textproduct: Seattle/Tacoma

UPDATE

Fog development underway in the Chehalis valley, the south Sound and western Whatcom county, with isolated patches elsewhere. Otherwise, generally clear skies with cold temperatures already setting up over western Washington, with some locations already reaching the freezing mark at the time of this writing. Inherited forecast is on track.

SYNOPSIS

A strong upper level ridge maintains dry conditions across western Washington through the rest of the week. A weak system will pass through on Thursday for increased cloud cover. High pressure rebuilds Friday and into the weekend, with little in the way of rainfall chances well into next week.

SHORT TERM /TODAY THROUGH THURSDAY NIGHT/

High pressure remains steadily in place through the short term period maintaining a stretch of dry conditions. Fog has scattered out across most of the region with just a few stubborn pockets of low clouds, mostly in the south Sound and around the San Juan Islands. Expect another round of fog and freezing fog (especially in the South Sound and outlying areas, where temperatures reach the upper 20s to around 30) for the start of Wednesday. Otherwise, expect minimal change with morning clouds and cool overnight temperatures. A weak disturbance brings additional mid level cloud cover into Thursday, but any chances for rain remain low (15% or less) for the coast and near-zero inland.

LONG TERM /FRIDAY THROUGH MONDAY/

Ensemble guidance continues to strongly favor the ridge rebounding over the region late this week into the weekend. While the majority of ensemble members maintain dry conditions across the region, there is a subset (around 15%) that would open the door for some light precipitation to arrive by late Sunday or Monday. So while this is a less likely outcome, this scenario would result in the ridge weakening enough to be undercut by an upper low that could bring some moisture into the region. Otherwise, the majority of ensemble guidance favors the return of mostly dry conditions and high pressure over the region into early next week.

AVIATION

Northwesterly flow aloft with the persistent upper level ridge along the West Coast. Satellite imagery and observations showing fog has lingered into the evening mainly along the I-5 corridor south of Puget Sound. Fog has redeveloped over KOLM, with another round of fog and LIFR visibilities through much of the interior again tonight. Northerly winds 5-10 kt will ease with many locations becoming northeasterly again tonight or just going calm.

KSEA...VFR conditions will linger through the evening into early Wednesday morning. High resolution ensembles showing less widespread fog tomorrow morning, with uncalibrated ensembles indicating a 20- 30% odds of LIFR conditions redeveloping near the terminal, with very similar conditions expected overnight tonight, have indicated LIFR visibilities again between 12-15Z. Northerly winds 6-8 kt will become northeasterly and east to 3-5 kt around 10z.

MARINE

High pressure remains in place across the region this evening. Seas are benign but a concern of note is the development of fog again later tonight into tomorrow morning to restrict visibilities at or below 1 NM, especially across Puget Sound and around the San Juan islands. Overall, little change to the overall pattern though the balance of the week with a few approaching systems potentially weakening the offshore flow but no major weather systems. Meanwhile, seas will generally hold around 5 ft this week.

HYDROLOGY

No river flooding is expected during the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

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