textproduct: Seattle/Tacoma

SYNOPSIS

A disturbance will continue across the area with additional snow for the mountains and lowland rain showers through Friday. A trend towards drier conditions emerge over the weekend and continue into the first half of next week before the next front arrives around midweek.

UPDATE

As of 0845 radar indicated light shower activity moving onshore along the coast and north interior. Rain shadowing was dominating across most of the Puget Sound. Satellite indicated standing waves, especially east of the Cascades. Showers will continue across the north Cascades and the Winter Weather Advisory remains in place. The current forecast is on track and no updates are planned this morning. -Wolcott-

SHORT TERM /TODAY THROUGH SATURDAY/

The upper-level pattern has remained fairly unchanged the last 24 hours or so. A cut off low continues to meander over the NE PAC in concert with a ridge nosing into the Bering Sea. Downstream of the low center, additional ridging is extending off the US West coast as its axis parallels the CA coast. In the midst of this complicated wave pattern, the PNW is in the regime of WNW flow aloft, serving as a conduit for lowland rain showers and mountain snow.

For today, rain showers are in the forecast along the coast and windward Olympic Peninsula. A few may sneak into the W WA lowland interior but rain shadowing will help to keep them at bay. Mountain snow will persist for the Cascades & Olympics with snow levels around 2,500 to 3,000 ft. Breezy southerly winds with gusts up to 30-35 mph are also in store with the strongest expected for areas adjacent to Puget Sound, San Juans and W Whatcom. A frontal system over interior BC will interact with a surface high positioned just off the Oregon Coast - highlighted well on the Olympia - Bellingham gradient analysis.

This pattern will likely remain through Friday. An additional 5-10" of snow is in the forecast for the North Cascades including Mt. Baker Ski area. 850mb flow will turn more zonal here as well. As a result, high-res model reflectivity is picking up on convergence zone activity over the Puget Sound region. This feature will increase PoPs for lowland locations of the interior but only for Snohomish county and adjacent areas.

Signs of pattern progression into Saturday as the aforementioned low center becomes ingested in the mean-flow. Here, it'll loom just off the CA/OR coast with upper-level ridging over the Gulf of Alaska. Conditions will dry out and trend that way into next week. High temperatures are to top out in the upper 40s to near 50 F with overnight lows cooling lower into the 30s each night. A 40-60% chance of overnight lows dropping below 32 F throughout the Chehalis River valley along with a 30-40% chance for W Whatcom lowlands by Saturday morning.

McMillian

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Ensembles suggests the upper-level pattern will continue to progress in the long-term forecast. The aforementioned cut off low will track inland over N. California/S. Oregon on Sunday. An upper- ridge will gradually position over the PNW by early next week. Daytime highs are slated to remain around average with cool overnight lows in the lower to mid 30s. The next front looks to arrive around midweek with a chance of widespread lowland rain and mountain snow.

AVIATION

West to northwest flow aloft continues across Western Washington today as the region remains on the periphery of a frontal system moving into British Columbia. The low level flow is onshore. Rather widespread MVFR ceilings across the area this morning are expected to lift to VFR most areas this afternoon. Ceilings are then expected to sink back to MVFR across the region again overnight into Friday morning. Locally gusty south to southwest surface winds will ease this evening.

KSEA...High end MVFR ceilings are expected to linger into midday before lifting to BKN040-060 later this afternoon into the evening hours. Ceilings will deteriorate again tonight with a general OVC015- 025 layer expected 09Z-20Z Friday before lifting again. Surface winds S/SW 10-15 gusting 20-25 knots easing to 8 to 11 knots after 00Z.

27

MARINE

Moderate onshore flow continues through Friday with lower pressure over British Columbia and high pressure located around the NE Pacific. Small Craft Advisory south to southwest surface wind gusts will continue into this morning for the Coastal Waters before slowly dissipating later today. SCA southwest to west winds are also expected through the Strait of Juan de Fuca. Onshore flow will also result in periodic SCA gusts for the northern inland waters and Puget Sound through the afternoon. Otherwise, winds will ease on Friday as high pressure builds over the waters. Northerly surface winds return over the weekend.

Seas will range between 7 to 9 feet today, before building to 8 to 11 feet tonight through Friday. Seas will then subside to 6 to 8 feet on Saturday. JD

HYDROLOGY

No river flooding is expected during the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 4 AM PST Friday for Cascades of Snohomish and Northern King Counties-Cascades of Whatcom and Skagit Counties.

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

Small Craft Advisory until 6 PM PST this evening for Northern Inland Waters Including The San Juan Islands-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.