The 2009 Pacific Northwest heat wave was a heat wave which affected the Pacific Northwest region in Oregon and Washington west of the Cascade Mountains in late July, and is well-known for setting many new heat records across the area, especially breaking the previous all-time record high temperature in Seattle by three Fahrenheit degrees (1.7 Celsius degrees). The heat wave was made worse by the fact that heat of this magnitude is rare in the Northwest; thus, air conditioners and other methods for cooling are not common in this region.
Temperatures soared to record highs in late July from Medford, Oregon to Seattle, Washington, and as far north as the Northwest Territories in Canada. The hottest weather occurred from July 26 to August 2, with the peak of the heat from July 28 to July 30. The high of 103 °F (39 °C) at Seattle-Tacoma International Airport on July 29 set the record for the hottest temperature ever in Seattle since records began in 1894, breaking the old record of 100 °F (38 °C) set on July 16, 1941 when the official weather station was located in downtown Seattle, and tied on July 20, 1994 at Seattle-Tacoma International Airport. Nighttime low temperatures were unusually high as well — in the upper 60s and lower 70s °F (about 21 °C). The morning of July 29, Seattle also set a new record warmest low temperature, 71 °F (22 °C), breaking the old record of 69 °F (21 °C) set on September 2, 1974 and tied a day earlier on July 28, 2009.
The heat was not confined to Seattle. Portland International Airport in Oregon reached 106 °F (41 °C) on two consecutive days on July 28 and 29, nearly tying the all-time record there of 107 °F (42 °C). Portland tied its warmest low temperature on record with a low of 74 °F (23 °C) on July 28.
The heat was part of a summer that was overall very warm in the Northwest. Seattle recorded its second warmest July on record with an average temperature of 69.45 °F (20.81 °C), trailing only July 1941 (average temperature of 69.55 °F (20.86 °C)), and the third warmest month on record, trailing July 1941 and August 1967 (average temperature of 71.1 °F (21.7 °C)). The June 1 to July 31 time period was the warmest on record in Seattle with an average temperature of 66.7 °F (19.3 °C), besting the previous warmest June/July period of 1958 (average temperature of 66.4 °F (19.1 °C)).
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