textproduct: Seattle/Tacoma

SYNOPSIS

Dry and warmer conditions as an upper level ridge offshore moves inland Wednesday and Thursday. A thermally induced trough will develop tonight and remain over the region through Wednesday, resulting in northeasterly winds advecting smoke to Western Washington. The ridge will push eastward late Thursday. Smokey conditions should improve by the end of the week with increased onshore flow. Cooler temperatures favored for the weekend.

SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/

Current satellite showing smoke aloft over much of W WA this evening, with the exception of the Olympics. The main impact this evening and most of tonight is expected to be some haze for most locations. A side benefit to this smoke aloft is that it kept daytime high temps anywhere from 1-4 degrees shy of what was forecast. A consideration for future shifts will be if persistent smoke will further affect temps as the current warming trend continues. That said, forecast remains on track with no need for an evening update. For addtional forecast details, please refer to the Previous Discussion section below. 18

From Previous Discussion...An upper level ridge continues to sit offshore this afternoon. It will begin to slowly push inland midweek. A thermally induced trough is expected to develop tonight along the coast and remain in place through midweek. As a result, surface winds will shift north-northeasterly on Tuesday and smoke will descend to the surface from British Columbia and Eastern Washington fires. A gradual warming trend is expected through the latter part of the week. However, with smoke filtering to the surface, it presents a challenging forecast as to how it will affect high temperatures. The current forecast has high temperatures in the 70s along the coast and 80s in the interior on Tuesday. As for Wednesday, temperatures are forecasted in the 70s for the coast and 80s to 90s in the interior. It is possible that observed temperatures are lower than what is in the forecast. In addition, there is uncertainty as to how far west surface smoke will reach. With increasing temperatures, Minor to Moderate (Seattle-Tacoma Metro & Southwest Interior) HeatRisk on Tuesday. More widespread Moderate HeatRisk is expected by Wednesday.

LONG TERM /THURSDAY THROUGH MONDAY/

Long-term guidance is in good agreement that the ridge axis directly over Western Washington will start to push eastward later on Thursday, along with the thermal trough. This will lead to increased onshore flow Thursday night, continuing into next week. Increased onshore flow should improve smokey conditions over the region. However, there is uncertainty as to how quickly conditions will improve. Another hot day expected on Thursday for the interior with temperatures in the mid 80s to 90s. As for the coast, cooler temperatures in the upper 60s to low 70s. A weak upper level trough and cooler temperatures are favored this weekend.

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AVIATION

Northwesterly flow aloft this evening and remaining that way for the remainder of the TAF period. Surface winds generally from the north with some variation to the NE or NW depending on location. Most terminals expected to retain this direction with a slight favoring of NE for much of the TAF period, although some will go light and variable overnight. Speeds mostly 4-8 kts into Tuesday morning before increasing somewhat in the afternoon to 5-10 kts.

VFR conditions area-wide this evening and expected to persist for much of the TAF period. Smoke from nearby wildfires in both Canada and eastern WA will remain aloft this evening, but could start making its way to the surface as early as overnight tonight, but latest model data suggests this may hold off until Tuesday as the atmosphere begins to mix better. Currently, the only impact expected is some haze this evening and tonight. Additional impacts are possible tomorrow as smoke mixes down closer to the surface, which will likely impact visibilities. It is important to remember this smoke may limit both horizontal and slant-range visibilities...and thus experienced vis may be less than what is being reported in obs.

KSEA...VFR throughout the TAF period. Elevated smoke will increase, becoming more surface-based Tuesday. This may result in vis reductions. To anticipate this, have included HZ after 18Z Tuesday afternoon with a more impactful TEMPO for the afternoon hours reducing vis to 5SM. This could be more, depending on daytime mixing, but feel this is a good starting point for future amendments/updates. North to northwesterly winds this evening before favoring more northeasterly overnight and Tuesday morning. A slight turn to northerly is expected in the afternoon. Speeds 5-10 kts tonight and remaining that way into Tuesday.

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MARINE

A strong area of high pressure will remain offshore throughout midweek as a thermally induced low pressure expands northward along the interior coast. Northwesterly winds will prevail across the coastal waters this week. With increasing confidence, a small craft advisory has been issued for the southernmost coastal water zones this afternoon. Breezy conditions briefly expected Tuesday morning across Puget Sound, however winds should remain below small craft thresholds. The surface trough is expected to push inland on Thursday, resulting in increased onshore flow across the waters. Small craft winds are possible through the Strait of Juan de Fuca as early as Thursday evening, continuing into the weekend. Latest guidance suggests the strongest winds will be on Friday, with a 70- 85% chance of small craft winds and 25-40% chance for gales. Another strong push expected on Saturday, with a 65-80% chance for small craft winds and 15-20% chance for gales.

Seas expected to remain below 10 ft for the majority of week and will trend toward 10 feet toward the weekend. Steep seas possible Tuesday evening at times over the coastal waters. 29

FIRE WEATHER

A warming and drying trend will continue across western Washington through much of the week as a high pressure ridge builds offshore and a surface thermal trough builds northward along the Pacific coast. Several days of drying and warming will drive minimum relative humidity values to near critical levels, especially in the western Cascades and Olympics where humidities may reach critical values as low as the teens to upper single digits by Wednesday and Thursday. While no significant wind or instability is expected through the week, maximum caution should be emphasized.

The thermal trough will shift inland on Thursday with the arrival of a frontal system late Thursday into Friday. Some uncertainty remains over potential instability and incoming wind in combination with the movement of the thermal trough which may briefly promote fire growth. Increased onshore flow by Friday is expected to bring cooler temperatures and increased moisture with healthier humidity recoveries.

In addition, north to east winds throughout the week will continue to bring in wildfire smoke from eastern Washington and British Columbia. This may cause localized impacts to fire operations at times.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...Air Quality Alert until 5 PM PDT Wednesday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County- City of Seattle-Downtown Everett / Marysville Area- Eastern Kitsap County-Eastside-Foothills and Valleys of Central King County-Foothills and Valleys of Pierce and Southern King Counties-Foothills and Valleys of Snohomish and Northern King Counties-Foothills and Valleys of Thurston and Lewis Counties-Foothills and Valleys of the North Cascades-Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties- Lowlands of Western Skagit and Northwestern Snohomish Counties-Northern Hood Canal-Olympia and Southern Puget Sound-Shoreline / Lynnwood / South Everett Area-Willapa and Black Hills.

PZ...Small Craft Advisory until 3 AM PDT Tuesday for Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm- Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.