textproduct: Seattle/Tacoma

UPDATE

Widespread showers are pushing across W WA this evening. Hi-Res guidance has this activity maintaining overnight into Friday as a frontal system works its way throughout the region. Notably, this is the first measurable precipitation the area has observed in a couple of weeks. For tonight, minimum temperatures are forecast to range throughout the 50s. The previous discussion remains below along with an updated aviation section:

SYNOPSIS

A weakening front will move into the coast of WA this afternoon/early tonight before progressing inland early Friday. The general flow will shift from south to west-northwest by tomorrow afternoon as a surface low evolves east into British Columbia. The upper low prompting the current pattern change is forecast to dive south to Vancouver Island tomorrow moving then more over the Pacific NW Saturday keeping conditions cool and unsettled. This upper low and associated trough will shift slowly east into the Northern Intermountain West on Sunday as upper-level high pressure builds well offshore. Meantime, surface high pressure will build this weekend reinforcing northwest flow.

SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/

A warm front is currently moving onshore with the latest water vapor imagery showing a shortwave trough diving east offshore the OR/WA border. This feature will continue to track to southwest OR tonight and is promoting showers. The latest radar display shows showers moving into the Olympics that will become more widespread tonight across the interior. Meantime, the upper low's main shortwave trough will dive to Vancouver Island tomorrow reinforcing the unsettled weather with additional large scale forcing for ascent.

Modest lightning chances exist tomorrow afternoon focused across the northern Olympics and central/northern slopes of the WA Cascades. Confidence currently low owing to forecast sounding showing shallow instability with limited tap into -10C and colder supporting lightning/ice development. This matches current thinking with chances 15-20% at this time. Precip chances will be lowest across the non-mountain areas of the southern interior tomorrow as chances start tapering off in the afternoon. Otherwise, reinforcing cooler air mass on Friday will see highs drop another 2-6 degrees. The cooler air mass will continue to prevail Saturday under cloudy skies and precip chances before chances start to wane Saturday night.

LONG TERM /SUNDAY THROUGH THURSDAY/

Unsettled weather will persist Sunday into the new week as upper-level troughing dominates the weather pattern overhead with an upper-level ridge in the eastern North Pacific. Synoptic northwest flow with embedded lows/upper disturbances moving through early-mid week will keep conditions largely reading few-several degrees below normal with lows near normal early-mid week. Overall limited sensible weather concerns beyond repeated chances for rain in the mountains, lower chances in the non-mountain areas.

AVIATION

Latest mosaic radar shows bands of rain showers moving across the southern edge of Puget Sound with another band forming along the coast. Intermittent rain shower activity will continue through the night. MVFR conditions will develop for the interior terminals with 10-20% chances of IFR ceilings between 12z and 15Z. Coastal terminals have 50-80% chances of IFR conditions (and 10-15% of LIFR) until 18Z Friday. Models show the bulk of rain shower activity waning down by 12Z Friday. There are 15-20% for thunder for from KPAE north and east to the Cascades between 18Z Friday and 06Z Saturday. Southerly winds will largely prevail, except for KCLM where winds will be light and variable.

KSEA...VFR conditions in place then transitioning to the low end of MVFR with ceilings less than 2k ft after 05Z. Light rain showers will continue on and off through the night period. MVFR conditions expect to return between 18Z and 20Z. Southerly winds will continue with the elevated winds during the afternoon period.

MARINE

Complex MSLP pattern over the next 24-hrs with increasing surface pressure differences late Friday as surface high pressure builds well offshore with surface low pressure inland. This will promote gusty northwest winds by late Friday continuing through Friday night, highest winds across the Juan de Fuca. Meantime, outer coastal water wave heights will trend upward to 7-9 ft. Otherwise, low chances for embedded isolated thunderstorms tomorrow over the coastal waters (10-15%) as a front moves through Friday. Onshore flow regime continues this weekend with increasing wave heights Saturday-Sunday building to 10-12 feet by Saturday night.

FIRE WEATHER

Fire weather concerns revolve around a marginal lightning threat tomorrow. Current thinking is there remains a low risk (15-20%) for embedded isolated thunderstorms capable of infrequent CG lightning. Main areas of fire weather concern will be focused on lee-side of Olympics and areas that dont get any initially wetting rain after the hot, dry spell. Otherwise, cool and unsettled conditions prevail Saturday with elevated afternoon humidity expected over the next 7-days alongside a very low lightning risk.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 3 PM Friday to 2 AM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.