textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper level trough moving through late tonight into Monday morning with a Puget Sound convergence zone also present, dissipating as the trough exits by Monday afternoon. Strong front approaching the area Tuesday bringing the possibility of heavy snow in the Cascades Tuesday night. Cool upper level low behind the front Wednesday and Wednesday night before drying northerly flow aloft for the end of the week. Splitting system arriving late in the weekend.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

W WA socked in with clouds as per latest satellite imagery this evening. Looking at radar, a Puget Sound convergence zone has developed over much of King county and southern portions of Snohomish. While not quite showing up on radar just yet...starting to see some weak development in a line extending from Vancouver island to the Skagit/Snohomish county line. Any precip from this activity should remain light.

Inherited forecast generally on track with no need for an evening update. For forecast details, please refer to the Previous Discussion section below. 18

From Previous Discussion...Satellite imagery shows mostly cloudy skies over Western Washington. Doppler radar has a few showers in the South Cascades. Temperatures at 3 pm/22z in the lower to mid 50s.

Clouds starting to bubble up late this afternoon but with plenty of cloud cover will be hard to generate enough surface heating to create much in the way of shower activity this evening. Surface gradients turned onshore this afternoon with onshore flow increasing tonight. A Puget Sound convergence zone will develop mostly likely over King county tonight with the convergence zone drifting north early Monday morning. Weak upper level trough also arriving overnight increasing the shower chances outside of the convergence zone. Lows in the 40s.

Weak upper level trough moving through Monday morning. Convergence zone over Snohomish county weakening and moving into the foothills. Low level onshore keeping skies at least mostly cloudy. Highs in the lower to mid 50s.

Upper level low and frontal system moving out of the Gulf of Alaska Monday night into Tuesday morning. Warm front brushing the north coast late Monday night. Cloud cover will keep lows in the lower to mid 40s.

Strong cold front approaching Tuesday with the front still offshore late Tuesday afternoon. Rain starting on the coast in the morning spreading inland in the afternoon. Winds picking up along the coast in the afternoon. Cool with highs near 50.

Have issued a winter storm watch beginning late Tuesday afternoon for the Cascades. Snow level near 3000 feet with heavy snow possible Tuesday night ahead and with the front. Breezy/windy conditions developing with the front for the lowlands. Strongest winds along the coast and over the Northwest Interior. Felton

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Cool upper level low behind the front over Western Washington Wednesday moving east Wednesday night. Snow levels could get as low as 1500 feet. Winter storm watch continuing for the Cascades into Wednesday night for additional significant snow accumulations.

Drying northwesterly flow aloft behind the upper level low Thursday and Friday. Very cool mornings both Thursday and Friday with the colder locations getting below freezing. Frost advisories will likely be needed especially Thursday morning.

Weak upper level ridge moving through Saturday ahead of a splitting front Sunday.

AVIATION

Weak flow aloft this evening and tonight as an upper level trough moves through W WA, before turning more north to northwesterly Monday morning. Surface winds at terminals largely southwesterly, however westerly to northwesterly winds flowing down the Strait has given rise to some PSCZ action over King county earlier this evening and latest radar trends suggest that another round of activity may be setting up over portions of Skagit and Snohomish counties.

Generally MVFR cigs in place for much of W WA, although VFR conditions still in place over northern portions of the San Juans and Whatcom and northern Skagit counties. Should see cigs diminish over these areas as well overnight, however there is some question as to whether or not they will join the remainder of the area in MVFR cigs. Latest model data suggests that cigs should become uniformly MVFR by Monday morning and as such, forecast for BLI may need to have cigs lowered for the 06Z issuance. As troughiness a low level moisture linger over the area throughout Monday, some terminals may see cigs lift somewhat, but probably not enough to lift them out of MVFR, before falling again Monday evening.

KSEA...MVFR conditions in place with showers as the terminal begins to find itself on the backside of an evening PSCZ. There may be a bit of a break for the remainder of the evening and into early tonight, however with activity possibly developing to the north, will need to keep an eye on how well it holds together if and when it should move southward. Winds SW this 4 to 8 knots this evening into tonight before increasing to 8-12 kt with gusts up to 20 kt by Monday morning. MVFR/IFR cigs are to remain into Monday as well.

18/McMillian

MARINE

Surface low pressure just off the N CA coast as a high pressure system develops offshore in the northeastern Pacific. Onshore flow is increasing as a small craft advisory remains in effect for westerlies down the Strait of Juan de Fuca this evening and likely well into Monday. A stronger frontal system arriving late Tuesday into Wednesday will likely yield more widespread headlines as we will see increased winds and seas and possible gales through the strait. Surface high pressure looks to rebuild across the region towards the second half of the week.

Coastal seas 4 to 6 feet throughout the weekend. Seas will then start to build upwards to 8 to 10 feet by Tuesday evening and remaining elevated through Thursday.

McMillian

HYDROLOGY

No river flooding the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Storm Watch from Tuesday afternoon through late Wednesday night 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 PDT Monday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.