textproduct: Seattle/Tacoma

SYNOPSIS

Dry and hot conditions through Thursday as an upper level ridge moves over Western Washington. A thermal trough over the coast will bring offshore winds at times. Onshore flow returns late in the week as the thermal trough pushes inland late Thursday. Cooler conditions return this weekend.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

The latest forecast remains on track this evening, with an updated aviation section below. Smokey skies across Western Washington this evening from fires in British Columbia and Eastern Washington impacting visibility and air quality. Latest HRRR smoke guidance suggests high near- surface smoke concentrations will continue to filter into Western Washington this evening and overnight. For the latest information and air quality please refer to your local clean air agency.

Warmer conditions anticipated as an upper level ridge offshore prepares to move inland across Western Washington Wednesday into Thursday. High temperatures will peak in the 70s along the coast and mid 80s to mid 90s as the ridge axis is directly over the region. This will result in Moderate HeatRisk for the majority of the region, for which a Heat Advisory has been issued. In addition, warm temperatures overnight will bring bring limited relief for interior locations, especially for urban heat islands. The ridge axis will move away from the region, weaken and move into Eastern Washington late Thursday night.

Offshore flow will continue at times through Thursday with a thermal trough along the coastline. This will keep advecting smoke into the region. The thermal trough is expected to move inland later on Thursday, marking the return for onshore flow, which should ameliorate smokey conditions.

LONG TERM /FRIDAY THROUGH TUESDAY/

Onshore flow is favored late in the week and should help improve smoke conditions. However, uncertainty remains on how quick this improvement will be as it will be dependent on the strength of the winds. Zonal flow will develop over Western Washington late this week and into the weekend. This will help moderate temperatures across the region to the 60s along the coast and mid 70s to low 80s in the interior. By Monday, there is good agreement between ensemble that a weak upper trough will settle over the region.

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AVIATION

Northwesterly flow aloft as upper level ridging continues offshore. Surface wind direction mostly northerly, although some terminals remain easterly this evening. Speeds are pretty uniform across the area, generally ranging 5-10 kts. Most locations should see a shift to north or northeasterly tonight before becoming light and variable overnight.

Majority of terminals reporting VFR conditions however widespread smoke in place over W WA has allowed some locations to dip down into MVFR conditions at times. This drop in conditions due mainly to vis restrictions imposed by smoke as opposed to any cigs or smoke layer. Vis should rebound somewhat to around 6SM tonight for most locations as the atmosphere decouples and smoke becomes more of an aloft feature with the main impact being continued hazy conditions. That said, it is important to remember that any smoke may limit both horizontal and slant-range vis, therefore, experienced vis may be less than what is being reported in observations. The problem of surface-based smoke will return for Wednesday afternoon at a level similar to today.

KSEA...Visibility impacts present but so far have not dipped below 6SM. Will limit vis reductions to this for 06Z issuance unless obs dictate otherwise. Still worth noting that brief dips down to 4SM remain possible this evening and early tonight. Also, refer to above statement regarding slant-range vis. Have also accounted for smoke likely playing a role in ASOS cig of OVC between 040 and 050. This should dissipate tonight as atmosphere decouples, relegating smoke to higher levels. Latest smoke model guidance suggests Wednesday being very similar to today.

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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.

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FIRE WEATHER

A warming and drying trend will continue across western Washington through 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 continue to 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 over new fire starts should be emphasized.

The thermal trough will shift inland later on Thursday with the arrival of a frontal system late Thursday into Friday. Some uncertainty still remains over potential instability and incoming wind in combination with the eastward movement of the thermal trough which may briefly promote fire growth late Thursday into early Friday. Hot and dry conditions may hold on on Friday east of the Puget Sound, though increased onshore flow by Saturday is expected to bring cooler temperatures and increased moisture with healthier humidity recoveries across the region.

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

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SEW WATCHES/WARNINGS/ADVISORIES

WA...Heat Advisory from 10 AM Wednesday to 10 PM PDT Thursday for City of Seattle-Downtown Everett / Marysville Area- Eastern Kitsap County-Eastern Strait of Juan de Fuca- 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- Foothills of the Western and Southern Olympic Peninsula- Island County-Lake Crescent Area Including US 101-Lower Chehalis River Valley-Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties-Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County- Middle Chehalis River Valley-Northern Hood Canal-Olympia and Southern Puget Sound-Port Townsend Area-San Juan County-Shoreline / Lynnwood / South Everett Area-Southern Hood Canal-Willapa and Black Hills.

Air Quality Alert until 5 PM PDT Thursday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County- Cascades of Whatcom and Skagit Counties-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- Foothills of the Western and Southern Olympic Peninsula- Grays Harbor County Coast-Lower Chehalis River Valley- Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties-Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County-Middle Chehalis River Valley- Northern Hood Canal-Olympia and Southern Puget Sound- Olympics-Shoreline / Lynnwood / South Everett Area- Southern Hood Canal-Willapa and Black Hills.

PZ...Small Craft Advisory until 2 AM PDT Wednesday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.