textproduct: Seattle/Tacoma

SYNOPSIS

A low pressure system will traverse western Washington tonight into Sunday, bringing rain and chances for isolated thunderstorms this afternoon. Drier conditions return Monday with temperatures gradually warming back up throughout the course of the week.

SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/

Satellite shows a low pressure system centered off the Oregon coast this afternoon beginning to work its way eastward. Scattered showers are occuring along the coast. Chances for showers will begin to work their way inland this afternoon, as well as a 10-20% chance of some isolated thunderstorms, mainly along the coast, Olympics, and portions of southwestern Washington west of I-5. A few lightning strikes have been observed over coastal waters, and soundings do show some marginal elevated instability.

The main bulk of the rain will move across the area overnight into Sunday morning. Areas north of the Strait of Juan de Fuca will likely remain mostly dry. The areas expected to see the most rainfall will be across the south Cascades where localized amounts of up to 1.5-2.0" of rain will be possible. There is around a 40-60% chance of seeing 2" of rain over the Cascades of King County and southward. Even portions of the lowlands may see significant rain, with a 40-80% chance of seeing 1" of rain from Seattle southward and east of I-5.

Rain will slowly taper off from west to east, first along the coast in the morning then through the lowlands by the afternoon. Winds will also increase Sunday as the system departs, but remain mostly benign with peak gusts in the 20-25 mph range.

High pressure will begin to build in the wake of the system on Monday, with the supporting upper level trough well to the east and northwesterly flow aloft.

Temperatures through Monday will remain below normal, with highs in the mid to upper 60s and lows in upper 40s to mid 50s.

LONG TERM /TUESDAY THROUGH SATURDAY/

A shortwave trough will move through the flow Monday night into Tuesday, bringing a reinforcing push of clouds, through little to no precipitation is expected with this system. Following this system, the upper level ridge building over the northeast Pacific will move over the Pacific Northwest through the end of the week, bringing a drier, sunnier, and warmer pattern. A surface thermal trough will develop, allowing winds to turn lightly offshore. Highs look to rise by Thursday into the upper 70s to low 80s.

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AVIATION

An upper level low will shift inland tonight with increasing clouds, rain and MVFR to IFR conditions expected. The flow aloft is southerly, becoming northerly by 12z Sunday. Rain will taper off west to east Sunday afternoon but the low level air mass will remain moist with low level clouds lingering. 33

KSEA...Increasing clouds, MVFR conditions and rain by 06-09z. Rain decreasing after 21z but low clouds and MVFR conditions linger through 00z Monday. NW winds backing to SW and increasing to 10 kt by 12z Sunday. 33

MARINE

Surface low pressure will shift into OR tonight with highest winds in western Washington through the Strait of Juan de Fuca - a Small Craft Advisory is in effect. Onshore flow will increase on Sunday as the low moves farther inland. SCA winds are possible over Puget Sound/Hood Canal and the Northern Inland Waters (as well as the strait). Onshore flow will ease moving into Monday. The flow will turn offshore Tuesday and Wednesday as a thermal trough forms along the coast. 33

FIRE WEATHER

Fire weather concerns remain minimal through the next week. Showers and a chance for isolated thunderstorms this evening will be followed by wetting rains across most of the southern half of the region overnight into Sunday. The pattern turns drier and warmer mid to late week, with the surface winds going light onshore. Daytime minimum relative humidity look to drop below 30% which will allow more reactive fuels to start to dry out.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 6 PM this evening to 6 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.