The climate of Seattle is temperate, classified in the warm-summer (in contrast to hot-summer) subtype of the Mediterranean zone by the most common climate classification (Köppen: Csb) although some sources put the city in the oceanic zone (Trewartha: Do). It has cool, wet winters and warm, dry summers, covering characteristics of both.
The climate is sometimes characterized as a "modified Mediterranean" climate because it is cooler and wetter than a "true" Mediterranean climate, but shares the characteristic dry summer and the associated reliance upon cooler-season precipitation (which has a strong influence on the region's vegetation). The city is part of USDA hardiness zone 9a, with surrounding pockets falling under 8b.
Records for the Seattle City area date back to 1894, with records at Seattle-Tacoma International Airport beginning in 1945, a location notably not within Seattle. Prior to 1945 the official temperatures were observed in locations in downtown Seattle, which tends in general to be somewhat warmer and drier than the airport location. The hottest officially recorded temperature was on June 28, 2021; the coldest recorded temperature was on January 31, 1950; the record cold daily maximum is on January 14, 1950, while, conversely, the record warm daily minimum is on June 27, 2021.
Seattle generally does not experience many extremes of weather. However, the 21st century has seen a trend towards more extreme high-temperature and large-precipitation events. In July 2009 Seattle's all-time high temperature was broken by a margin of 4 degrees Fahrenheit (2.2 Celsius), then broken again by a margin of 5 F (2.8 C) in June 2021. The single-day precipitation record set in October 2003 saw higher precipitation by nearly 2 inches (50mm) than any other day on record. However, thunderstorms are still rare, as the city reports thunder on just seven days per year. Similarly, the city typically receives at least light snowfall every year, though heavy snowfall is uncommon.
Regional setting and synoptic drivers
Seattle's climate is governed at the largest scale by the semi-permanent pressure centers of the North Pacific and by the position of the polar jet stream. During summer, the North Pacific High expands northward and dominates the eastern Pacific, deflecting the storm track well to the north of Washington and producing the region's characteristic warm, dry, and stable summer regime. From late autumn to early spring, the high weakens and retreats southward while the Aleutian Low, a semi-permanent low-pressure system centered near the Aleutian Islands, intensifies and shifts south, becoming one of the dominant centers of action in Northern Hemisphere atmospheric circulation.
Extratropical cyclones forming in the sub-polar latitudes of the North Pacific characteristically slow and reach peak intensity in the vicinity of the Aleutian Low, after which the polar jet stream steers many of them eastward toward the Pacific Northwest coast. The resulting cool-season storm track delivers the great majority of Seattle's annual precipitation between October and March, with the wettest months — November, December, and January — accounting for roughly half of the city's annual rainfall by volume.
When winter ridging over the eastern Pacific deflects the jet stream northward and the polar vortex remains contained over the Arctic, the storm track is shunted away from western Washington, producing extended dry or fair spells. When the polar vortex is disrupted and the jet stream buckles southward into the central or eastern United States, Seattle is typically spared the resulting cold waves, which are channeled east of the Cascades rather than across the Puget Sound lowland.
Temperature
The city's regime of temperature features small seasonal swings, due to its proximity to the ocean. The Pacific Ocean, Puget Sound and Lake Washington serve as moderators of the temperature meaning the city is milder than areas inland during the winter and cooler during the summer. Extreme heatwaves are rare, as are cold temperatures. Hot temperature extremes are enhanced by dry, compressed wind from the west slopes of the Cascades, while cold temperatures are generated mainly from the Fraser Valley in British Columbia. Records are taken from the Seattle City area from 1894 to 1944 and at Sea-Tac Airport from 1945.
Averages In an average year, the temperature will usually be between and with temperatures greatly exceeding these values being uncommon. Temperatures above and below are very rare, with the last occurrences being June 28, 2021 and November 24, 2010, respectively.
Highest daily temperatures
PeriodRecord temperatureDate Jan 28, 1931 Feb 27, 1968 Mar 20, 2019Mar 19, 2019 Apr 18, 2016 May 21, 1963 Jun 28, 2021 Jul 29, 2009 Aug 9, 1981Aug 9, 1960 Sep 2, 1988 Oct 1, 1987 Nov 3, 2010Nov 4, 1949 Dec 10, 2014
Lowest daily temperatures PeriodRecord temperatureDate Jan 31, 1950 Feb 1, 1950 Mar 4, 1955 Apr 5, 1975Apr 1, 1953Apr 7–8, 1952Apr 21, 1951 May 1, 1954 Jun 12, 1952 Jul 4, 1949Jul 24, 1953Jul 2, 1954 Aug 13–14, 1955Aug 27, 1952Aug 29, 1951Aug 4, 1950Aug 21, 1947 Sep 27, 1972 Oct 19, 1949 Nov 15, 1955 Dec 30, 1968
Daily record warm minima
PeriodRecord temperatureDate Jan 24, 1935 Feb 28, 1901 Mar 19, 2019 Apr 28, 1976 May 15, 2023 May 9, 1940 Jun 27, 2021 Jul 29, 2009 Aug 14, 2023 Sep 2, 1974 Oct 18, 1940 Nov 4, 2020 Dec 26, 1980Dec 28, 1917
Daily record cold maxima
PeriodRecord temperatureDate Jan 14, 1950 Feb 2, 1989 Mar 4, 1955 Apr 3, 1920Apr 11, 1911 May 15, 1894 Jun 1, 1908 Jul 2, 1966 Aug 2, 1956 Sep 26, 1948 Oct 30, 1935 Nov 12, 1955 Dec 29, 1968
Highest averages
PeriodRecord meanYearYear 1940, 2015 Spring (March–May) 1934 Summer (June–August) 2015 Autumn (September–November) 1967 Winter (December–February) 1940–1941 January 2010 February 2015 March 1941 April 2016 May 2018 June 2015 July 2015 August 1967 September 1967 October 2014 November 1899 December 1939
Lowest averages PeriodRecord meanYearYear 1955 Spring (March–May) 1955 Summer (June–August) 1954 Autumn (September–November) 1985 Winter (December–February) 1949–1950, 1948–1949 January 1950 February 1956 March 1955 April 1955 May 1962 June 1953 July 1955 August 1910 September 1972 October 1946 November 1985 December 1990
NOTE: in the tables below, all numbers before 1945 come from locations in downtown Seattle, which tends to be somewhat warmer than the current official location of Sea-Tac airport. To distinguish these older numbers we mark them below in italics.
Precipitation
The city sees frequent, though light rainfall between October and May, with rainfall becoming lighter and sparser between June and September. With many more "rain days" than other major American cities, Seattle has a well-earned reputation for frequent rain. In an average year, at least of precipitation falls on 150 days, more than nearly all U.S. cities east of the Rocky Mountains. In November, Seattle averages more rainfall than any other U.S. city of more than 250,000 people; it also ranks highly in winter precipitation. Conversely, the city receives some of the lowest precipitation amounts of any large city from June to September. Seattle is one of the five rainiest major U.S. cities as measured by the number of days with precipitation.
Because Seattle often has merely a light drizzle falling from the sky for many days, it actually receives significantly less rainfall (or other precipitation) overall than many other U.S. cities like New York City, Miami, or Houston. Seattle experiences its heaviest rainfall during November, December, and January, receiving roughly half of its annual rainfall (by volume) during this period. Light rain and drizzle are the predominant forms of precipitation during the remainder of the year. On average, less than of rain falls in July and August combined when rain is less common.
Annually, total precipitation averages , with winter being the wettest season and July the driest month. At Sea-Tac, rain has fallen in every month since records began there in January 1945, previously in the Seattle City area, the Julys of 1896 and 1922 reported no precipitation. Long stretches of little precipitation can occur. No measurable precipitation, greater than , fell between June 18 and August 13, 2017.
Seattle also sees snow, primarily in winter, but sometimes in the late autumn and early spring. Snowfall averages per year but is highly variable between winter seasons. The most rainfall in 24 hours was on October 20, 2003, and the most snowfall was on February 2, 1916. Seattle typically receives some snowfall on an annual basis but heavy snow is rare. Average annual snowfall, as measured at Sea-Tac Airport, is . From winter season to winter season, amounts can be extremely variable.
One of many exceptions to Seattle's reputation as a damp location occurs in El Niño years, when marine weather systems track as far south as California and less than the usual precipitation falls in the Puget Sound area. However, the El Nino of 2015-2016 caused an increase in rainfall. Since the region's water comes from mountain snow packs during the dry summer months, El Niño winters can not only produce substandard skiing but can result in water rationing and a shortage of hydroelectric power the following summer.
Orographic precipitation gradient
Seattle's modest annual rainfall belies its position within one of the steepest precipitation gradients in the contiguous United States. The city lies in the lee of the Olympic Mountains, which rise abruptly to nearly from near sea level only about to the west and intercept moisture-laden flow arriving from the open Pacific. The resulting orographic uplift produces extreme precipitation totals on the windward slopes: the Hoh Rainforest on the western flank of the range averages roughly annually, with upper elevations of Mount Olympus estimated to receive in excess of , making the windward Olympics among the wettest locations in the conterminous United States. For context, to the south of Seattle, the state capital Olympia, which is out of the Olympic Mountains' rain shadow, receives an annual average precipitation of , and the city of Bremerton, about west of downtown Seattle on the other side of Puget Sound, receives annually.
The descending, drying air on the leeward side of the range produces a pronounced rain shadow. Within roughly of the Hoh Rainforest, annual precipitation collapses to about at Sequim in the Dungeness Valley — comparable to semi-arid locations such as Los Angeles — producing a so-called "blue hole" in which sunshine frequently persists while the surrounding region is overcast. The gradient between these extremes is exceptionally compressed: heading west from Port Angeles () along U.S. Highway 101, annual precipitation rises to roughly at Elwha within and to about at Sappho another farther west, with all three locations at comparable elevations.
Seattle itself lies near the eastern edge of the Olympic rain shadow, which only partially shelters the city, since incoming flow more often arrives from the southwest, threading between the Olympics and the southern Washington coast. Even so, the rain shadow contributes materially to the city's reputation for frequent but light precipitation, as the heaviest precipitation of incoming systems is typically deposited on the Olympic, Willapa Hills, and Cascade slopes before the residual moisture reaches the Puget Sound lowland. A similar but larger-scale rain shadow east of the Cascades produces the semi-arid climate of the Columbia Basin, with several locations receiving less than of annual precipitation.