667
FXUS66 KSEW 190313
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
813 PM PDT TUE AUG 18 2026
SYNOPSIS
WEAK UPPER LEVEL RIDGE OVER WESTERN WASHINGTON THROUGH
THURSDAY AS LOW LEVEL ONSHORE FLOW CONTINUES. UPPER LEVEL LOW SLOWLY
MOVING NORTHEAST AND OFF THE COAST FRIDAY BEFORE PASSING THROUGH THE
AREA SATURDAY COULD BRING SHOWERS THIS WEEKEND. WEAK UPPER LEVEL
RIDGE WITH LOW LEVEL ONSHORE FLOW RETURNS FOR THE START OF NEXT
WEEK.
SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/
WEAK UPPER LEVEL RIDGING WILL LEAD TO DRY AND MILD WEATHER ACROSS
WESTERN WA THROUGH THURSDAY. THE LOW LEVEL FLOW REMAINS ONSHORE.
TEMPERATURES WILL TRACK CLOSE TO AVERAGE WITH A SLIGHT UPTICK ON
THURSDAY. 33
LONG TERM /FRIDAY THROUGH TUESDAY/
AN UPPER LEVEL LOW WILL SPIN OFFSHORE ON FRIDAY, THEN SHIFT INLAND
ON SATURDAY FOR INCREASING SHOWERS AND COOLER WEATHER. THE AIR MASS
WILL BE SLIGHTLY UNSTABLE FOR A CHANCE OF THUNDERSTORMS IN THE
MOUNTAINS SATURDAY AFTERNOON (EASTERN OLYMPICS AND NORTHERN
CASCADES). HIGHEST CHANCES FOR WETTING RAINS WILL BE IN THE NORTHERN
CASCADES NEAR THE CANADIAN BORDER. WITH THE TROUGH OVERHEAD, EXPECT
COOLER CONDITIONS ON SATURDAY WITH HIGHS IN THE 60S TO LOWER 70S.
WE'LL SEE LINGERING SHOWERS ON SUNDAY AS THE TROUGH EXITS NE.
OTHERWISE, WERE BACK TO DRY AND MILD WEATHER MOVING INTO NEXT WEEK
AS WEAK HIGH PRESSURE RETURNS. 33
AVIATION
VFR CONDITIONS CONTINUE ACROSS THE INTERIOR TERMINALS WITH MVFR
ALONG THE COAST THIS EVENING. CONDITIONS WILL DETERIORATE TONIGHT
INTO WEDNESDAY MORNING AS ANOTHER ROUND OF MARINE STRATUS PUSHES
INLAND. GUIDANCE CONTINUES TO STRUGGLE WITH HOW LOW THE CEILINGS
WILL GET, BUT BASED ON PERSISTENCE, IT IS POSSIBLE THAT THAT
CEILINGS WILL TREND TOWARD IFR/LIFR AGAIN. HIGH-RES GUIDANCE SHOWS
THERE IS A 50-70% CHANCE FOR IFR/LIFR FOR THE INTERIOR TERMINALS.
ADDITIONALLY, REDUCTIONS IN VISIBILITY ARE POSSIBLE WITH THE
STRATUS. UNCERTAINTY REMAINS AS TO HOW LOW VISIBILITY WILL BE, BUT
COULD SEE VISIBILITIES LOWER THAN 3SM AT TIMES. THE STRATUS WILL
SCATTER OUT AND RETREAT TOWARDS THE COAST AROUND 19Z. VFR CONDITIONS
ARE EXPECTED FOR INTERIOR TERMINALS IN THE AFTERNOON/EVENING, WITH
ONLY MINIMAL IMPROVEMENT TO MVFR EXPECTED ALONG THE COAST.
GENERALLY W TO SW WINDS PERSISTING ACROSS THE TERMINALS THIS
EVENING, WITH THE EXCEPTION OF N WINDS AT KPAE. WINDS REMAIN OUT OF
THE SW AT KSEA AND KBFI AS OF 03Z, BUT A BRIEF TRANSITION TO WNW
REMAINS POSSIBLE THROUGH 06Z. PROBABILITIES ARE SLIGHTLY HIGHER FOR
BFI TO SEE WINDS TRANSITION TO THE WNW, AT AROUND 70-80%. OTHERWISE
EXPECT CALM TO LIGHT W/SW WINDS FOR THE MAJORITY OF THE AREA
TERMINALS OVERNIGHT.
KSEA...VFR CONDITIONS TONIGHT. WHILE GUIDANCE CONTINUES TO STRUGGLE
WITH HOW LOW THE CEILINGS WILL BE WEDNESDAY MORNING, HIGH-RES
GUIDANCE HAS A 50% CHANCE FOR LIFR AND 65% FOR IFR AFTER 12Z. BASED
OFF PERSISTENCE, WILL KEEP THE TRENDS FOR THE TAF TOWARDS IFR/LIFR
FOR THE MORNING. CEILINGS WILL SLOWLY IMPROVE THROUGHOUT THE LATE
MORNING AND SCATTER OUT BY THE AFTERNOON FOR A RETURN TO VFR
CONDITIONS. GENERALLY SW WINDS PERSIST AT 3-6 KT TONIGHT. WINDS MAY
STILL BRIEFLY SHIFT TO THE WSW THIS EVENING, BUT EXPECT WINDS TO
SHIFT BACK TO THE SW AGAIN OVERNIGHT.
29/14
MARINE
BROAD HIGH PRESSURE OVER THE NORTHEAST PACIFIC WILL CONTINUE
BUILDING INTO AREA WATERS THROUGH TOMORROW. A WEAK SYSTEM WILL
APPROACH THE WATERS TODAY, BUT IS NOT EXPECTED TO HAVE ANY
SIGNIFICANT IMPACTS. INCREASED NW SURFACE WINDS THROUGH WEDNESDAY
AND ARE EXPECTED TO REMAIN BELOW SCA THRESHOLDS. HIGH PRESSURE WILL
THEN WEAKEN THROUGH LATE IN THE WEEK AS A STRONGER WEATHER SYSTEM
APPROACHES AND MOVES OVER THE WATERS ON FRIDAY. THIS MAY RESULT IN
SCA WINDS THROUGH THE STRAIT OF JUAN DE FUCA FRIDAY EVENING, WITH
LATEST PROBABILITIES AROUND 60-70% THROUGH THE CENTRAL PORTION. OVER
THE WEEKEND, HIGH PRESSURE WILL REBUILD AND STRENGTHEN OVER AREA
WATERS.
SEAS WILL REMAIN BELOW 10 FEET THROUGHOUT THE WEEK. HOWEVER, SEAS
MAY BECOME STEEP AT TIMES FOR THE OUTER COASTAL WATERS TONIGHT
THROUGH THURSDAY AT 5-7 FT WITH A DOMINANT PERIOD OF 7 SECONDS.
29
FIRE WEATHER
CONTINUED ONSHORE FLOW THROUGH THURSDAY WILL MAINTAIN MILD
TEMPERATURES PAIRED WITH EXCELLENT OVERNIGHT RELATIVE HUMIDITY
RECOVERIES, LIMITING FIRE WEATHER CONCERNS ACROSS WESTERN WASHINGTON.
A STORM SYSTEM APPROACHING THE REGION WILL CAUSE ELEVATED FIRE
WEATHER CONCERNS ON FRIDAY, BRINGING INCREASED POTENTIAL FOR
THUNDERSTORMS OVER THE OLYMPICS AND CASCADES. WHILE THERE IS STILL A
BIT OF UNCERTAINTY WITH THE TIMING AND INTENSITY OF THIS WEATHER
SYSTEM, ANY THUNDERSTORMS THAT DEVELOP MAY BE CAPABLE OF PRODUCING
LOCALLY WETTING RAINS ALONGSIDE GUSTY AND ERRATIC OUTFLOW WINDS AND
ABUNDANT LIGHTNING. WHILE CONVECTION MAY CONTINUE OVERNIGHT, A
FRONTAL SYSTEM WILL CROSS THE REGION SOMETIME BETWEEN FRIDAY EVENING
AND SATURDAY, BRINGING IN A MUCH COOLER AND MORE MOIST AIR MASS.
WIDESPREAD GUSTY WINDS ARE NOT EXPECTED BEHIND THE FRONTAL PASSAGE,
THOUGH MODELS CONTINUE TO SHOW POTENTIAL FOR CONVECTION AND A
POTENTIAL PUGET SOUND CONVERGENCE ZONE ON SATURDAY.
FIRE WEATHER CONCERNS DECREASE INTO SUNDAY, AS A DRIER AND CALMER
PATTERN SETS UP OVER WESTERN WASHINGTON. THIS WILL LIKELY BRING A
RETURN TO ONSHORE FLOW WITH MORNING LOW STRATUS AND AFTERNOON SUN,
NEAR NORMAL TEMPERATURES, AND GENERALLY CALM WINDS.
15
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...NONE.
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