textproduct: Seattle/Tacoma

SYNOPSIS

A frontal system will cross western Washington tonight, bringing in widespread lowland rain, mountain snow, and breezy winds. Cooler and unsettled conditions will linger into Monday, with increased potential for a wetter and cooler system to enter the region mid week into the Thanksgiving holiday.

SHORT TERM /TODAY THROUGH MONDAY/

A frontal system stalled over British Columbia will maintain light shower activity along the Canada border and Pacific Coast for much of the day today. Onshore flow will promote cloudy skies throughout the day, with an increase in winds this afternoon along the northern coast and Whidbey Island northward. The frontal boundary will shift southward later this evening, with more widespread precipitation filling in across western Washington overnight into Sunday. Whlie the bulk of the moisture with this system will shift eastward by the afternoon Sunday, post frontal showers will continue to stream inland through Sunday night with a convergence zone developing over the northern Puget Sound and central Cascades. Stronger showers that move inland later Sunday may even produce a few lightning strikes, with the highest chances along the Pacific Coast. Winds will also become breezy as this system digs into the Pacific Northwest, with gusts reaching 20 to 35 mph. Snow levels around 5500-6000 feet will fall to near 2000-2500 feet by Sunday night, with a high chance (60% to 70%) of 1 to 2 inches of accumulating snowfall through the Cascade mountain passes.

The upper trough axis will shift inland early Monday morning, bringing in another round of showers across western Washington. Snow levels will remain near 2000-2500 feet, with an additional 2 to 4 inches of snow accumulation possible through the Cascade passes. Precipitation will taper off Monday afternoon and evening as weak ridging builds inland, giving way to some occasional sun breaks among cloudy skies.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

High pressure will continue to build northward into Tuesday, providing a brief break in wet conditions across western Washington. Tuesday morning will see the coldest morning temperatures of the period, with many lowland locations especially away from the water dipping to near freezing.

Ensembles and operational forecast models remain in good agreement over the next weather system entering the Pacific Northwest by late Tuesday, with potential to bring in significant lowland rain and mountain snow alongside breezy winds. Snow levels will fluctuate mid week, opening the door for accumulating mountain snowfall and even a slight chance of freezing rain through the Cascade passes ahead of the Thanksgiving holiday. While the details of this incoming atmospheric river remain somewhat uncertain at this time, conditions continue to be closely monitored ahead of holiday travel. Ensembles also highlight active and potentially impactful weather continuing into next weekend.

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AVIATION

An upper level ridge will move out of the region this morning as a trough approaches late tonight/Sunday with a frontal system. Flow aloft will increase out of the W/SW. For this morning: MVFR ceilings are currently ongoing across most terminals (with pockets of IFR ceilings/mist along the coast and the south interior). With the increased low-level moisture ahead of the next system, the ceilings will only marginally improve to upper-end MVFR/low-end VFR late this afternoon and evening (best chance for VFR improvements are terminals from KBFI south to KOLM). Ceilings will lower again to MVFR late tonight/Sunday (pockets of IFR possible). Rain showers will arrive on the coast early this afternoon, with organized rain showers encompassing most terminals early Sunday morning. SW winds will pick up to 8-12 kt today (with a few gusts to 20-25 kt for terminals along interior waterways, and the coast).

KSEA...MVFR ceilings are expected to remain in place through around 21Z (with a brief improvement to VFR decks just above 3,000 ft from 21Z-03Z before lowering back to MVFR). Showers arriving by around 06Z Sunday. Winds SW 8-12 kt (occasional gusts to 20 kt possible from 10Z Sunday onwards through the afternoon).

HPR

MARINE

An upper level region will move south of the region, giving way to a trough and frontal system approaching the waters from the west late this afternoon into Sunday. The seas in the coastal waters will remain around 10-13 ft Saturday through Sunday as this system moves through. Increasing S winds are expected with increasing confidence for 20-25 kt gusts and seas remaining above 10 ft through Monday resulted in the small craft advisory (SCA) being extended through Monday. A couple of high-resolution models also increased confidence in gusty S winds to 25 kt from the Northern Inland Waters, down to the northern portions of the Puget Sound waters (including the east Strait of Juan de Fuca and Admiralty Inlet). An additional SCA was issued for the frequent gusts Saturday evening through late Sunday morning. Post-front, a strong push may produce frequent gale gusts for the central/east strait Sunday evening into Monday morning (gale watch was issued). After Monday, seas will decrease down to 6 to 8 ft through midweek before another system brings seas just above 10 ft and gusty winds just above 20 kt.

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 6 AM PST Sunday 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-Grays Harbor Bar.

Small Craft Advisory until 4 PM PST this afternoon for West Entrance U.S. Waters Strait Of Juan De Fuca.