textproduct: Seattle/Tacoma

SYNOPSIS

Generally quiet weather is expected through Wednesday night or Thursday. The door will open to a series of weather systems beginning Thursday and continuing into next week. Initially the concerns will be gusty winds and the potential for coastal flooding, then transition to a river flood threat with a continued coastal flood threat as we move through the weekend into next week. Active weather is likely to continue through at least the first half of next week.

SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/

Generally quiet conditions are expected in the short term. A weak frontal will push through tonight into early Tuesday bringing with in some light precipitation with little impact expected. A ridge will nose into the region Wednesday, keeping the area relatively dry. Patchy fog will be possible each morning.

LONG TERM /THURSDAY THROUGH MONDAY/

By Thursday, weak waves moving over the top of the ridge could begin to increase precipitation chance Thursday, but the ridge breakdown is more likely to occur Friday. Beginning Friday, the door will open to periods of moderate to heavy precipitation associated with moisture plumes originating from the subtropics. While each of these periods alone appear unlikely to cause issue, the successive nature could result in a increased risk of flooding as early as this weekend, but perhaps more likely by early next week. In addition, rainfall-driven landslide indices are also approaching seasonal thresholds and by early next week the landslide risk could become the highest yet seen this season.

In addition, hazards associated with coastal flooding and wind will become elevated late this week, likely peaking late this weekend or early next week. Currently, the coastal flood risk appears to peak along the outer coast Thursday through Saturday this week, but at this time, only appears to flirt with minor flood thresholds. The concern is higher for the inner coast of the Puget Sound and Salish Sea from Friday into early next week. Current models indicate multiple days of water levels well above minor flood stage, and a few locations (particularly Cherry Point and Friday Harbor) flirting with moderate or higher flood levels. The actual level of impact will strongly depend on the overlap of low pressure and elevated winds which are a challenge to nail down at this lead time.

Looking out beyond the next 7 days, the Climate Prediction Center has included a moderate (40-60%) chance of hazardous rain, high elevation snow, and winds continuing through December 10. -Wolcott-

AVIATION

West to northwest flow aloft will continue with an upper level ridge centered well offshore. A weak frontal system will reach the area tonight with deteriorating ceilings and areas of light precipitation. VFR ceilings will lower to MVFR overnight. A mix of MVFR and IFR will prevail areawide on Tuesday with light surface flow and a moist air mass in place.

KSEA...VFR ceilings will lower to MVFR tonight as a weak system spreads some light rain across the area. Precipitation will dissipate on Tuesday, but low MVFR or occasional IFR ceilings will persist throughout the day tomorrow. Light and variable surface winds will become southerly 7 knots or less tonight, then return to light and variable or very weak northerly during the day on Tuesday.

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MARINE

A weak frontal system will cross the Pacific Northwest tonight into Tuesday resulting in small craft winds beyond 10 nm offshore. Seas will also increase from 4-6 ft Monday to 9-12 ft Tuesday through Wednesday. Benign conditions are expected behind this system, with light winds, and seas returning to 4-6 ft. A stronger system Friday through the weekend may bring small craft advisory level winds to the coastal waters, and a few of the north interior waters. There is also a medium chance at this time for gale winds in the Strait of Juan de Fuca. Seas will also increase to 9-13 ft with this system Friday through the weekend into early next week.

Wolcott/HPR

HYDROLOGY

A series of at least two (and potentially more) atmospheric river events with subtropical origins is currently expected to impact the area beginning roughly Friday and continuing through the weekend into early next week. Given the lead time, there is still quite a range of potential outcomes, but there is at least a chance of a significant flood event beginning as early as this weekend, but more likely early next week. Based on the latest HEFS guidance, the median (50th percentile) forecast would result in only a few rivers reaching action stage, while the higher end scenarios indicate the potential for significant flooding. The 90th percentile (reasonable high end) in particular indicates the potential for a few rivers to reach major flood stage. These higher end scenarios would also bring with it an elevated rainfall-driven landslide risk. -Wolcott-

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from midnight tonight to noon PST Tuesday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm.