textproduct: Seattle/Tacoma

UPDATE

The going forecast largely on track with updates limited to capturing latest trends.

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

Latest surface obs show mix of IFR-MVFR conditions across much of the sound and coast with improvement apparent in the south with VFR at KOLM terminal/vicinity. Latest satellite shows high clouds streaming eastward obscuring low stratus covering areas, however, an improvement trend will take hold with high-end VFR/high clouds expected overhead. Elsewhere, MVFR will persist across the coast. Timing of precip is 4-6Z for much of the interior terminals with coastal terminals seeing chances ramp up 22-01Z. The upcoming front is anticipated to reach the interior late today/00-06Z Friday with showers spreading and becoming widespread west to east.

KSEA...MVFR conditions currently in place will be lifting through early this afternoon. Latest ACARS soundings at KSEA showed dry air in the low-mid troposphere with this being realized with daytime mixing as low ceilings lift/improve over 17-19Z across the interior. Current thinking is best chances for showers after 03Z Friday with HREF hourly chances ramping up to 40% with showers becoming then more widespread. With upcoming frontal system, ceilings will deteriorate with ceilings becoming MVFR with increasing chances for low-end MFR (less than 2kft) at 30-50% based on latest HREF guidance. Otherwise, southerly winds today with confidence low (25-40%) for winds greater than 17 kts, however, cannot rule out an isolated gust. West/southwest winds will pick up after 00Z Friday with gusts up to 20 kts more likely (30-50%).

ET

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.