043
FXUS66 KSEW 120323
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
823 PM PDT TUE AUG 11 2026
SYNOPSIS
AN UPPER LEVEL TROUGH WILL MOVE INTO THE REGION ON WEDNESDAY
BRINGING COOLER TEMPERATURES ALONGSIDE SHOWERS AND THUNDERSTORMS
OVER THE NORTH CASCADES WEDNESDAY AND THURSDAY. AN UPPER LEVEL
RIDGE BEGINS TO REBUILD INTO THE REGION LATE IN THE WEEK, BUT
ONSHORE FLOW IS EXPECTED TO KEEP TEMPERATURES NEAR OR ONLY
SLIGHTLY ABOVE NORMAL FOR MID AUGUST.
SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/
LESS IN THE WAY OF MORNING CLOUD COVER IS RESULTING IN SOMEWHAT
WARMER TEMPERATURES THIS AFTERNOON WITH DAYTIME HIGHS EXPECTED
TO LAND RIGHT AROUND SEASONAL NORMALS FOR MOST SPOTS. AN UPPER
LEVEL TROUGH WILL DIP SOUTHWARD OUT OF BRITISH COLUMBIA ON
WEDNESDAY WITH CLOUDS AND MODESTLY COOLER TEMPERATURES. THE MAIN
CONCERN WITH THIS SYSTEM WILL BE IN THE FIRE WEATHER REALM WITH THE
THREAT OF SHOWERS AND THUNDERSTORMS IN THE NORTH CASCADES, AND
TO A SOMEWHAT LESSER EXTENT IN THE OLYMPICS, ON WEDNESDAY
AFTERNOON AND EVENING. BROAD UPPER TROUGHING REMAINS OVER THE
REGION ON THURSDAY. LOWLAND AREAS WILL SEE MAINLY DRY CONDITIONS
WITH A SLIGHT BUMP IN TEMPERATURES FOR INTERIOR AREAS. THE
PRESENCE OF THE TROUGH WILL RETAIN SOME DEGREE OF INSTABILITY
FOR THE CASCADES WITH A CONTINUED THREAT OF CONVECTION AGAIN IN
PRIMARILY THE AFTERNOON AND EVENING HOURS.
LONG TERM /FRIDAY THROUGH TUESDAY/
UPPER RIDGING CENTERED WELL TO OUR EAST BEGINS TO EXERT A LITTLE
MORE INFLUENCE ON THE REGION ON FRIDAY. DESPITE A SMALL RECOVERY
IN HEIGHTS, THE LOW LEVEL FLOW REMAINS FIRMLY ONSHORE FOR
LITTLE APPRECIABLE CHANGE IN TEMPERATURES FROM THURSDAY. THERE
IS STILL SOME UNCERTAINTY IN THE FORECAST FOR THE WEEKEND AND
BEYOND, BUT ENSEMBLES ARE CONVERGING ON A SOMEWHAT CLEARER
PICTURE. A BROAD UPPER TROUGH IS EXPECTED TO DIG SOUTHWARD WELL
OFFSHORE WHILE STRONG UPPER RIDGING CENTERED EAST OF THE ROCKIES
BUILDS WESTWARD SOMEWHAT. THIS LEADS TO A MODEST WARMING TREND
TOWARD THE WEEKEND AND EARLY NEXT WEEK, BUT ONSHORE FLOW
PREVAILS THROUGH THE PERIOD. AS A RESULT, THE PROBABILITIES
FOR MODERATE HEATRISK HAVE LOWERED FOR FRIDAY THROUGH MONDAY FOR
INTERIOR AREAS WITH A GENERAL 20% TO 40% RISK AROUND SEATTLE
METRO. CORRESPONDING HIGH TEMPERATURES FOR THE WARMEST PORTIONS
OF THE INTERIOR ARE IN THE LOWER TO MID 80S WHILE COASTAL AREAS
ARE HELD MOSTLY IN THE 60S. AT SOME POINT DURING THE EXTENDED
PERIOD, SOUTHERLY FLOW ALOFT COULD IMPORT SOME MONSOONAL
MOISTURE NORTHWARD NEAR THE CASCADE CREST AND WE'LL NEED TO
WATCH FOR ADDITIONAL THUNDERSTORM POTENTIAL IF THIS DOES INDEED
COME TO FRUITION.
27
AVIATION
WESTERLY FLOW ALOFT WILL BECOME SOUTHWESTERLY ON
WEDNESDAY AS AN UPPER LEVEL TROUGH MOVES INTO THE REGION. FLOW ALOFT
THEN LIKELY TO TREND MORE VARIABLE WEDNESDAY EVENING. ONSHORE FLOW
CONTINUES IN THE LOW LEVELS THROUGHOUT THE PERIOD. LOW CLOUDS WILL
PUSH BACK INLAND AGAIN TONIGHT AHEAD OF AN APPROACHING TROUGH WITH
LIFR-IFR CEILINGS NEAR THE COAST AND MVFR CEILINGS LIKELY WEDNESDAY
MORNING FOR MUCH OF THE INTERIOR. CURRENTLY, STRATUS HAS FILLED THE
COAST AND IT'S LEADING EDGE IS NEAR KSHN. CEILINGS ARE EXPECTED TO
LIFT BACK TO VFR FOR INTERIOR AREAS WEDNESDAY AFTERNOON. INSTABILITY
ASSOCIATED WITH THE TROUGH WILL LEAD TO SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS FOR THE NORTH CASCADES AND THE OLYMPICS
WEDNESDAY AFTERNOON AND EVENING.
KSEA...VFR IS EXPECTED FOR THE REMAINDER OF THE EVENING. LOW CLOUDS
ARE EXPECTED TO RETURN ON WEDNESDAY MORNING, BUT CEILINGS WILL MORE
LIKELY FALL INTO MVFR CATEGORY. SURFACE WINDS W/SW UNDER 7 KNOTS
WILL VEER N/NW AGAIN BRIEFLY THIS EVENING BEFORE BACKING S/SW AGAIN
TONIGHT.
27/41
MARINE
BROAD SURFACE HIGH PRESSURE WILL REMAIN CENTERED OVER THE OFFSHORE
WATERS WITH LOWER PRESSURE EAST OF THE CASCADES THROUGH MUCH OF THE
WEEK AHEAD. THIS WILL MAINTAIN VARYING DEGREES OF ONSHORE FLOW
OVER THE NEXT SEVERAL DAYS AND BRING DIURNALLY DRIVEN SMALL
CRAFT ADVISORY STRENGTH WESTERLIES TO THE CENTRAL AND EAST
STRAIT OF JUAN DE FUCA AT TIMES.
SEAS ACROSS THE COASTAL WATERS OF 2-4 FEET TODAY ARE EXPECTED
TO INCREASE TO 5-7 FEET NEAR MIDWEEK BEFORE SUBSIDING BACK TO
3-5 FEET HEADING INTO THE WEEKEND.
27
FIRE WEATHER
A DRY, ONSHORE FLOW PATTERN WITH SEASONABLE TEMPERATURES AND
WILL CONTINUE THIS AFTERNOON. ENSEMBLES CONTINUE TO VERY
CONFIDENTLY SHOW AN UPPER LEVEL SHORTWAVE CLIPPING THE AREA
WEDNESDAY AFTERNOON AS RECENT MODELS HAVE BROUGHT THE SHORTWAVE
A BIT MORE SOUTH OVER THE CWA. THIS SHORTWAVE WILL PROVIDE
INSTABILITY TO HELP DEVELOP THUNDERSTORMS WEDNESDAY AFTERNOON
AND EVENING FOR THE NORTHERN AND CENTRAL CASCADES. LATEST
GUIDANCE CONTINUES TO SHOW GENERALLY A 20-30% PROBABILITY OVER
THE NORTH AND CENTRAL CASCADES, WITH THE HIGHER PROBABILITIES
ALONG THE BORDER AND ON THE EAST SIDE. WARM AND CONTINUED DRY
CONDITIONS REMAIN ON TRACK THROUGH THE SECOND HALF OF THE WEEK,
ALTHOUGH IN LATEST GUIDANCE, TEMPERATURES HAVE SLIGHTLY GONE
DOWN FROM PREVIOUS FORECASTS. IT IS A LITTLE EARLY TO BACK AWAY
FROM EXPECTING WARMER AND DRY CONDITIONS INTO THE START OF NEXT
WEEK...RESULTING IN ELEVATED FIRE WEATHER CONCERNS.
ADDITIONALLY, POTENTIAL SOUTHERLY FLOW OVER THE WEEKEND COULD
CONTINUE LOW CHANCES FOR THUNDERSTORMS OVER THE CASCADES. WE'LL
CONTINUE TO MONITOR THE EVOLUTION OF THE PATTERN AS IT DEVELOPS.
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 5 AM PDT WEDNESDAY FOR CENTRAL U.S.
WATERS STRAIT OF JUAN DE FUCA-EAST ENTRANCE U.S. WATERS
STRAIT OF JUAN DE FUCA.
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