textproduct: Seattle/Tacoma

SYNOPSIS

The thunderstorm threat will widely diminish today, outside of a lingering threat across the North Cascades this afternoon and evening. A trough will move across the region late Monday into Tuesday. A warmer and drier trend will commence mid to late week as a ridge starts to build into the region, however shower and thunderstorm chances increase again heading into next weekend as the next system approaches the region.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

Forecast is on track for the evening update. -HPR

Broad troughing resides over the northeastern Pacific this afternoon, while upper level ridging starts to nudge into the Pacific Northwest from the southeast. Latest satellite imagery shows some high cloud cover still streaming overhead across western Washington this afternoon, along with marine stratus scattering out across the interior lowlands. It also shows some cumulus starting to develop along the Cascade crest. While the overall threat for thunderstorms continues to diminish, a 15-25 percent chance of thunderstorms does remain possible for the North Cascades this afternoon and evening. Thunderstorm chances will wane heading into tonight. Elsewhere across western Washington, expect dry conditions to continue with afternoon highs in the upper 70s to low 80s across the interior and in the mid 60s to low 70s along the coast.

A rather typical summertime pattern consisting of onshore low- level flow will continue on Monday and Tuesday. Onshore flow will bring morning clouds into portions of the interior, which will scatter by the afternoon for some sun. A trough will pass through the area Monday night into Tuesday, though conditions are expected to remain dry across western Washington. Highs will be rather seasonable, topping out in the mid 70s to lower 80s across the interior lowlands and in the 60s to near 70 along the coast. Overnight lows will be in the 50s areawide.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Ensemble guidance suggest near normal temperatures and onshore flow at the low levels maintaining morning clouds with afternoon clearing for the midweek time period. Dry conditions likely continue through the end of the week. Around Friday, expect that the ridge over the Rockies/Great Basin amplifies with the next upper trough approaching the west coast. This will likely bring a shift in the pattern with the potential for precipitation and cooler temperatures, along with thunderstorm chances increasing into Saturday.

AVIATION

A northeast-oriented trough will drag southward, with neutral flow aloft transitioning to weak north flow. Surface winds generally from the north or northwest...with a handful of obs leaning more westerly. Speeds generally 5-10 kts. Light and variable winds less than 5 kts expected at most terminals overnight, although eastern Puget Sound terminals /SEA, BFI, PAE/ will favor more northerly winds with similar speeds. Most terminals return to north to northwesterly winds by late morning/early afternoon with speeds generally 4-8 kts.

Widespread VFR conditions over W WA this evening although some coastal sites still trapped under marine stratus, hanging on to MVFR conditions. Another marine push is likely overnight into Monday morning. Consistency of the stratus west of Puget Sound is likely to consist of IFR or LIFR ceilings, with more likelihood of lower visibilities along the coast and Strait of Juan de Fuca. There is some uncertainty with how far the lower ceilings will track into Puget Sound, but Paine Field has the best chance (20-30%) of seeing IFR/LIFR in the morning. VFR conditions expected to return around 18- 21Z Monday.

KSEA...VFR conditions this evening, with west winds around 5 kt becoming north to northeast under 5 kt tonight and overnight. Some uncertainty remains again with how low the stratus will get at the terminal with the models, but MVFR ceiling probability remains highest at 30%, with IFR not far behind at 20%.

HPR/18

MARINE

High pressure continues to build offshore early this week, strengthening over the next few days. Onshore flow through the Strait of Juan de Fuca each afternoon and evening will generally weaken through the next few days. Have maintained the existing advisory for the central and eastern Strait with an eye toward frequent gusts tonight, but chances for additional advisories in the Strait early this week are generally below 40%. As the high pressure builds, expect persistent northwesterly winds over the coastal waters to build steep seas, reaching around 6-7 ft with a dominant period around 7 seconds. The next stronger system may approach the waters late in the week.

FIRE WEATHER

Localized elevated fire weather conditions continue this evening, primarily along the North Cascades, where there is isolated convection moving out to the northeast. Onshore flow and excellent overnight humidity recoveries will help mitigate fire weather conditions across western Washington through the first half of the week before a warming and drying trend commences mid to late week.

While some uncertainty remains in the long term pattern, guidance continues to suggest that the next storm system will arrive by the weekend, increasing the chances for shower and thunderstorm activity across the area and potentially resulting in the return to elevated to critical fire weather conditions.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 3 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.