textproduct: Seattle/Tacoma

SYNOPSIS

A lifting frontal system will continue to spread across the region, maintaining showers and mountain snow tonight. Another disturbance tomorrow brings heavier snow to the mountains and a cooler air mass to the region. Expect the pattern to shift by early next week as a ridge builds over the Pacific Northwest. This will maintain drier and cooler conditions with patchy fog into the start of next week.

SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/

The latest forecast remains on track this evening with no updates. Rain and mountain snow showers continue to lift north behind the warm front that occluded as it moved across the region. Showers continue overnight with a sight chance (10-15%) of an isolated thunderstorm near the coastline. The next disturbance pushes through tomorrow bringing another round of widespread rain to the lowlands and heavier snow to the mountains. Winter Weather Advisories remain in effect for the Cascade zones with generally 6-12 inches expected from the passes into the higher elevations. Accumulations are expected to be lesser below the passes, but with snow levels falling through the day to around 1500 ft or so by Friday night, could even see a light snow falling into these lower elevations. One other area to note would be that a Puget Sound Convergence Zone will be favored to develop late Friday behind the stronger onshore flow, and with the colder air moving in there is some potential for a burst of snow down to even lower elevations under this band. Will need to closely monitor high res guidance and radar/satellite trends through the day Friday for this potential impact. Otherwise, moisture decreases into Saturday with high pressure building over the region. This will likely maintain some cooler mornings with temperatures dipping down to near or below freezing across many portions of the interior lowlands. While these temperatures aren't especially far below normal for this time of year, they will be quite cool compared to the mild temperatures we've seen recently and thus this may be more notable.

LONG TERM /SUNDAY THROUGH THURSDAY/

Ensemble guidance supports a strong ridge building across the region and holding through the first half of next week. This will maintain dry conditions across the area with the stormy weather pointed well north into the central British Columbia coast. Meanwhile, expect fog to develop in the more stable air mass. Confidence begins to lower with the evolution of the breakdown the ridge with the ensemble guidance more spread.

AVIATION

Southerly flow will persist aloft tonight with an upper level low situated off the coast of OR/northern CA. Flow aloft will gradually transition to northwesterly by Friday afternoon as the upper trough moves inland over the region.

Showers continue to move northward across western Washington this evening as a frontal system lifts across the area, with conditions at area terminals ranging from VFR outside of shower activity to MVFR and LIFR in shower activity. May see some overall improvement towards VFR again later tonight in the wake of shower activity, however expect MVFR conditions to redevelop across the majority of the terminals again by Friday morning as another round of rain moves into the region. Localized IFR/LIFR conditions will be possible in any heavier bands of rain and latest hi-res guidance indicates a convergence zone developing across King/Snohomish counties during the afternoon and evening hours. Winds remain out of the N/NE winds 5-10 kt will shift to the S/SE this evening and ease to 3-7 kt. Winds will increase again to 10-15 kt between 15-18Z Friday.

KSEA...Mostly VFR so far this evening, before MVFR ceilings redevelop overnight. There remains a slight chance (20-25%) of IFR ceilings developing between 09Z-18Z Friday as a second round of rain pushes through the terminal. N/NE winds 5-10 kt this afternoon will shift to the S/SE this evening and weaken to 3-6 kt. S/SW winds will increase to 10-15 kt between 15-18Z Friday.

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MARINE

Offshore flow continues so far this evening with a broad surface trough across the area and high pressure situated over the interior of British Columbia and Alberta. Winds will gradually turn southerly as a frontal system continues to lift northward across the area waters tonight and become northwesterly by Friday as the trough pushes inland. Southerly winds may occasionally gust to around 25 kt across the coastal waters this evening. Winds will then increase to 20-30 kt and shift to the northwest across the coastal waters tonight before spreading to the inner coastal waters by Friday morning. Westerly winds along the Strait of Juan de Fuca will also increase to 20-30 kt on Friday morning, before easing Friday night. Seas will increase to 10-13 ft across the area waters tonight and towards 13-16 ft through the day on Friday.

High pressure will then build over the coastal waters over the weekend and remain situated across the region through early next week. Seas will gradually subside to below 10 ft again by Saturday afternoon and into Saturday evening. Weak fronts may brush the area to the north at times early next week, however no significant impacts are expected with this at this time. Dense fog may develop over the interior waters at times this weekend under the influence of high pressure and light winds, likely restricting visibilities at times.

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HYDROLOGY

No river flooding is expected over the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory from 4 AM to 10 PM PST Friday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County-Cascades of Whatcom and Skagit Counties.

PZ...Small Craft Advisory until 6 AM PST Saturday for Grays Harbor Bar.

Small Craft Advisory from 6 AM Friday to 1 AM PST Saturday for Central U.S. Waters Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 1 PM Friday to 1 AM PST Saturday for East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 6 PM PST Saturday 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.