textproduct: Seattle/Tacoma

SYNOPSIS

Western Washington will remain positioned between an upper level low over the northeastern Pacific and upper level ridging to the southeast through the weekend. Isolated thunderstorms will be possible across the mountains this afternoon and evening, with the thunderstorm threat diminishing on Sunday. Another trough will move across the region early next week. A warmer and drier trend is then expected mid to late week as a ridge starts to build into the region, however shower chances may increase again by Friday as a disturbance approaches from the west.

SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/

Western Washington will remain positioned between a weak upper level low over the northeastern Pacific and upper level ridging building in from the southeast through the weekend. The low will allow for a weak shortwave to move across the area today, with the main highlight remaining the shower and isolated thunderstorm threat for the mountains this afternoon and evening.

Latest satellite imagery this afternoon shows stratus persisting along the Pacific Coast and Southwest Interior, while gradually scattering out across the interior lowlands. Meanwhile, mid- level clouds associated with shower activity continue to work their way up along the south-central Cascades. Isolated thunderstorms will remain possible for the mountains this evening as conditions remain unstable until dusk. Elevated fire weather conditions will continue for the Cascades as a result due to lightning and gusty outflow winds in any storms that do fire up. Highs today will be in the 70s to low 80s across the interior lowlands and in the mid 60s along the coast.

Ridging will build into the region from the southeast on Sunday, allowing for overall thunderstorm chances to diminish across the area with only a slight chance (15-20%) expected across the North Cascades during the afternoon and evening hours. Otherwise, expect another round of marine stratus to make its way inland during the morning and to scatter out by the afternoon. Highs on Sunday will be in the upper 70s to low 80s across the interior lowlands and in the mid to upper 60s along the coast. Monday will be similar as the the low offshore weakens and sinks southwestward. Another round of morning stratus across the area will scatter for some afternoon sun, with highs expected to be similar to Sunday - in the upper 70s to low 80s across the interior and in the mid 60s to low 70s along the coast.

LONG TERM /TUESDAY THROUGH SATURDAY/

Ensemble guidance has become somewhat more consolidated with respect to the pattern for the middle to late portions of next week, with a lean toward high pressure building across the Great Basin and extending into the Pacific Northwest. This would bring temperatures to near to slightly above seasonal normals into the late stages of next week. While the pattern of onshore low-level flow maintaining morning clouds and clearing in the interior by early afternoon each day will prevent relative humidity values from trending too low, there is still not a significant signal for rainfall for the midweek period. This may begin to shift toward Friday as ensemble guidance suggests the next disturbance may get close enough to bring another round of shower chances. That said, there's still around 40% of ensemble members that would favor more of a ridge pattern over the west coast that could continue to block the region from any significant weather systems.

AVIATION

Marine stratus remains in place over points west of the Olympic Mtns and as far east as SHN, resulting in persistent MVFR conditions. VFR conditions present elsewhere this evening. Widespread MVFR conditions expected to return very late tonight/early Sunday morning as another marine push gets underway. That said, terminals currently seeing MVFR conditions will see cigs lower overnight, getting down into IFR/LIFR conditions by Sunday morning. Improvement to VFR conditions expected throughout the area by the early afternoon, including along the coast.

Winds aloft generally southwesterly for the TAF period, although split flow may impact areas in the far northern reaches of the CWA. Westerly surface winds, with some variances to the NW or SW, this evening becoming light and variable overnight before returning to west northwesterly Sunday afternoon. Speeds this evening and Sunday afternoon generally 5-10 kts.

KSEA...VFR conditions this evening. MVFR cigs possible (30-40% chance) again Sunday morning, with a 20% chance of IFR cigs after 12z Sunday. Conditions should improve in the afternoon to VFR. SW winds this evening 5-7 kt winds will shift more W/NW by 06z around 4-5 kt. Light SW winds overnight into Sunday morning. Guidance suggests WNW return Sunday afternoon.

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MARINE

Broad high pressure over the Pacific will maintain light winds over the coastal waters. High pressure will then expand into area waters on Sunday and strengthen into next week. This will lead to increased NW winds over the coastal waters into next week, but are expected to remain below SCA thresholds. Seas will be around 3-6 ft through Monday. Seas will build slightly on Tuesday and become steep around 6-8 ft with a dominant period of 7 seconds over the outer coastal waters. Steep seas look to continue through Thursday.

Diurnally driven westerly winds will continue to occur each afternoon and evening through the Strait of Juan de Fuca this weekend with gusts likely (70%+ chance reaching SCA thresholds. The westerly pushes look to start to weaken starting on Monday into late in the week with a 30% or lower chance of winds reaching SCA thresholds.

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FIRE WEATHER

Elevated fire weather conditions will continue across the Cascades today with slight chances (15-20%) of thunderstorms possible along the Cascade crest this evening. Overall chances for convection will diminish into Sunday as upper level ridging builds into the area from the southeast. By Sunday, expect convection to largely be confined along the far Northern Cascade crest and areas generally east of the crest.

Continued onshore flow and excellent overnight humidity recoveries will then help mitigate fire weather conditions through early next week. While some uncertainty remains in the long term pattern, ensemble guidance has increasingly come to consensus that a warming and drying trend is likely mid to late week, before the potential for shower and thunderstorm chances increases again late week as the next disturbance approaches from the west. This could potentially reintroduce some elevated fire weather concerns to the area.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 5 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.