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 /TODAY THROUGH MONDAY NIGHT/

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

The marine stratus continues to erode this afternoon across the interior. VFR conditions expected for interior terminals this afternoon, while MVFR will continue along the coast. Another push of marine stratus will bring lowered conditions to the interior terminals Sunday morning. Latest guidance suggests majority of terminals will trend towards MVFR. The coast and Strait of Juan de Fuca have the highest chances (40-60%) for IFR ceilings tonight into Sunday morning. Improvement expected in the afternoon for interior terminals and minor/brief improvement along the coast. Shower and thunderstorm activity has quieted down this afternoon across the majority of the area. However, thunderstorm activity still remains possible north of PAE towards BLI and across the North Cascades. Thunderstorms will have the potential for frequent lightning, gusty/erratic winds, heavy downpours, and reduced visibility. Thunderstorm activity will dissipate tonight. Breezy winds along the Strait of Juan de Fuca, Kitsap Peninsula, and Northern Interior.

KSEA...VFR will prevail today. MVFR ceilings are likely Sunday morning with another push of marine stratus. Improvement to VFR after 21z. Southwesterly winds today through Sunday morning at 5-8 kt. Winds shift WNW Sunday afternoon at 5-6 kt.

MARINE

A weak surface low will continue to move across the coastal waters today and 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. Today's westerlies may approach 21 kt at times, but look to overall remain below small craft criteria.

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

Felton/29

FIRE WEATHER

Scattered showers and thunderstorms have been observed so far today. Coverage has decreased into the afternoon but additional storms are slated to develop, primarily for the North Cascades where HREF guidance has 1,000 to 1,200 j/kg of surface based CAPE modeled in concert with southerly diffluence aloft. As a result, Red Flag Warnings remain for the North Cascades but should be allowed to expire this evening as conditions trend drier. 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...Red Flag Warning until 8 PM PDT this evening for West Slopes of the North Cascades Generally above 1500 Feet.

Red Flag Warning until 5 PM PDT this afternoon for West Slopes of the Central Cascades Generally above 1500 Feet.

PZ...None.