517
FXUS66 KSEW 202242
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
342 PM PDT SUN APR 20 2025
SYNOPSIS
A TROUGH WILL BRING WIDESPREAD SHOWERS TO WESTERN
WASHINGTON THIS AFTERNOON. A CONVERGENCE ZONE THIS EVENING MAY
LEAD TO ADDITIONAL INCHES OF SNOW ALONG THE PASSES. CONDITIONS
WILL BEGIN TO DRY OUT ON MONDAY. HIGH PRESSURE WILL BRING DRY AND
WARMER CONDITIONS WEDNESDAY AND THURSDAY. PRECIPITATION CHANCES
RETURN FRIDAY INTO THE WEEKEND.
SHORT TERM /TONIGHT THROUGH WEDNESDAY/
A TROUGH CONTINUES TO
PUSH ACROSS WESTERN WASHINGTON THIS AFTERNOON, BRINGING WIDESPREAD
SHOWERS. THE TROUGH AXIS WILL PUSH EAST LATER THIS AFTERNOON.
BEHIND THE TROUGH AXIS,THERE IS GENERAL AGREEMENT AMONG HIGH-
RESOLUTION MODELS THAT A CONVERGENCE ZONE WILL DEVELOP ALONG THE
CENTRAL SOUND AND CASCADES (ALONG SNOQUALMIE AND STEVENS PASS)
THIS AFTERNOON/EVENING (MAINLY AFTER 5 PM). SNOW LEVELS THIS
AFTERNOON (~4000 FT) WILL DROP OVERNIGHT TO 2500 FT. RAINFALL AND
SNOW ACCUMULATIONS WILL BE LIGHT, WITH 0.10 INCHES OR LESS OF RAIN
IN THE LOWLANDS AND AROUND 2-3 INCHES OF SNOW ALONG THE PASSES.
LOCALIZED HIGHER AMOUNTS EXPECTED ALONG CONVERGENCE ZONE SHOWERS,
WHICH MAY LEAD TO ADDITIONAL INCHES OF SNOW ALONG THE PASSES. IN
ADDITION, WEAK INSTABILITY MAY LEAD TO A RUMBLE OR TWO (10-15%
CHANCE) WITH THESE SHOWERS. POST-TROUGH SHOWERS WILL CONTINUE ON
MONDAY AND GRADUALLY TAPER OFF THROUGHOUT THE DAY.
A WEAK SHORTWAVE TROUGH WILL MOVE OVER WESTERN WASHINGTON DURING
THE EARLY HOURS OF TUESDAY. MODELS HAVE GENERALLY BEEN CONSISTENT
FOR THE PAST RUNS IN KEEPING THIS SYSTEM RELATIVELY DRY. IF THERE
IS ANY PRECIPITATION, IT WILL BE LIGHT AND LIMITED TO THE
MOUNTAINS. WHILE RELATIVELY DRY, THIS TROUGH WILL BRING COOLER
OVERNIGHT TEMPERATURES. LOW TEMPERATURES IN THE SOUTHERN INTERIOR
WILL DROP INTO THE MID-30S. THE COMBINATION OF LIGHT WINDS,
LINGERING MOISTURE, AND MOSTLY CLEAR SKIES OVERNIGHT MAY PROMOTE
THE DEVELOPMENT OF PATCHY FROST ACROSS THE SOUTHERN INTERIOR
VALLEYS. THE SHORTWAVE WILL PUSH OUT OF THE REGION LATE TUESDAY
NIGHT.
AN UPPER LEVEL RIDGE WILL BRING DRY AND WARMER CONDITIONS FOR
WESTERN WASHINGTON STARTING ON WEDNESDAY. HIGH TEMPERATURES ON
WEDNESDAY WILL BE IN THE LOW-TO-MID 60S WITH MOSTLY CLEAR SKIES.
LONG TERM /THURSDAY THROUGH SUNDAY/
UPPER LEVEL RIDGING ON
THURSDAY WILL KEEP WARM AND DRY CONDITIONS OVER WESTERN
WASHINGTON. THURSDAY TEMPERATURES WILL BE SLIGHTLY WARMER, WITH
HIGH TEMPERATURES IN THE UPPER 60S TO LOW 70S ALONG THE INTERIOR.
THE RIDGE AXIS WILL MOVE EAST INTO THE ROCKIES LATE THURSDAY
AFTERNOON AHEAD OF AN INCOMING DIGGING TROUGH.
THERE IS GENERAL AGREEMENT AMONG ENSEMBLE GUIDANCE AND
DETERMINISTIC MODELS THAT COOLER AND UNSETTLED WEATHER WILL LIKELY
RETURN FRIDAY AND CONTINUE THROUGH THE WEEKEND, WITH AN UPPER
LEVEL TROUGH EXPECTED TO TRAVERSE THE REGION. HOWEVER, CURRENT
ENSEMBLE GUIDANCE IS STRUGGLING WITH THE INTENSITY, TROUGH AXIS
LOCATION, AND TIMING WITH THIS SYSTEM.
29
AVIATION
NORTHWESTERLY FLOW WILL PERSIST ALOFT THROUGH THE TAF
PERIOD AS AN UPPER LEVEL TROUGH SWINGS THROUGH WESTERN WA TONIGHT
AND EXITS THE REGION ON MONDAY. TERMINALS ARE PRIMARILY MVFR TO
VFR ACROSS THE REGION THIS AFTERNOON, WITH RADAR SHOWING SCATTERED
SHOWER ACTIVITY ACROSS THE MAJORITY OF THE REGION. EXPECT OVERALL
SHOWER ACTIVITY TO GRADUALLY TAPER TONIGHT, OUTSIDE OF A CONVERGENCE
ZONE THAT LOOKS TO DEVELOP ACROSS THE CENTRAL SOUND LATE THIS
AFTERNOON AND PERSIST INTO EARLY MONDAY. MOST INTERIOR TERMINALS
ARE SEEING SOUTHWEST WINDS AT 7-12 KT THIS AFTERNOON, THOUGH THE
PSCZ MAY CAUSE SPORADIC SHIFTS IN WINDS TO THE WEST AND NORTHEAST
BETWEEN 23Z-06Z FOR THE CENTRAL SOUND TERMINALS. WINDS FOR KHQM
AND KCLM REMAIN WEST/NORTHWESTERLY, WITH GUSTS TO 20-25 KT
POSSIBLE THROUGH THIS EVENING. EXPECT WINDS FOR INTERIOR TERMINALS
TO TRANSITION BACK TO SOUTH/SOUTHWESTERLY OVERNIGHT.
KSEA...VFR CEILINGS WITH SHOWERS IN THE VICINITY OF THE TERMINAL.
WINDS GENERALLY PERSISTING AT 7-12 KT OUT OF THE SOUTHWEST THIS
AFTERNOON. A CONVERGENCE ZONE LOOKS TO DEVELOP CLOSE TO OR OVER
THE TERMINAL BETWEEN 23Z-04Z MONDAY. WIND SHIFTS TO THE WEST AND
THEN NORTHEAST ARE EXPECTED WITHIN THE CONVERGENCE ACTIVITY,
BEFORE WINDS RETURN BACK TO THE SOUTHWEST BETWEEN 07-09Z.
CEILINGS AND VIS MAY BRIEFLY DROP TO MVFR IN SHOWER ACTIVITY AT
TIMES TONIGHT, THOUGH EXPECT CEILINGS TO LOWER TO MVFR BY AROUND
12Z MONDAY.
14
MARINE
HIGH PRESSURE CURRENTLY RESIDES OVER THE NORTHEASTERN
PACIFIC, WHILE A WEAK DISTURBANCE CONTINUES TO PROGRESS ACROSS THE
AREA WATERS TONIGHT. NORTHWESTERLY WINDS ACROSS PORTIONS OF THE
COASTAL WATERS WILL INCREASE TONIGHT AGAIN AS A RESULT AND ARE
GENERALLY EXPECTED TO PERSIST BETWEEN 20 TO 25 KT. ONSHORE FLOW
WILL INCREASE TONIGHT AGAIN AS WELL, WITH ANOTHER ROUND OF SMALL
CRAFT STRENGTH WESTERLIES EXPECTED THROUGH THE STRAIT OF JUAN DE
FUCA. WINDS LOOK TO PEAK AROUND 25-30 KT THIS EVENING, BEFORE
EASING LATE TONIGHT. HIGH PRESSURE WILL BUILD BACK INTO THE
COASTAL WATERS ON MONDAY AND PERSIST THROUGH MIDWEEK, BRINGING
GENERALLY CALMER CONDITIONS TO THE AREA WATERS. THE NEXT FRONTAL
SYSTEM LOOKS TO APPROACH THE AREA WATERS ON FRIDAY.
SEAS ACROSS THE COASTAL WATERS ARE GENERALLY PERSISTING AT 8-10
FT THIS AFTERNOON. EXPECT SEAS TO FALL TOWARDS 6-8 FT ON MONDAY
AND CLOSER TO 4-6 FT BY MIDWEEK WITH HIGH PRESSURE OVER THE
REGION. SEAS THEN LOOK TO INCREASE TOWARDS 7-9 FT AGAIN LATE IN
THE WEEK IN THE WAKE OF THE NEXT FRONTAL SYSTEM.
14
HYDROLOGY
NO RIVER FLOODING EXPECTED IN THE NEXT 7 DAYS.
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 3 AM PDT MONDAY FOR CENTRAL U.S.
WATERS STRAIT OF JUAN DE FUCA-EAST ENTRANCE U.S. WATERS
STRAIT OF JUAN DE FUCA-WEST ENTRANCE U.S. WATERS STRAIT OF
JUAN DE FUCA.
SMALL CRAFT ADVISORY UNTIL 11 AM PDT MONDAY FOR COASTAL WATERS
FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60 NM-COASTAL
WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 10 NM-COASTAL
WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10 TO 60 NM-
COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE OUT 10
NM.
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