667
FXUS66 KSEW 171027
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
227 AM PST TUE DEC 17 2024
SYNOPSIS
AN ACTIVE WEATHER PATTERN IS EXPECTED OVER THE NEXT
ONE TO TWO WEEKS ACROSS WESTERN WASHINGTON. AN ATMOSPHERIC RIVER
WILL BRING HEAVIER RAIN, GUSTY WINDS, AND MOUNTAIN SNOW AND
FREEZING RAIN TODAY THROUGH WEDNESDAY. ADDITIONAL WARM AND WET
SYSTEMS WILL MOVE ACROSS THE AREA AT TIMES INTO THE WEEKEND.
SHORT TERM /TODAY THROUGH THURSDAY/
SHOWERS CONTINUE THIS
MORNING ACROSS THE AREA, AHEAD OF THE NEXT SIGNIFICANT WEATHER
SYSTEM TO MOVE THROUGH LATER TODAY INTO EARLY WEDNESDAY. WIDESPREAD
STRATIFORM PRECIPITATION WILL SLIDE NORTHWARD ACROSS WESTERN
WASHINGTON LATER THIS MORNING AND CONTINUE THROUGH TONIGHT AS A
WARM FRONT SLOWLY LIFTS NORTHWARD. SNOW LEVELS WILL INITIALLY BE
AROUND 3000 TO 4000 FEET AT THE ONSET OF PRECIPITATION, ALLOWING
FOR ACCUMULATING SNOWFALL ACROSS THE CASCADE PASSES. AT THIS TIME,
APPROXIMATELY 2 TO 4 INCHES OF SNOW ARE FORECAST FOR SNOQUALMIE
PASS, WITH 3 TO 5 INCHES OF SNOW FOR STEVENS PASS INTO THIS
AFTERNOON. HOWEVER, SNOW LEVELS WILL THEN RISE RAPIDLY TONIGHT,
PEAKING NEAR 8000 FEET, ALLOWING FOR A TRANSITION TO MORE LIQUID
PRECIPITATION. EASTERLY WINDS AT THE CASCADE PASSES WILL ALLOW FOR
CONTINUED TEMPERATURES NEAR FREEZING TONIGHT FOR BOTH STEVENS AND
SNOQUALMIE PASS, RESULTING IN FREEZING RAIN CONCERNS WITH WARMER
AIR ALOFT. WILL HAVE TO MONITOR THE EXTENT OF ICE ACCUMULATION
WITH THE WINTER WEATHER ADVISORY REMAINING IN EFFECT THROUGH
WEDNESDAY MORNING.
IN ADDITION TO THE MOUNTAIN WINTRY MIX, SOUTHERLY WINDS WILL
INCREASE IN THE WARM SECTOR TONIGHT. LATEST NBM GUIDANCE SUGGESTS
THE HIGHEST PROBABILITIES (60 TO 90 PERCENT) OF GUSTS EXCEEDING
45 MPH ALONG THE COAST, AS WELL AS OVER ADMIRALTY INLET, WESTERN
WHATCOM AND SKAGIT COUNTIES, AND THE SAN JUAN ISLANDS. HREF IS
SIMILAR REGARDING THE REGION OF HIGHER WIND GUSTS TONIGHT. THE
WIND ADVISORY HAS BEEN EXPANDED TO INCLUDE THE AFOREMENTIONED
AREAS, WITH GUSTS OF 40 TO 55 MPH IN THESE AREAS. WITH THAT SAID,
BREEZY WINDS ARE EXPECTED ELSEWHERE AROUND PUGET SOUND, AND
FURTHER EXPANSION OF HEADLINES MAY NEED TO BE MONITORED,
ESPECIALLY AHEAD OF THE COLD FRONTAL PASSAGE TONIGHT. WEST WINDS
WILL ALSO RAPIDLY INCREASE POST-FRONT EARLY WEDNESDAY MORNING
ACROSS THE STRAIT OF JUAN DE FUCA, LEADING TO CONTINUED GUSTY
WESTERLY WINDS FOR WHIDBEY ISLAND INTO MID-WEDNESDAY MORNING.
LOCALIZED POWER OUTAGES AND TREE DAMAGE ARE POSSIBLE.
A POST-FRONTAL CONVERGENCE BAND MAY SLIDE ACROSS SNOHOMISH AND
NORTHERN KING COUNTIES WEDNESDAY MORNING FOR A BRIEF PERIOD OF
HEAVIER RAINFALL AFTER THE FRONTAL PASSAGE. OTHERWISE, SHOWERS
FOLLOWING THE FRONT WILL BE RATHER LIMITED BY WEDNESDAY AFTERNOON,
MOSTLY CONFINED TO THE CASCADES. A SLIGHT UPTICK IN THE RISK FOR
POTENTIAL COASTAL FLOODING EXISTS ON WEDNESDAY, MAINLY NORTH OF
ADMIRALTY INLET. OTHERWISE, THE RISK THEN SLOWLY DECREASES FOR
THE REST OF THE WEEK.
A BRIEF PERIOD OF DRIER WEATHER LIKELY CONTINUES IN WEDNESDAY
EVENING BEFORE ANOTHER ROUND OF LIGHTER RAIN SLIDES NORTHWARD INTO
EARLY THURSDAY. HIGHS THROUGH MIDWEEK WILL BE SLIGHTLY ABOVE
NORMAL WITH TEMPERATURES INTO THE LOW TO MID 50S.
LONG TERM /FRIDAY THROUGH MONDAY/
RIDGING BUILDS FURTHER ACROSS
THE INTERIOR WEST FRIDAY INTO SATURDAY. HOWEVER, WESTERN
WASHINGTON WILL BE ON THE WESTERN FRINGE OF THE UPPER RIDGE,
ALLOWING FOR AT LEAST SOME PRECIPITATION POTENTIAL LATER IN THE
WEEK. A FRONTAL SYSTEM MOVES TOWARDS THE PACIFIC NORTHWEST LATER
SATURDAY INTO SUNDAY, INCREASING THE RISK FOR LOWLAND RAIN AND
HIGHER ELEVATION MOUNTAIN SNOWFALL. THE ACTIVE PATTERN WILL THEN
CONTINUE NEXT WEEK, AS HIGHLIGHTED BY THE CPC WEEK-2 HAZARD
OUTLOOK FOR A MODERATE RISK IN HEAVY PRECIPITATION, HIGH WINDS,
AND HEAVY MOUNTAIN SNOWFALL. JD
AVIATION
LIGHT SOUTHWEST FLOW ALOFT THIS MORNING WILL SHIFT TO
THE WEST AND INCREASE DURING THE DAY. A STRONG SURFACE WARM FRONT
WILL LIFT NORTH THROUGH THE DAY TODAY, WHICH WILL BE FOLLOWED BY A
COLD FRONT TUESDAY NIGHT INTO WEDNESDAY MORNING. THE WARM MOIST AIR
MASS BEING BROUGHT IN WITH THE WARM FRONT IS EXPECTED TO BRING
CEILINGS DOWN TO MVFR GOING THROUGH THE MORNING HOURS THROUGH
TONIGHT/WEDNESDAY MORNING (ISOLATED IFR CIGS WILL LOCALLY MIX IN AT
TIMES). HIGH RESOLUTION GUIDANCE SHOWS MORE WIDESPREAD RAIN TRACKING
NORTH ACROSS ALL TERMINALS LATE THIS MORNING INTO THE EVENING, WITH
ANOTHER ROUND EARLY WEDNESDAY MORNING. HEAVIER RAINFALL RATES MAY
REDUCE VISIBILITIES AT TIMES TO 3-5SM. GUSTY SOUTH WINDS WILL PICK
UP TUESDAY AFTERNOON INTO WEDNESDAY MORNING. THE HIGHEST WINDS ARE
EXPECTED FOR COASTAL TERMINALS, PARTICULARLY FROM EVERETT TO
BELLINGHAM, AND FROM HOQUIAM UP TO NEAH BAY (GUSTS UP TO AND
EXCEEDING 40-45 KT POSSIBLE IN THESE AREAS). LOW LEVEL WIND SHEAR
WILL BE A POSSIBILITY ALOFT AT AROUND 2,000 FEET.
KSEA...OFF AND ON RAIN SHOWERS WILL CONTINUE THIS MORNING. MORE
WIDESPREAD RAIN WILL ARRIVE 16Z-00Z TODAY/WEDNESDAY, AND AGAIN FROM
06Z-12Z WEDNESDAY. CIGS WILL GRADUALLY LOWER TO MVFR LATE THIS
MORNING. IFR CONDITIONS DUE TO LOCALLY MODERATE TO HEAVY RAIN
(REDUCING VISIBILITIES BRIEFLY) CANNOT BE RULED OUT GOING THROUGH
THE DAY. WINDS WILL BEGIN TO PICK UP EARLY WEDNESDAY MORNING,
PEAKING AT UP TO 20 KT WITH GUSTS UP TO 35 KT POSSIBLE. MVFR CIGS
WILL LIKELY CONTINUE INTO WEDNESDAY MORNING.
HPR
MARINE
A SURFACE LOW PRESSURE SYSTEM WILL TRACK A WARM AND COLD
FRONT COMBO ACROSS THE WATERS TODAY INTO TONIGHT/WEDNESDAY MORNING.
WINDS WILL THE PRIMARY CONCERNS WITH THIS SYSTEM. SOUTH WINDS WILL
PICK UP GOING INTO THE AFTERNOON, AND TRANSITION TO WEST WINDS
WEDNESDAY MORNING. THERE IS AN INCREASED LIKELIHOOD IN SEEING
FREQUENT WIND GUSTS TO GALES ACROSS A MAJORITY OF WATERWAYS WITH
THIS SYSTEM (ESPECIALLY WITH THE SOUTH WINDS TONIGHT OVER THE
COASTAL WATERS AND ADMIRALTY/EAST STRAIT OF JUAN DE FUCA/NORTHERN
INLAND WATERS). THE SOUTH WINDS WILL TRANSITION TO WEST WINDS LATE
WEDNESDAY MORNING, WITH GALE GUSTS REMAINING LIKELY IN THE EAST
STRAIT OF JUAN DE FUCA THROUGH EARLY WEDNESDAY AFTERNOON. ANOTHER
SYSTEM ON THURSDAY WILL BRING GUSTY WINDS OVER THE COASTAL WATERS,
AND ADDITIONAL SYSTEMS WILL SPREAD WINDS TO OTHER WATERWAYS THROUGH
NEXT WEEKEND. ADDITIONALLY, HEAVY RAIN RATES MAY REDUCE VISIBILITIES
SIGNIFICANTLY TO MARINERS.
SEAS WILL REMAIN ABOVE 10 FEET THROUGH THE FORECAST PERIOD, WITH THE
CHANCE OF SEAS APPROACHING 20 FEET NEXT WEEKEND WITH THE EXPECTED
PLETHORA OF SYSTEMS EXPECTED TO PASS THROUGH THE REGION.
HPR
HYDROLOGY
AN INCOMING ATMOSPHERIC RIVER EVENT WILL CAUSE
SIGNIFICANT RISES TO AREA RIVERS LATER TODAY INTO MIDWEEK. LATEST
DETERMINISTIC FORECASTS SHOW INCOMING LIQUID AMOUNTS OF 3 TO 5
INCHES OVER THE OLYMPIC PENINSULA BETWEEN TODAY AND WEDNESDAY,
WITH UP TO TO 4 INCHES OVER THE CASCADES. SNOW LEVELS WILL ALSO BE
ELEVATED DURING THIS EVENT, LIFTING AS HIGH AS 7500-8500 FT
TONIGHT. THIS AMOUNT OF RAINFALL HAS THE POTENTIAL FOR PRODUCING
FLOODING. THE EXACT DETAILS OF THIS MID- WEEK SYSTEM REMAIN
UNCERTAIN, BUT AT THIS TIME THE SKOKOMISH RIVER IS FORECAST TO
ENTER MODERATE FLOOD STAGE ON WEDNESDAY WITH OTHER AREA RIVERS
APPROACHING OR REACHING ACTION STAGE.
ADDITIONAL INCOMING ATMOSPHERIC RIVER PRODUCING PRECIPITATION
TOWARDS THE END OF THE WEEK INTO NEXT WEEK MAINTAINS THE THREAT OF
RIVER FLOODING ACROSS WESTERN WASHINGTON, BUT MODELS CONTINUE TO
SHOW LARGE DIFFERENCES AND THE FORECAST IS UNCERTAIN.
JBB/JD
SEW WATCHES/WARNINGS/ADVISORIES
WA...WIND ADVISORY FROM 4 PM THIS AFTERNOON TO 4 AM PST WEDNESDAY FOR
CENTRAL COAST-NORTH COAST-SAN JUAN COUNTY-WESTERN SKAGIT
COUNTY-WESTERN WHATCOM COUNTY.
WIND ADVISORY FROM 1 PM THIS AFTERNOON TO 7 AM PST WEDNESDAY FOR
ADMIRALTY INLET AREA.
FLOOD WATCH THROUGH LATE TONIGHT FOR HOOD CANAL AREA-LOWER
CHEHALIS VALLEY AREA-OLYMPICS.
WINTER WEATHER ADVISORY UNTIL 4 AM PST WEDNESDAY FOR WEST SLOPES
NORTH CASCADES AND PASSES-WEST SLOPES NORTH CENTRAL
CASCADES AND PASSES-WEST SLOPES SOUTH CENTRAL CASCADES AND
PASSES.
PZ...SMALL CRAFT ADVISORY UNTIL 10 AM PST WEDNESDAY FOR GRAYS HARBOR
BAR.
GALE WARNING FROM 10 AM THIS MORNING TO 10 AM PST WEDNESDAY 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-COASTAL WATERS FROM POINT GRENVILLE TO
CAPE SHOALWATER 10 TO 60 NM-COASTAL WATERS FROM POINT
GRENVILLE TO CAPE SHOALWATER OUT 10 NM-WEST ENTRANCE U.S.
WATERS STRAIT OF JUAN DE FUCA.
SMALL CRAFT ADVISORY FROM 10 AM THIS MORNING TO 10 PM PST THIS
EVENING FOR CENTRAL U.S. WATERS STRAIT OF JUAN DE FUCA.
GALE WARNING FROM 10 PM THIS EVENING TO 10 AM PST WEDNESDAY FOR
CENTRAL U.S. WATERS STRAIT OF JUAN DE FUCA.
GALE WARNING UNTIL 10 AM PST WEDNESDAY FOR ADMIRALTY INLET-EAST
ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA.
GALE WARNING FROM 10 AM THIS MORNING TO 7 AM PST WEDNESDAY FOR
NORTHERN INLAND WATERS INCLUDING THE SAN JUAN ISLANDS.
SMALL CRAFT ADVISORY FROM 10 AM THIS MORNING TO 7 AM PST
WEDNESDAY FOR PUGET SOUND AND HOOD CANAL.
SMALL CRAFT ADVISORY UNTIL 10 AM PST THIS MORNING 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-COASTAL WATERS FROM POINT GRENVILLE TO CAPE
SHOALWATER 10 TO 60 NM-COASTAL WATERS FROM POINT GRENVILLE
TO CAPE SHOALWATER OUT 10 NM.
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