A strong upper level ridge of high pressure began to build into the region around August 11, remaining in place through August 13 before pushing eastward. As the upper level ridge influence increased over the region, the lower level flow turned easterly, creating a downsloping effect across much of Western Washington. As a result of this setup, very warm temperatures were forecast for most of the area, with 850mb temperatures expected to peak near 25C during the heart of the event, with surface temperatures ranging from the mid 80s to near 100 and low temperatures during the overnight period ranging from the mid 60s to low 70s.
Excessive Heat Watch was issued on August 8, several days prior to the event, valid for all included zones beginning 12PM on Wednesday August 11 and continuing for some zones through Friday August 13 at 8PM and others Saturday August 14 at 7PM. The Watch was upgraded to a warning during the afternoon of August 10.
Fire weather was also a concern during the event, along with air quality impacts from smoke, however, no major impacts were noted during the period of unusually warm weather.

Heat in SEATTLE AND VICINITY, WASHINGTON
From 11-AUG-21 12:00:00 to 14-AUG-21 19:00:00 (PST-8)
Reported by: ASOS

A period of much above normal temperatures affected the Seattle area around mid-August. High temperatures ranged from the mid 80s to low 90s during this period, with the temperature peaking August 12-August 13 at 93 degrees, both of which were record high temperatures. Low temperatures during this period ranged from the mid to upper 60s.

Heat in SOUTHWEST INTERIOR, WASHINGTON
From 11-AUG-21 12:00:00 to 14-AUG-21 19:00:00 (PST-8)
Reported by: ASOS

The Southwest Interior experienced a stretch of heat towards mid August. The ASOS sensor at Olympia showed high temperatures in the low to mid 90s through the duration of the heat event, with a record high temperatures of 96 recorded on August 12. Low temperatures through the event ranged from the mid to upper 50s.
Centralia also recorded temperatures in the low to mid 90s. A high temperature of 95 was recorded on August 12 and August 13. Low temperatures stayed in the mid 60s.

Heat in WESTERN WHATCOM COUNTY, WASHINGTON
From 11-AUG-21 12:00:00 to 13-AUG-21 20:00:00 (PST-8)
Reported by: ASOS

A warm stretch of weather affected Western Whatcom County towards the middle of August, high temperatures consistently reached the mid to upper 80s for much of the duration of the heat stretch, however, temperatures soared to 100 degrees at the Bellingham Airport on August 12, setting a new record. Low temperatures during the event remained right around 60.

Heat in BREMERTON AND VICINITY, WASHINGTON
From 11-AUG-21 12:00:00 to 13-AUG-21 20:00:00 (PST-8)
Reported by: Fire Department/Rescue

Very warm weather affected the Bremerton Area and the Kitsap Peninsula around the middle of August. Observations from Bremerton Fire showed high temperatures ranging from the mid 80s to low 90s through the duration of the event. Temperatures peaked on August 13, where a record high temperature of 93 was set. Low temperatures ranged from the low to mid 60s.

Heat in EVERETT AND VICINITY, WASHINGTON
From 11-AUG-21 12:00:00 to 13-AUG-21 20:00:00 (PST-8)
Reported by: ASOS

A warm stretch of weather affected the Everett area towards mid August. Temperatures ranged from the upper 80s to low 90s during the event, with a record high temperature of 92 degrees. Low temperatures ranged from the mid to upper 60s.
Everett-Paine Field airport had temperatures range from the upper 80s to low 90s, with a record high temperature set on August 13 of 91.

Heat in TACOMA AREA, WASHINGTON
From 11-AUG-21 12:00:00 to 14-AUG-21 19:00:00 (PST-8)
Reported by: Unknown

Warm weather affected the Tacoma area around mid August. Temperatures ranged in the upper 80s through the duration of the event, with the highest temperature recorded on August 12 at 89 degrees. This set a new record high temperature for that day. Low temperatures ranged from the mid to upper 60s.

Heat in CASCADES OF SNOHOMISH AND KING COUNTIES, WASHINGTON
From 11-AUG-21 12:00:00 to 14-AUG-21 19:00:00 (PST-8)
Reported by: Unknown

A stretch of warm weather affected the valleys and lower elevations of the Central Cascades of King and Snohomish Counties. Monroe recorded temperatures in the upper 80s to low 90s during the event, with a record high of 91 on August 13 and August 14. Low temperatures ranged from the mid 50s to around 60.

Heat in CASCADES OF PIERCE AND LEWIS COUNTIES, WASHINGTON
From 11-AUG-21 12:00:00 to 14-AUG-21 19:00:00 (PST-8)
Reported by: Park/Forest Service

A stretch of unusually warm weather affected the valleys and lower elevations of the Cascades of Lewis and Pierce Counties. Temperature recordings at Longmire at Mount Rainier National Park showed temperatures ranging from the mid 80s to upper 90s through the event, with record high temperatures of 98 degrees set on August 13 and August 14. Low temperatures ranged from the mid to upper 50s.
Cedar Lake also recorded temperatures in the mid to upper 80s, with a record high of 87 on August 14.

Heat in LOWER CHEHALIS VALLEY AREA, WASHINGTON
From 11-AUG-21 12:00:00 to 13-AUG-21 20:00:00 (PST-8)
Reported by: Unknown

A stretch of unusually warm weather affected the Lower Chehalis valley in mid August. Temperatures ranged from the upper 90s to low 100s during the event. A record high temperature of 101 was set on both August 12 and August 13, per a source sited from Elma, WA. Low temperatures ranged from the mid to upper 50s.

Heat in CASCADES OF WHATCOM AND SKAGIT COUNTIES, WASHINGTON
From 11-AUG-21 12:00:00 to 14-AUG-21 19:00:00 (PST-8)
Reported by: Unknown

Warm weather affected The Cascades valleys during mid August. Temperatures observed at Darrington ranged from the upper 80s to mid 90s, with lows in the upper 50s to near 60. Concrete observed temperatures in the mid 80s to low 90s, with a record high of 93 on August 13. Low temperatures ranged from the upper 50s to mid 60s.