textproduct: Seattle/Tacoma

SYNOPSIS

A weak frontal system will move inland across Western Washington this morning. An upper level trough will then settle over the region tonight into Saturday maintaining cool and unsettled conditions. The trough will shift eastward later Sunday allowing drier conditions to develop into Monday. Another system is expected to drop southward into the area around the middle of next week for continued cool temperatures and another chance for showers.

SHORT TERM /TODAY THROUGH SUNDAY/

A weak front will move onshore across the area this morning with areas of light rain. Onshore flow will increase behind it for breezy conditions around Puget Sound and through the strait. This will allow a decent convergence zone to develop late this afternoon into tonight. Some modest instability this afternoon may lead to an isolated thunderstorm or two with CAPE values generally 250 J/kg or less. The best chances for any thunder (around 25 percent or so) will be in the convergence zone late this afternoon and evening. The convergence zone should diminish by Saturday morning as an upper trough settles over the region. The trough will gradually shift southward across the area through the day on Saturday keeping the threat of a few showers in the forecast and holding temperatures a handful of degrees below normal. By Saturday afternoon and evening, shower chances will gradually become confined to the Southwest Interior and higher terrain in the Cascades. The upper low is expected to shift eastward into the northern Great Basin and Rockies on Sunday allowing most of Western Washington to dry out. Heights only recover modestly and low level onshore will remain in place for continued cool conditions and plenty of cloud cover.

LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/

Model ensembles are in relative good agreement with keeping a mean trough position along the west coast through much of next week. They differ somewhat in minutiae as one might expect, but the overall pattern favors continued cool, unsettled conditions with perhaps another upper low dropping southward toward the region mid to late week for additional light precip.

27

AVIATION

Latest radar shows scattered showers moving across western Washington. Intermittent rain showers will continue through the morning becoming more isolated in the afternoon. Widespread LIFR to MVFR conditions are currently being observed across the region. Interior terminals are expected to generally remain MVFR this morning with a 20-40% chance for IFR ceilings through 17Z. Coastal terminals, including KPWT, have 50-80% chances of IFR conditions (and 10-15% of LIFR) until 18Z Friday. A Puget Sound Convergence Zone (PSCZ) is anticipated to develop in Snohomish County after 00Z, bringing higher confidence for showers at KPAE Friday evening. There is also a 15-20% for isolated thunderstorms for KSEA north and east to the Cascades between 18Z Friday and 06Z Saturday. Best chances will be with the PSCZ, but confidence is not high enough to include in any TAFs. Southerly winds will largely prevail with gusts up to 20 knots this afternoon, except for KCLM where winds will be light and variable.

KSEA...Predominantly MVFR conditions with ceilings expected to drop below 2000 feet after 12Z with a 20-30% chance for IFR conditions through 16Z. Light rain showers will continue on and off through the morning with chances decreasing in the afternoon. Ceilings will gradually improve to VFR in the late morning into the early afternoon. Southerly winds 6-10 knots this morning increasing to 8- 13 knots with gusts up to 20 knots this afternoon. There is moderate confidence (50% chance) for MVFR conditions to return after 06Z Saturday.

MARINE

A weak front will move inland early today. Onshore will increase behind the front this afternoon into tonight with small craft advisory winds expected in the central/east strait as well as Puget Sound. Broad high pressure will remain centered well offshore into midweek with lower pressure over the interior for varying degrees of onshore flow. Increasing northwest flow will lead to gusty winds and choppy seas out the outer coastal waters this weekend into early next week.

27

FIRE WEATHER

Main fire weather concerns revolve around a marginal lightning threat today with thunderstorm probabilities around 15-20%...greatest around the convergence zone. Otherwise, cool and unsettled conditions prevail over the next 7-days alongside a very low lightning risk.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 3 PM this afternoon 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.

Small Craft Advisory from 11 AM this morning to 8 PM PDT this evening for Puget Sound and Hood Canal.