textproduct: Seattle/Tacoma

SYNOPSIS

An upper level ridge will remain in place over Western Washington through midweek. This will maintain dry conditions and patchy fog. A more active weather pattern returns on Thursday, with a return of mountain snow and lowland rain late this week through the start of next week. An increased threat of coastal flooding due to larger high tides develops late this week.

SHORT TERM /TODAY THROUGH THURSDAY/

Latest satellite imagery shows some scattered high clouds moving over the area, but otherwise generally clear skies. Although many locations are seeing a 1-3 degree dewpt depression, there are some that are going to 0-1 degrees and as such, fog has developed...mainly over the south Sound and Chehalis River valley, but also in isolated spots near Arlington and Port Townsend. Of particular note is a pretty thick band that has formed along the south shores of the Strait stretching from near Neah Bay almost reaching Port Angeles. Timing on any fog lifting should be similar to previous days...most spots should clear out by late morning while others, especially around Olympia, will likely hang on until the early afternoon.

Forecast models remain consistent with the upper level ridge holding court over the Pac NW through at least mid-week, keeping conditions dry and overnight lows still on the chilly side...residing mainly in the lower to mid 30s. Some outliers in the SW interior could dip down into the upper 20s while some spots along the water may stay in the upper 30s to around 40. Daytime highs will remain in the mid to upper 40s in the lowlands. As the upper high has been over the area for some time now...making for a relatively stable atmosphere, the question of air stagnation concerns does arise. In collaboration with local air quality agencies, while isolated locations may see some degradation of air quality, overall there are no widespread concerns for west of the Cascades at this time.

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LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

Overall, model guidance is in good agreement with the next incoming system moving up into the area Thursday night. While a fair number /but not quite a majority/ of ensemble members want to sneak in some precip as early as Thursday afternoon, deterministic models and majority of ensemble members show chances rising Thursday evening with those chances rising to high-end Chance to Likely during the overnight hours and staying that way into Friday. A more organized system moves in quickly Friday afternoon as an upper level trough sinks southward over the eastern Pacific and churning moisture up into the area. As has been stated in prior discussions, this follow- up system continues to show some atmospheric river-like qualities, yet models remain on track keeping the bulk of this moisture well to the south of WA...targeting northern CA instead. This trough will slowly rotate northeastward, keeping precip going into Saturday. Just as this feature exits Saturday evening, yet another trough dips in behind it, spinning additional moisture up into the area to keep rain in the forecast for the remainder of the weekend. Deterministic and ensemble model data starts to hint at the potential for a small break in the action Sunday evening...maybe stretching into Monday morning...as a weak shortwave ridge tries to exert some influence over the area...but by Monday evening, the trough responsible for Sunday's precip begins to work toward the coast brining yet more rain.

Daytime highs in the long term remain fairly steady, mainly in the mid to upper 40s, although some isolated spots could reach 50 at times. It is the overnight lows that get a much needed boost, in the upper 30s to lower 40s.

Finally, the threat of minor coastal flooding will continue to increase late in the week. While there is uncertainty regarding the timing and strength of the upcoming weather systems, they could exacerbate coastal flooding concerns.

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AVIATION

Mostly VFR early this morning with passing high clouds and localized IFR/LIFR in fog and low stratus across the Chehalis Basin south of the Puget Sound. Fog/freezing fog and stratus are expected to expand through the morning for areas west and south of the Sound, with terminals east of the Sound expected to remain VFR as northeast winds persist. A slight improvement in conditions is possible by the late afternoon hours for terminals seeing fog early this morning, however expect improvement to be short-lived with fog and low stratus on track to redevelop once again tonight. Surface winds will generally remain out of the north to northeast around 4 to 8 kt this morning, gradually transitioning back to southerly by late this afternoon.

KSEA...VFR conditions expected to prevail through the period with NE winds around 4-8 kt early this morning. While a few low clouds will be possible within the vicinity of the terminal this morning, northeasterly winds will likely help limit fog potential and keep VFR conditions prevalent at the terminal. Winds will gradually transition to more southerly throughout the afternoon today.

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MARINE

High pressure will remain situated over the coastal waters through Wednesday, while a weak thermal trough remains situated along the coast today. This will continue to promote offshore flow through the first half of the week. Seas will continue to hover between 6-9 feet today with a west-northwest swell moving into the coastal waters. Seas will then gradually subside towards 5-7 feet by midweek.

Dense fog may develop over portions of the interior waters with clearing conditions and light winds during the overnight periods the next few days and may restrict visibilities across the waters at times.

A more active pattern returns the second half of the week, with the next frontal system expected to move into the area waters on Thursday, followed by a stronger system on Saturday. Increasing winds and seas building to 10-13 feet by the weekend will likely bring the next round of area-wide headlines.

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HYDROLOGY

No river flooding is expected during the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

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