textproduct: Seattle/Tacoma

SYNOPSIS

A weather system has pushed precipitation across western Washington. Precipitation will become showery through this evening and gradually diminish overnight. Significant snow is expected above 1500 feet. While a rain snow mix cannot be completely ruled out in the lowlands, accumulations are unlikely for most areas. Drier and conditions as well as morning fog are then expected through Tuesday before a return to more typical lowland rain and mountain snow later this week.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

Latest radar trends indicate most of the widespread precipitation associated with the current weather system is now moving into the mountains while shower activity has developed in its wake. As expected, a Puget Sound Convergence Zone is beginning to develop across Snohomish County, and this feature will likely persist for several more hours into the evening while potentially sagging south slightly towards northern King County. Snow levels in the mountains have fallen to around 3000 feet with snow near the summit of Snoqualmie Pass. Snow levels will continue to lower through Saturday morning and significant accumulations are expected above 1500 feet.

For the lowlands, mainly rain is expected through tonight. However, there is a chance of a rain-snow mix at times, especially in heavier showers. Snow accumulations are unlikely given warm ground/surface temperatures before the precipitation ends. However, temperatures will continue to cool through sunrise Saturday and there may be some areas that see temperatures around freezing, leading to patchy ice, particularly on elevated structures such as bridges and overpasses.

We will continue to monitor potential trouble spots between the mountains and the lowlands, including the valleys in the foothills of the Cascades as well as some roadways at somewhat higher elevations including SR-18 over Tiger Mountain, US 101 west of Lake Crescent, and perhaps I-5 in the Chuckanuts south of Bellingham. However, even in most of these locations, the probability of seeing more than an inch of accumulation is fairly low.

Another item to monitor overnight into Saturday will be some increased outflow through the Fraser River valley. With cooler temperatures and modest winds across western Whatcom County we could see some wind chill values between 12-20 degrees. As such we have opted to issue a Cold Weather Advisory for that area starting early Saturday morning through noon.

Conditions will remain cool and mostly dry through Tuesday/Tuesday night as upper level ridging dominates the pattern. Given recent rains, saturated conditions near the surface could lead to the development of fog the next few mornings. This will need to be monitored for dense fog potential.

LONG TERM /WEDNESDAY THROUGH FRIDAY/

Ridging begins to flatten Wednesday onward and the the door begins to open to additional weather systems moving through the region. At this point in time, it appears that this pattern will be fairly nominal for the area with chances for lowland rain, mountain snow, and occasional breezy conditions. At this point in time there isn't a strong signal for any major events over the next week. -Wolcott-

AVIATION

An upper level trough will continue to push inland tonight, making way for an upper level ridge to build in from the west on Saturday. As a result, westerly flow aloft will become more northwesterly by tonight and persist through the remainder of the TAF period. Onshore flow in the low levels will continue to strengthen through this afternoon, before easing tonight into Saturday.

Conditions are currently a mixed bag across the area terminals this afternoon with latest radar showing the main band of precipitation moving across the Sound and slowly pushing into the Cascades. Terminals in rain are primarily MVFR to IFR, while terminals outside of any shower activity have mostly rebounded to VFR. Hi-res guidance does continue to highlight the development of a convergence zone across King and Snohomish counties this afternoon and evening, which could bring a more prolonged period of reduced cigs and visibilities to terminals like KPAE through tonight. Shower activity will gradually taper across the majority of the area by around 06Z. Breezy southerly winds between 10-15 kt (with gusts to 25-30 kt) this afternoon will ease back to 5-10 kt by this evening and will persist into Saturday. Weak Fraser Outflow may bring some gusty northeasterly winds to 25 kt to KBLI late tonight through 18Z Saturday.

With low levels remaining moist from recent rains, any areas of clearing overnight may also bring periods of fog to some terminals by Saturday morning. Cool overnight temperatures near freezing may even promote some spots of freezing fog early Saturday morning- especially for terminals like KOLM south of the Sound.

KSEA...MVFR ceilings this afternoon with southerly winds persisting between 10-20 kt, with gusts to 25-30 kt. Southerly winds will ease to 5-10 kt again by this evening. Could see a brief improvement in ceilings towards VFR for a few hours this evening, but expect ceilings to fall back to down to MVFR overnight into early Saturday. At this time, convergence zone activity looks to stay north of the terminal.

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MARINE

Small craft strength northwesterly winds will continue across the coastal waters into Saturday in the wake of a weak surface trough that continues to push inland this afternoon. Westerly winds along the Strait of Juan de Fuca have also increased to 20-30 kt and will remain gusty through late tonight, before easing Saturday morning. Small craft conditions will persist across the coastal waters through Saturday, however, as seas currently at 10-12 ft continue to build towards 13-16 ft by late tonight.

A brief period of weak Fraser Outflow will also be possible tonight into Saturday morning and may bring 15-20 kt northeasterly winds, with occasional gusts to 25-30 kt, to the northern portions of the Northern Inland Waters at times.

High pressure will build back into the area waters this weekend and remain situated across the region into early next week. Dense fog may develop over the interior waters with clearing conditions and light winds during the overnight periods and will likely restrict visibilities at times. Seas will gradually subside to 3-6 ft for a brief period on Sunday, though systems passing by to the north will allow for seas to build back to 6-9 ft by Monday.

The ridge will weaken on Tuesday, with the next front looking to drop down into the coastal waters around midweek.

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HYDROLOGY

No river flooding is expected over the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 4 AM PST Saturday 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 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-Grays Harbor Bar.

Small Craft Advisory until 1 AM PST Saturday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of Juan De Fuca.