textproduct: Seattle/Tacoma

SYNOPSIS

An upper low offshore will move inland tonight and weaken through Sunday. Upper ridging builds into the Pacific Northwest Monday and Tuesday for warmer, dry conditions. An upper low will approach the region midweek for increasing onshore flow and light precipitation chances. Onshore flow into late week will continue cooler conditions into next weekend.

SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/

Thunderstorm activity has largely quieted over western Washington this evening, mostly over the Strait and moving northward. This northward progression has also allowed the inherited Red Flag Warning to expire earlier this evening. Inherited forecast remains on track and no updates needed this evening. For additional forecast details, please refer to the Previous Discussion section below.

18

From Previous Discussion...An upper low continues to meander towards the Western Washington coast this afternoon. A complicated pattern exists with onshore flow at the surface and continued southerly flow aloft. Scattered thunderstorms occurred this morning, especially over the Cascades. Increasing daytime heating and further destabilization has resulted in additional thunderstorms this afternoon over the North Cascades. In addition, we continue to monitor the threat for localized severe wind gusts (>58 MPH) this afternoon and evening with any stronger thunderstorm over the North Cascades. Elsewhere, POPs remain lower, but a few isolated showers and thunderstorm may exist King County northwards. The upper low will move inland and transition to an upper trough tonight into Sunday. Lingering light rain will continue at times over the Cascades into Sunday with widespread cloud cover into Sunday as well. Temperatures will remain near to slightly cooler than normal through the weekend due to the presence of the upper troughing.

An upper ridge slowly builds over the Pacific Northwest beginning Sunday night resulting in drier, warmer conditions on Monday. Surface flow also becomes more northerly, with a slight east component Monday night. This may allow for smoke transport from ongoing wildfires in BC and Eastern Washington towards Western Washington early next week, which continues to be monitored. Otherwise, highs will warm into the mid 70s to mid 80s on Monday.

LONG TERM /TUESDAY THROUGH FRIDAY/

The upper ridge remains over the Pacific Northwest on Tuesday, with the ridge axis focused over the Intermountain West. For this reason, Tuesday will be the warmest day of the week, with highs across the interior in the 80s and low 90s in the Cascade Valleys, resulting in areas of Moderate HeatRisk. An upper low will approach the Pacific Northwest offshore by Wednesday leading to increasing onshore flow and cooler conditions. Precipitation chances also increase Wednesday into Thursday associated with this system, and will need to monitor thunder potential over the Cascades (although current probabilities are less than 10%). Ensembles suggest the influence of troughing continues into Friday with onshore flow and temperatures a few degrees below normal. JD

AVIATION

An upper level trough will continue to move north with south flow aloft transitioning to west/northwest as it moves east. VFR decks remain this evening across most terminals, with lower stratus along the coast/just offshore. Additionally, thunderstorms continue to remain in the Strait of Juan de Fuca area as they slowly move north into Victoria this evening. Brief gusty winds and lightning are the main threats with this area of convection. The threat of thunder will diminish after sunset, although showers will continue through tonight across the north interior (affecting BLI). Stratus from the coast will fill into the interior overnight, reaching Puget Sound terminals by around 12Z. Model guidance holds onto the stratus past 18Z (likely 19-21Z). Probabilities favor MVFR for most, with the coast holding onto a 40-60% chance of IFR/LIFR conditions. Wind gusts will diminish tonight for most except the north interior out of the southwest around 5 kt. Winds Sunday will shift from the southwest in the morning, to the northwest in the afternoon/evening 4-8 kt.

KSEA...VFR through the evening with decks remaining overhead. MVFR stratus remains likely to arrive around 12Z, and remain through 19- 21Z. Southwest winds 4-8 kt tonight becoming northwest Sunday afternoon.

HPR

MARINE

A few areas of showers and thunderstorms will continue in the Strait of Juan de Fuca area through dusk, with showers continuing in the north interior tonight, and breezy winds diminishing by Sunday morning. The low will track inland on Sunday. High pressure will rebuild over area waters on Sunday and remain over the area through Tuesday. A weak frontal system will move across the waters midweek. Then high pressure will rebuild over the waters late in the week. Diurnal westerlies in the Strait of Juan de Fuca evening the next few days.

Seas 3 to 5 feet through the majority of next week.

29/HPR

FIRE WEATHER

Showers and thunderstorms wrapping up this evening. As the upper level low passes through tonight and early Sunday, showers and isolated showers may still be possible, but the probabilities for thunderstorms will be significantly reduced and as such may not warrant any additional headlines. Will leave that for overnight shift to evaluate with latest model data. Upper level ridge building in later Sunday. Brief low-level offshore flow possible on Monday as thermal trough builds along the coast before pushing inland overnight into Tuesday. Another upper level trough arriving in the middle of next week increasing the chances for more showers and the potential for thunderstorms. Upper-troughing may linger into Friday.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None.

PZ...None.