textproduct: Seattle/Tacoma

SYNOPSIS

Upper level ridge offshore moving over the area tonight into Tuesday. The ridge will shift east Wednesday. Thermally induced surface trough developing along the coast today moving inland Tuesday night and east of the Cascades Wednesday morning. Very warm temperatures over the interior the next three days. Much cooler Thursday with low level onshore flow. First weather system in two weeks moving through Friday. Upper level low moving over Western Washington Saturday will move east Sunday.

SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/

Water vapor imagery shows upper level ridging along the coast with a layer of high cirrus streaking across southern Washington. Marine stratus will stay along the coast for the remainder of the day due to offshore flow. The main story for the short term will be on the warm temperatures.

Upper level ridging will move inland today and slowly exiting the region by the end of tomorrow. A thermally induced surface trough has started to form along the coast. This combination of upper ridging and a thermal trough will raise temperatures up to the 80s to low 90s today through Wednesday. Tuesday will be the warmest day during this warm stretch. Expect Moderate HeatRisk for most of the lowland areas with 25% chances for Major HeatRisk for Seattle and Tacoma areas on Tuesday. Therefore, a Heat Advisory is in effect most of the lowlands locations except the coastal beaches through Tuesday night. The Heat Advisory will end for all locations by Wednesday night as the upper level ridge moves east. Temperatures will start to moderate with the return of onshore flow by Wednesday night to Thursday morning.

LONG TERM /THURSDAY THROUGH MONDAY/

Ensemble models are consistently showing a weather system that will start impacting the coastal areas by Thursday. This weather system will bring cooler temperatures and chances for rain showers. Chances for rain showers will expand on Friday for most of western Washington. There is potential for the convergence zone forming north of Seattle because of the onshore flow, leading to Seattle seeing little to no rain fall on Friday. By Saturday, the low pressure center will be over the state before descending into the Great Basin. Temperatures will be cooler with the onshore flow and the rain showers. Chances for precipitation will linger in the Cascades on Saturday and a little on Sunday.

By Sunday, ensembles show a minor ridging forming over the coast with another weather system upstream. There is model disagreement on the ridge placement due to the low pressure center over Rocky Mountains. Current model run leans the placement of the ridge near the coast, bringing a slight warming trend and drier conditions for the coastal areas.

AVIATION

VFR conditions at all terminals this evening. Current satellite shows stratus still hugging the coastline with minimal eastward progress while high clouds start to move over the Olympic Peninsula. While some high clouds may drift further east to the inland terminals, the only risk for MVFR to IFR conditions will remain HQM as marine stratus is expected to push eastward enough to impact that terminal. Otherwise, TAF sites to remain VFR throughout the terminal forecast period. Wind directions largely remain terrain influence and vary by location. Speeds generally ranging 8-12 kts this evening, dipping to 5-10 kts by 06Z before becoming light and variable between 09-12Z. North to northeasterly winds ranging 4-8 kts expected by Tuesday afternoon.

KSEA...VFR expected through the TAF period. Northerly winds ranging 8-12 kts this evening before switching to northeasterly at around 5 kt overnight. Tuesday afternoon winds will return to northerly at 5- 10 kts.

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MARINE

Offshore flow prevails across western WA with a thermal trough along the coast. The thermal trough will shift inland on Tuesday then east of the Cascades on Wednesday with a push of westerly winds. Winds down the Strait of Juan de Fuca will be strong with gales possible. Seas will also build to 10-11 ft over the outer Coastal Waters on Wednesday. The flow remains onshore through the end of the week with highest winds/waves in the strait and outer Coastal Waters. 33

FIRE WEATHER

High pressure aloft combined with a thermally induced surface trough over the area will create elevated fire conditions through Tuesday. In addition to the warm, dry, and conditionally unstable air mass, low level offshore flow will drop minimum relative humidity values into the twenties, and even the teens today and Tuesday afternoon. The upper level ridge and thermally induced trough moving east Wednesday but elevated fire weather concerns could linger. Fuels are approaching critical levels, but the main concern remains dry grasses or shrubs, or dead piles of fuels where brush fires may be able to start (as seen the last few days). Onshore flow will return with improved low level moisture Wednesday night and Thursday. Weather system moving into the area will bring a chance of wetting rains for the first time in a couple of weeks Thursday night and Friday. Cool weather will continue through the weekend.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Heat Advisory until 11 PM PDT Tuesday for Eastern Strait of Juan de Fuca-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-Lower Chehalis River Valley-Lowlands of Lewis and Southern Thurston 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-Southern Hood Canal-Willapa and Black Hills.

Heat Advisory until 11 PM PDT Wednesday for 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-Lowlands of Pierce and Southern King Counties-Shoreline / Lynnwood / South Everett Area.

PZ...None.