textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper level ridge over the area through Thursday with varying degrees of low level onshore flow. Upper level low well offshore slowly moving northeast and off the coast Friday. Weak system spinning out of the low could produce some showers over the area Friday. Low moving through Saturday keeping a chance of showers in the forecast for some locations. Weak upper level ridge with low level onshore flow returns later Sunday into the first part of next week.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

Weak upper level ridging will lead to dry and mild weather across western WA through Thursday. The low level flow remains onshore. Temperatures will track close to average with a slight uptick on Thursday. 33

LONG TERM /FRIDAY THROUGH TUESDAY/

An upper level low will spin offshore on Friday, then shift inland on Saturday for increasing showers and cooler weather. The air mass will be slightly unstable for a chance of thunderstorms in the mountains Saturday afternoon (eastern Olympics and northern Cascades). Highest chances for wetting rains will be in the northern Cascades near the Canadian border. With the trough overhead, expect cooler conditions on Saturday with highs in the 60s to lower 70s. We'll see lingering showers on Sunday as the trough exits NE. Otherwise, were back to dry and mild weather moving into next week as weak high pressure returns. 33

AVIATION

VFR this afternoon for interior terminals and MVFR along the coast. Conditions will deteriorate tonight into Wednesday morning as another round of marine stratus pushes inland. Guidance continues to struggle with how low the ceilings will get, but based of persistence, it is possible that that ceilings will trend toward IFR/LIFR again. High-res guidance shows there is a 50-70% chance for IFR/LIFR for the interior terminals. Additionally, reductions in visibility are possible with the stratus. Uncertainty remains as to how low visibility will be, but could see visibilities lower than 3SM. The stratus will retreat towards the coast/scatter out in the afternoon after 19Z. VFR for interior terminals in the afternoon/evening and minimal improvement along the coast to MVFR.

Generally SW winds this afternoon. Winds may transition WNW at BFI/SEA this evening between 00-06Z. Probabilities are slightly higher for BFI to see WNW, with probabilities trending towards 70- 80%. Otherwise, calm to light SW winds overnight.

KSEA...VFR will prevail today. While guidance continues to struggle with how low the ceilings will be Wednesday morning, some high-res guidance has a 50% chance for LIFR and 65% for IFR after 12Z. Based off persistence, will trend the TAF towards IFR/LIFR for tomorrow morning. Ceilings will slowly improve throughout the late morning and scatter out by the afternoon, with VFR for Wednesday afternoon. Generally SW winds 3-6 kt today. Winds will shift more towards the WSW this evening and may switch WNW (60% chance) between 01-06Z, but should remain light around 5 kt. Light WSW winds overnight.

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MARINE

Broad high pressure over the northeast Pacific will continue building into area waters through tomorrow. A weak system will approach the waters today, but is not expected to significant impacts. Increased NW surface winds through Wednesday and are expected to remain below SCA thresholds. High pressure will then weaken through late in the week as a stronger weather system approaches and moves over the waters on Friday. This may result in SCA winds through the Strait of Juan de Fuca Friday evening, with latest probabilities around 60-70% through the central portion. Over the weekend, high pressure will rebuild and strengthen over area waters.

Seas will remain below 10 feet throughout the week. However, seas may become steep at times for the outer coastal waters today through Thursday at 5-7 ft with a dominant period of 7 seconds.

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FIRE WEATHER

Continued onshore flow through Thursday will maintain mild temperatures paired with excellent overnight relative humidity recoveries, limiting fire weather concerns across western Washington.

A storm system approaching the region will cause elevated fire weather concerns on Friday, bringing increased potential for thunderstorms over the Olympics and Cascades. While there is still a bit of uncertainty with the timing and intensity of this weather system, any thunderstorms that develop may be capable of producing locally wetting rains alongside gusty and erratic outflow winds and abundant lightning. While convection may continue overnight, a frontal system will cross the region sometime between Friday evening and Saturday, bringing in a much cooler and more moist air mass. Widespread gusty winds are not expected behind the frontal passage, though models continue to show potential for convection and a potential Puget Sound Convergence Zone on Saturday.

Fire weather concerns decrease into Sunday, as a drier and calmer pattern sets up over western Washington. This will likely bring a return to onshore flow with morning low stratus and afternoon sun, near normal temperatures, and generally calm winds.

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SEW WATCHES/WARNINGS/ADVISORIES

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