Climate

Seattle's climate is classified as oceanic or temperate marine, with cool, wet winters and mild, relatively dry summers.^{[72] [73]} The city and environs are part of USDA hardiness zone 8b, with isolated coastal pockets falling under 9a.^[74]
Temperature extremes are moderated by the adjacent Puget Sound, greater Pacific Ocean, and Lake Washington. Thus extreme heat waves are rare in the Seattle area, as are very cold temperatures (below about 15 F). The Seattle area is the cloudiest region of the United States, due in part to frequent storms and lows moving in from the adjacent Pacific Ocean. Despite having a reputation for frequent rain, Seattle receives less precipitation than many other US cities like Chicago or New York City. However, unlike many other US cities, Seattle has many more "rain days", when a very light drizzle falls from the sky for many days.^[75] In an average year, at least 0.01 inches (0.25 mm) of precipitation falls on 150 days, more than nearly all U.S. cities east of the Rocky Mountains.^[76] It is cloudy 201 days out of the year and partly cloudy 93 days.^[71] Official weather and climatic data is collected at Seattle–Tacoma International Airport, located about 19 km (12 mi) south of downtown in the city of SeaTac, which is at a higher elevation, and records more cloudy days and fewer partly cloudy days per year.^[71]
Hot temperature extremes are enhanced by dry, compressed wind from the west slopes of the Cascades,^[77] while cold temperatures are generated mainly from the Fraser Valley in British Columbia.^[78]
From 1981 to 2010, the average annual precipitation measured at Seattle–Tacoma International Airport was 37.49 inches (952 mm). Annual precipitation has ranged from 23.78 in (604 mm) in 1952 to 55.14 in (1,401 mm) in 1950; for water year (October 1 – September 30) precipitation, the range is 23.16 in (588 mm) in 1976–77 to 51.82 in (1,316 mm) in 1996–97.^[79] Due to local variations in microclimate, Seattle also receives significantly lower precipitation than some other locations west of the Cascades. Around 80 mi (129 km) to the west, the Hoh Rain Forest in Olympic National Park on the western flank of the Olympic Mountains receives an annual average precipitation of 142 in (3.61 m). Sixty miles (95 km) to the south of Seattle, the state capital Olympia, which is out of the Olympic Mountains' rain shadow, receives an annual average precipitation of 50 in (1,270 mm).^[79] The city of Bremerton, about 15 mi (24 km) west of downtown Seattle on the other side of the Puget Sound, receives 56.4 in (1,430 mm) of precipitation annually.^[79]
Conversely, the northeastern portion of the Olympic Peninsula, which lies east of the Olympic Mountains is located within the Olympic rain shadow and receives significantly less precipitation than its surrounding areas. Prevailing airflow from the west is forced to cool and compress when colliding with the mountain range, resulting in high levels of precipitation within the mountains and its western slopes. Once the airflow reaches the leeward side of the mountains it then lowers and expands resulting in warmer, and significantly dryer air. Sequim, Washington, nicknamed "Sunny Sequim", is located approximately 40 miles northwest of downtown Seattle and receives just 16.51" of annual precipitation, more comparable to that of Los Angeles. Oftentimes an area devoid of cloud cover can be seen extending out over the Puget Sound to the north and east of Sequim. On average Sequim observes 127 sunny days per year in addition to 127 days with partial cloud cover. Other areas influenced by the Olympic rain shadow include Port Angeles, Port Townsend, extending as far north as Victoria, British Columbia.^[80]
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, and it receives some of the lowest amounts of annual sunshine among major cities in the lower 48 states, along with some cities in the Northeast, Ohio and Michigan. Thunderstorms are rare,^[81] as the city reports thunder on just seven days per year.^[82] By comparison, Fort Myers, Florida, reports thunder on 93 days per year, Kansas City on 52, and New York City on 25.
Seattle experiences its heaviest rainfall during the months of November, December and January, receiving roughly half of its annual rainfall (by volume) during this period. In late fall and early winter, atmospheric rivers (also known as "Pineapple Express" systems), strong frontal systems, and Pacific low pressure systems are common. Light rain & drizzle are the predominant forms of precipitation during the remainder of the year; for instance, on average, less than 1.6 in (41 mm) of rain falls in July and August combined when rain is rare. On occasion, Seattle experiences somewhat more significant weather events. One such event occurred on December 2–4, 2007, when sustained hurricane-force winds and widespread heavy rainfall associated with a strong Pineapple Express event occurred in the greater Puget Sound area and the western parts of Washington and Oregon. Precipitation totals exceeded 13.8 in (350 mm) in some areas with winds topping out at 209 km/h (130 mph) along coastal Oregon.^[83] It became the second wettest event in Seattle history when a little over 130 mm (5.1 in) of rain fell on Seattle in a 24-hour period. Lack of adaptation to the heavy rain contributed to five deaths and widespread flooding and damage.^[84]
Autumn, winter, and early spring are frequently characterized by rain. Winters are cool and wet with December, the coolest month, averaging 40.6 °F (4.8 °C), with 28 annual days with lows that reach the freezing mark, and 2.0 days where the temperature stays at or below freezing all day;^[79] the temperature rarely lowers to 20 °F (−7 °C).^[79] Summers are sunny, dry and warm, with August, the warmest month, with high temperatures averaging 76.1 °F (24.5 °C), and reaching 90 °F (32 °C) on 3.1 days per year. In 2015 the city recorded 13 days over 90 °F.^[79] The hottest officially recorded temperature was 103 °F (39 °C) on July 29, 2009;^[85] the coldest recorded temperature was 0 °F (−18 °C) on January 31, 1950;^[86] the record cold daily maximum is 16 °F (−9 °C) on January 14, 1950, while, conversely, the record warm daily minimum is 71 °F (22 °C) the day the official record high was set.^[79] The average window for freezing temperatures is November 16 through March 10, allowing a growing season of 250 days.^[79]
Seattle typically receives some snowfall on an annual basis but heavy snow is rare. Average annual snowfall, as measured at Sea-Tac Airport, is 6.8 inches (17.3 cm). Single calendar-day snowfall of six inches (15 cm) or greater has occurred on only 15 days since 1948, and only once since February 17, 1990, when 6.8 in (17.3 cm) of snow officially fell at Sea-Tac airport on January 18, 2012. This moderate snow event was officially the 12th snowiest calendar day at the airport since 1948 and snowiest since November 1985.^[79] Much of the city of Seattle proper received somewhat lesser snowfall accumulations. Locations to the south of Seattle received more, with Olympia and Chehalis receiving 14 to 18 in (36 to 46 cm).^[87] Another moderate snow event occurred from December 12–25, 2008, when over one foot (30 cm) of snow fell and stuck on much of the roads over those two weeks, when temperatures remained below 32 °F (0 °C), causing widespread difficulties in a city not equipped for clearing snow. The largest documented snowstorm occurred from January 5–9, 1880, with snow drifting to 6 feet (1.8 m) in places at the end of the snow event. From January 31 to February 2, 1916, another heavy snow event occurred with 29 in (74 cm) of snow on the ground by the time the event was over.^[88] With official records dating to 1948, the largest single-day snowfall is 20.0 in (51 cm) on January 13, 1950.^[89] Seasonal snowfall has ranged from zero in 1991–92 to 67.5 in (171 cm) in 1968–69, with trace amounts having occurred as recently as 2009–10.^[79] The month of January 1950 was particularly severe, bringing 57.2 in (145 cm) of snow, the most of any month along with the aforementioned record cold.^[79]
The Puget Sound Convergence Zone is an important feature of Seattle's weather. In the convergence zone, air arriving from the north meets air flowing in from the south. Both streams of air originate over the Pacific Ocean; airflow is split by the Olympic Mountains to Seattle's west, then reunited to the east. When the air currents meet, they are forced upward, resulting in convection.^[90] Thunderstorms caused by this activity are usually weak and can occur north and south of town, but Seattle itself rarely receives more than occasional thunder and small hail showers. The Hanukkah Eve Wind Storm in December 2006 is an exception that brought heavy rain and winds gusting up to 69 mph (111 km/h), an event that was not caused by the Puget Sound Convergence Zone and was widespread across the Pacific Northwest.
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 little precipitation falls in the Puget Sound area.^[91] 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.^[92]