textproduct: Seattle/Tacoma

SYNOPSIS

An upper ridge offshore will bring dry, warmer conditions through Wednesday. Troughing develops over the Pacific Northwest Thursday with an upper low bringing cooler conditions and precipitation potential into the weekend.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

Mostly clear skies prevail across Western Washington this evening. Forecast trends look on track for a smidge of warming tomorrow as an upper ridge nudges into the region before a cooler, unsettled stretch finishes off the week as a trough settles over the Pacific Northwest. Previous discussion follows with updates to marine and aviation portions. 27

Mostly sunny conditions today with a few clouds mainly over the higher elevations. An upper ridge offshore builds into Western Washington on Wednesday, resulting in warmer temperatures and continued dry conditions. Highs on Wednesday are expected to range in the 70s to low 80s. The ridge begins to flatten Wednesday night as troughing develops on Thursday. Widespread clouds Thursday AM will result in cooler conditions. Increasing onshore flow may also result in the development of weak convergence by late Thursday over central Puget Sound and into the Cascades.

LONG TERM /FRIDAY THROUGH TUESDAY/

Troughing further develops over the Pacific Northwest on Friday with a continued cooling trend late in the week. Ensembles are in general agreement that an upper low moves southwards offshore Friday before moving inland over WA/OR during the weekend. Rain potential increases as the upper low moves inland, particularly Saturday into Saturday night. There is also a 10 to 15 percent probability for thunderstorms Saturday afternoon. Some uncertainty in the ensembles as to how quickly the troughing moves eastwards Sunday into Monday. However, lingering precipitation is likely at least over the Cascades on Sunday before a slow warming trend begins early next week with ridging offshore. JD

AVIATION

VFR conditions will continue across western Washington through tonight with mostly clear skies. Models are hinting at some marine stratus development early Wednesday morning, but the most likely area remain the coast, with less than a 20-25% chance elsewhere. Some patchy valley fog may develop for the more sheltered terminals as well (OLM, PAE). Low clouds and fog will dissipate quickly again tomorrow morning, through high clouds will begin to build in from the north. Winds remain northerly 10 kt or less through the period.

KSEA...VFR conditions prevail through the TAF period. Northerly winds between 3-6 kts overnight, increasing slightly tomorrow afternoon at around 4 to 8 kt.

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MARINE

High pressure will continue to build over the coastal waters through early Thursday. Northwesterly winds will prevail over the coastal waters, but will increase moreso Wednesday night as a weak frontal system from the north tightens the pressure gradients. Northwest winds over the far outer coastal waters could reach up to around 20 kt (30-50% chc), with better chances of seeing Small Craft Advisory winds being through portions of the Strait of Juan de Fuca (60-90% chc). Winds look to slowly ease Thursday night. Another weather system may move over the waters late Friday into Saturday, but no headline winds are anticipated at this time.

Seas around 3 to 5 feet will gradually build through Wednesday, reaching 7 to 10 feet by Thursday afternoon. This will also bring steep seas as short period waves from the stronger winds over the coastal waters begin to dominate. Seas will ease to around 5 to 6 feet by Friday, then further easing into the weekend.

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FIRE WEATHER

Dry, warm weather continues through Wednesday. Given the recent cool conditions, this will generally return fire weather conditions back to seasonal levels of concern, with localized elevated conditions Wednesday. Light offshore flow Tuesday night into Wednesday will yield moderate RH recoveries along the ridges of the Cascades and Olympics. Daytime RH values will approach the low to mid 20 percent range Wednesday over the Cascades and nearby valleys. However, this period of warmer, dry conditions will remain short-lived as onshore flow returns later Thursday for increasing RH values. Cooler conditions, along with precipitation chances, then continues over the weekend. JD

SEW WATCHES/WARNINGS/ADVISORIES

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