUND 17-19Z. LIGHT ONSHORE WINDS WILL INCREASE TO AROUND 5-10 KT
LATE MORNING, WITH NORTHERLY WINDS THROUGH THE PUGET SOUND INTERIOR.
OUTSIDE OF THE MAIN TERMINALS, THERE WILL BE CHANCES OF SHOWERS AND
ISOLATED THUNDERSTORMS (25-35%) ALONG THE CASCADE CREST, THOUGH
THERE IS ALSO A LOW (10-15%) CHANCE OVER THE OLYMPICS, AS WELL. HAZE
WILL PRIMARILY REMAIN ALOFT AND IS NOT EXPECTED TO PRODUCE IMPACTS
AT THE SURFACE. THE PERSISTENT PATTERN WILL BRING SIMILAR
PROBABILITIES FOR LOW IFR/LIFR CEILINGS AND VISIBILITIES FRIDAY
MORNING (UP TO INCLUDING THE INTERIOR/PUGET SOUND TERMINALS) WITH
LIKELIHOODS OF IFR AT 40-50%, AND LIFR PROBABILITIES AT 10-20%.
KSEA...LIFR/FOG REMAINS OVER THE TERMINAL THIS MORNING, BUT AS
TEMPERATURES INCREASE, WILL LIKELY BURN OFF FROM 17-19Z FOR VFR THE
REMAINDER OF THE DAY. SIMILAR PROBABILITIES FOR FRIDAY MORNING, WITH
IFR AT 40-50% AND LIFR AT 10-20% (CAN'T RULE OUT MORE FOG FOR FRIDAY
MORNING). SW WINDS LESS THAN 5 KT BEFORE 17Z, THEN SWITCHING TO W
THAN NW THROUGH THE LATE MORNING AND REMAIN N 5-8 KT THROUGH
TONIGHT BEFORE TURNING BACK TO THE SW UNDER 5 KT. 62/HPR
MARINE
HIGH PRESSURE IN THE NORTHEAST PACIFIC WILL WEAKEN GOING INTO THE
END OF THE WEEK AS IT GETS PUSHED SOUTH INTO THE WEEKEND BY SYSTEMS
OVER THE OPEN WATERS. DIURNAL WESTERLY PUSHES THROUGH THE STRAIT OF
JUAN DE FUCA REMAINS THE PRIMARY IMPACT, ALTHOUGH CHANCES FOR SMALL
CRAFT ADVISORY CONDITIONS REMAIN LOW UNTIL FRIDAY EVENING AND
STRONGER PUSHES WILL CONTINUE THROUGH THE WEEKEND. WINDS IN THE
COASTAL WATERS WILL DECREASE TONIGHT AS PRESSURE GRADIENTS DECREASE.
HIGH PRESSURE LOOKS TO REBUILD GOING INTO NEXT WEEK.
SEAS REMAIN 3-5 FT THROUGH THE FORECAST PERIOD. 62
FIRE WEATHER
FIRE WEATHER CONCERNS REMAIN ELEVATED TO CRITICAL AT TIMES
THROUGH FRIDAY NIGHT. UPPER LEVEL TROUGH OVER THE AREA. AIR MASS
BECOMING UNSTABLE OVER THE HIGHER TERRAIN IN THE AFTERNOON AND
EARLY EVENING HOURS CREATING A CHANCE FOR THUNDERSTORMS. THERE
IS A HIGH PROBABILITY THESE STORMS WILL NOT PRODUCE WETTING
RAINS BUT COULD PRODUCE GUSTY ERRATIC OUTFLOW WINDS. THE FIRE
WEATHER WATCH TODAY FOR THE OLYMPICS AND CASCADES WILL BE
UPGRADED TO A WARNING. IF FORECAST TRENDS CONTINUE ANOTHER FIRE
WEATHER WATCH COULD BE ISSUED FOR FRIDAY AFTERNOON AND EVENING
WITH LATER FORECAST PACKAGES.
UPPER LEVEL TROUGH REPLACED BY AN UPPER LEVEL RIDGE OVER THE
WEEKEND LESSENING BUT NOT ELIMINATING THE CHANCES FOR CONVECTION
OVER THE MOUNTAINS ESPECIALLY THE CASCADES. ANOTHER UPPER LEVEL
TROUGH MOVING INTO THE AREA NEXT WEEK WILL ONCE AGAIN INCREASE
FIRE WEATHER CONCERNS. FELTON
SEW WATCHES/WARNINGS/ADVISORIES
WA...RED FLAG WARNING UNTIL 9 PM PDT THIS EVENING FOR EAST PORTION
OF THE OLYMPIC MOUNTAINS-WEST PORTION OF THE OLYMPIC
MOUNTAINS-WEST SLOPES OF THE CENTRAL CASCADES GENERALLY
ABOVE 1500 FEET-WEST SLOPES OF THE NORTH CASCADES
GENERALLY ABOVE 1500 FEET.
PZ...NONE.
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