textproduct: Seattle/Tacoma

SYNOPSIS

Mostly zonal flow will persist over western Washington as high pressure amplifies over the Great Basin region to the south. Seasonably warm conditions are expected throughout the week, with morning clouds and patchy fog, giving way to sunny afternoons. Rain chances increase into Friday, but the incoming system doesn't appear to be particularly strong.

SHORT TERM /TODAY THROUGH THURSDAY/

Stratus and fog are expected this morning again as temperatures and dewpoints near one another and lightly onshore flow persists. Early morning observations across the region indicate visibilities as low as a quarter of a mile along the coast and through the Strait of Juan de Fuca. This will continue inland throughout the morning, with in-situ development in the Puget Sound region. Stratus and fog that develops will likely burn off by the early afternoon. Gradients become more northerly by this afternoon into Wednesday, dampening the widespread stratus and fog potential through the middle of the week, though the threat of patchy fog development will continue.

Given the zonal flow to weakly amplified high pressure in Washington, temperatures will remain on the mild to warm side. Through midweek, temperatures will be in the upper 60s, with a bump in temperatures come Thursday where the low 70s are expected due to weakly offshore flow.

LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/

By late in the week, the influence of the western CONUS ridge weakens slightly and may allow for broad troughing over the NE Pacific. The ensembles generally keeping the ridge in place through Thursday, with some deviation toward a trough by Friday and the weekend. There is a bit more consensus on broader troughing over the weekend, but low placement is not generally agreed upon. For sensible weather in western Washington, this brings a weak system into the area on Friday, with light rain amounts expected into Saturday. At this time, Sunday looks drier.

Accompanying the troughing will be a dip in the low temperatures over the weekend. Lows will be in the low to mid 40s throughout Puget Sound, dipping into the upper 30s in the Cascades and adjacent foothills.

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AVIATION

A mostly flat upper level ridge will remain overhead through the TAF period, promoting westerly flow aloft. Low level flow remains lightly onshore, but will transition to more northerly throughout the day. Widespread low cloud and/or fog formation is expected for much of the lowlands this morning, with LIFR ceilings and reduced surface visibility below 3SM expected to become more widespread by 12Z. Low clouds will gradually lift and scatter for most areas between 19-21Z for a return to VFR conditions. Another round of low clouds/fog will be possible on Wednesday morning.

KSEA...VFR conditions will deteriorate after 09Z, with LIFR ceilings and visibilities below 2SM expected to develop at the terminal by 12Z. Low ceilings and visibilities will hang around through most of the morning, with improvement towards VFR expected between 19-21Z. Winds this morning are light out of the north/northeast at 5 kt or less, but will increase to 5-10 kt this afternoon after 18Z.

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MARINE

High pressure will remain situated over the coastal waters with lower pressure inland over the next several days. Areas of fog are likely to develop over the waters again this morning and will continue to be likely over the next few days. Latest HREF guidance indicates the the greatest probabilities (between 45-65%) of visibilities dropping below 1 NM this morning are along the central and eastern Strait of Juan de Fuca, Admiralty Inlet, and the Puget Sound waters. Expect visibilities to improve again into the afternoon as fog slowly dissipates.

A thermal trough expanding northward along the Oregon coast today and along the Washington coast Wednesday will turn flow more northerly across the waters through Thursday. Periods of gusty northerly winds may develop across the southern coastal water zones at times this evening into Wednesday, but expect the winds to primarily remain between 15-20 kt at this time. Seas will largely range between 4-8 ft through Thursday.

Onshore flow will then resume Thursday night into Friday. A frontal system approaching the area waters from the north Friday into Saturday will likely bring small craft strength winds to the coastal waters and the Strait of Juan de Fuca. Seas will build in the system's wake Friday night into Saturday, with GEFS guidance showing a 50-60% probability of seas exceeding 9 ft.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...None.