textproduct: Seattle/Tacoma

SYNOPSIS

High pressure will maintain drier conditions for much of the region today before the next system brings in widespread lowland rain, mountain snow, and breezy winds on Sunday. Cooler and unsettled weather will continue into early next week, with potential for additional systems to bring moderate to heavy rain and mountain snow next week.

SHORT TERM /TODAY THROUGH SATURDAY/

Weak ridging will continue to build inland today as isolated showers continue near the Canadian border. Areas of fog have already begun to develop early this morning, and light winds and some clearing will continue to promote patchy fog this morning, especially for areas south and west of the Puget Sound. A frontal system will stall over British Columbia into the start of the weekend, resulting in rain at times across the northern Olympic Peninsula and northern interior, while conditions generally south of Everett will remain dry and cloudy. As the upper ridge flattens on Saturday, onshore flow will increase, with breezy winds reaching 20 to 35 mph along the northern coast and Whidbey Island northward. Temperatures will stay fairly steady today in the low 50s across the lowlands, warming a few degrees into Saturday.

LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/

The frontal system stalled over British Columbia will swing southward across western Washington late Saturday night into Sunday, bringing widespread precipitation and breezy winds to the region into Monday. Widespread precipitation will spread into the region by early Sunday, with snow levels initially around 5500-6000 ft lowering to 2000-2500 ft by Sunday evening. The lowlands are on track to see generally up to a half inch of rain through Monday morning, with 4 to 6 inches of snow accumulation through most mountain passes. Forecast models continue to show a post frontal convergence zone developing into Monday, which will generate locally higher snowfall amounts over the central Cascades.

A weak shortwave is on track to maintain showers mainly over higher terrain on Monday, with light additional accumulations. Weak ridging is favored to briefly dry out conditions on Tuesday ahead of a potentially more active weather system mid-week. Preliminary forecasts show significant incoming moisture to the Pacific Northwest, with rainfall totals of up to an inch across the lowlands and up to a foot or more of snow generally above 3000 ft. However, the amount of incoming precipitation will be highly dependent on the track of the weather system which remains fairly uncertain at this time. This storm system may cause travel impacts towards the end of next week, particularly through the Cascade Passes, which will continue to be closely monitored as the Thanksgiving holiday approaches.

15

AVIATION

A flat east-tilted ridge will pass over western WA this morning from north to south, with weak to light low level flow aloft. This has kept the region mostly clear overnight. Given some moisture content in the lower levels and a very shallow inversion (combined with the clear skies and very little dew point-temperature spread), patchy radiational mist/fog has formed and is currently in progress in pockets of the Kitsap Peninsula/South Interior areas. Additional pockets of localized mist/fog are possible across the region this morning, and may cause variable visibilities from MVFR all the way down to LIFR through late morning (especially for terminals with light/calm winds). A passing front to the north will produce showers in the northern terminals this afternoon/evening, and the coast through Saturday. Outside of the isolated fog/mist this morning, clouds will fill in with the front this afternoon, with VFR decks gradually lowering to MVFR into Saturday morning (with IFR from Kitsap down the Chehalis corridor). Winds remain out of the S 5-10 kt later today into Saturday (could see a couple terminal along coastal waters see a gust or two to 15-20 kt.).

KSEA...A couple high-resolution models have been picking up on mist between downtown and the KSEA terminal early this morning. Believe this will be primarily confined to the KBFI terminal (given the lighter winds and valley nature of the area, plus the terminal has begun to see sporadic drops in VIS down to 7SM as of 10Z). The models however have the wind at KSEA dropping to under 5 kt through late morning, so cannot rule out a possible TEMPO scenario of mist down to IFR between now and 18Z (but the confidence remains too low to include it at this time). Ceilings will fill in and gradually lower from VFR down to MVFR Saturday morning (roughly after 10Z). Winds pick up out of the S 5-10 kt (couple gusts to 15 kt are possible).

HPR

MARINE

An flat upper level ridge will pass through the waters today, with a frontal system approaching from the northwest through the weekend. Main concern in the short-term is seas - they remain at 11 to 15 ft in the coastal waters and Grays Harbor bar this morning. They are expected to decrease down to 9 to 12 ft into the evening (but remain high enough to keep the small craft advisories going for the coastal waters, Grays Harbor bar and the west Strait of Juan de Fuca). The wave height forecast keeps seas 9 to 12 ft through Monday, before decreasing down to 6 to 8 ft through Wednesday and increasing again to 9 to 12 ft Wednesday through the end of next week as additional systems pass through. The next windy period for the waters is with the Sunday/Monday system with a medium chance of winds exceeding 20 kt in the coastal waters (high chance in the Strait of Juan de Fuca), followed by another system Tuesday/Wednesday.

Additionally, there are pockets of mist/fog in the waters close to the Kitsap Peninsula this morning that may limit visibility at times to mariners.

HPR

HYDROLOGY

A weather system will move across western Washington late Saturday into Monday for lowland rain and mountain snowfall. No river flooding is expected during this period, but rivers will rise. The potential exists for additional, heavier precipitation during the middle of next week. However, uncertainty exists in both the location and duration of precipitation. River levels will continue to be monitored during this mid to late week period.

JD

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until noon PST today for Grays Harbor Bar.

Small Craft Advisory until 4 AM PST early 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-West Entrance U.S. Waters Strait Of Juan De Fuca.