textproduct: Seattle/Tacoma

SYNOPSIS

A significant change in the weather pattern will begin to take place today. A frontal system will arrive early Friday and will be followed by a cool and unsettled period across Western Washington that will persist into the middle of next week.

SHORT TERM /TODAY THROUGH SATURDAY/

After an extended period of time in which 12 of the past 13 days featured above normal temperatures and an absence of any measurable precipitation, we're in for a major shift in the weather pattern beginning today. Stratus is creeping inland from the coast and will reach at least some of the interior lowlands by daybreak. The accompanying marine air mass will knock 10 to 20 degrees off high temperatures across interior lowland locations over those of yesterday with mid and high level clouds steadily increasing throughout the day. Some light rain ahead of an approaching front will reach coastal areas by late afternoon or early evening. It'll likely be slow to spread inland to Puget Sound, but we should see a few showers around by early Friday morning as a dissipating front drags itself slowly inland.

After a sojourn at or near record high temperature territory in recent days, Seattle will probably only narrowly miss a record low maximum on Friday. Hope you kept your hoodie near the front of the closet. A post-frontal convergence zone is likely to form Friday afternoon and evening. Thunder chances look fairly slim during the period...mainly 10 to 20 percent. The convergence zone will diminish by early Saturday as an upper low closes off and gradually shifts southward along the coast. It will, however, remain unsettled on Saturday with shower chances in the forecast as moisture wraps around the low just offshore. Temperatures will be held below seasonal norms for a third consecutive day.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Looking ahead into the early portion of next week, cooler and unsettled conditions are likely to stay in the picture. Model ensembles maintain a general mean upper trough position over the western third of the lower 48 with upper ridging centered well offshore around 150 W. While not a particularly wet scenario, it keeps onshore flow in place with heights never really recovering much. That keeps temperatures a little below average, skies a little gloomier, and a lingering threat of showers in the forecast now and then. Longer term ensembles point toward a warming trend...after the 4th. Ah, a small taste of normality.

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AVIATION

Onshore flow has brought IFR stratus to coastal terminals early this morning with nighttime microphysics satellite showing marine stratus spreading inland. The arrival of high level clouds will make it difficult to monitor trends over the next couple of hours, but expecting stratus to fill into SEA/BFI around 13Z. Winds will remain out of the south-southwest 5-12 kt, except for near the Strait of Juan de Fuca where winds will be 10-15 kts with gusts up to 25-30 kts through 20z Thursday. A warm front approaches the coast Thursday, with showers increasing as early as 18Z for the coast. Showers become more widespread over western WA by 03Z Friday.

KSEA...VFR conditions initially with moderate confidence (50% chance) for ceilings to drop below 2000 feet between 13-17Z before lifting after 17Z. Isolated showers Thursday afternoon will become more widespread in the evening. Conditions are expected to remain VFR through 05Z Friday, with MVFR probabilities increasing from 20% at 05Z to 60% by 12Z. Winds expected to remain out of the southwest at 5-10kt through most of the period. West/southwest winds will pick up after 00Z Friday with gusts up to 20 kts.

MARINE

High pressure will weaken over the coastal and offshore waters today as a weak surface low and associated frontal system approach the north end of Vancouver Island. The front will move onshore across Western Washington on Friday morning then gradually dissipate as it moves inland. A broad area of weak low pressure will remain over the inner coastal and inland waters into Saturday while high pressure begins to rebuild well offshore. This will likely lead to gusty northwesterly winds over the outer coastal waters along with increasingly choppy seas building to 8 to 10 feet.

Varying degrees of onshore flow can be expected for the first half of next week as low pressure remains in place east of the Cascades with broad surface high pressure anchored well offshore.

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FIRE WEATHER

Cooler and wetter conditions will prevail beginning today and persist into early next week that will limit fire weather concerns. There is a low end chance of thunder (10-20%) on Friday, but is likely to be accompanied by wetting rain...especially in and around the convergence zone and in the higher elevations of the Cascades.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 11 AM PDT this morning for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.