Climate of Seattle

Seattle has a temperate climate, classified in the Mediterranean zone by the main climatic classification (Köppen: "Csb") but some sources put the city in the oceanic zone ("Cfb"). It has cool, wet winters and mild, relatively 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 (which has a strong influence on the region's vegetation). The city and environs are part of USDA hardiness zone 8b, with isolated coastal pockets falling under 9a.
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 ). 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. 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. However, due to the fact that Seattle often has merely a light drizzle falling from the sky for many days, Seattle actually receives significantly less rainfall (or other precipitation) overall than many other U.S. cities like New York City, Miami, or Houston. Seattle is cloudy 201 days out of the year and partly cloudy 93 days. (Official weather and climatic data is collected at Seattle–Tacoma International Airport, located about 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.)
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.
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 in 1952 to in 1950; for water year (October 1 – September 30) precipitation, the range is in 1976–77 to in 1996–97. Due to local variations in microclimate, Seattle also receives significantly lower precipitation than some other locations west of the Cascades. Around 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 . 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 . The city of Bremerton, about west of downtown Seattle on the other side of the Puget Sound, receives of precipitation annually.
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 expands and cools as it goes over 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 compresses and warms, and is significantly dryer. Sequim, Washington, nicknamed "Sunny Sequim," is located approximately northwest of downtown Seattle and receives just of annual precipitation, more comparable to that of Los Angeles. Often 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.
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, as the city reports thunder on just seven days per year. 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 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 and drizzle are the predominant forms of precipitation during the remainder of the year. For instance, on average, less than of rain falls in July and August combined when rain is less common. 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 in some areas with winds topping out at along coastal Oregon. It became the second wettest event in Seattle history when a little over of rain fell on Seattle in a 24-hour period. Lack of adaptation to the heavy rain contributed to five deaths, widespread flooding and damage.
Autumn, winter, and early spring are frequently characterized by rain. Winters are cool and wet with December, the coolest month, averaging , 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; the temperature rarely lowers to . Summers are sunny, dry and warm, with August, the warmest month, with high temperatures averaging , and reaching on 3.1 days per year. In 2015 the city recorded 13 days over 90 °F. The hottest officially recorded temperature was on July 29, 2009; 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 the day the official record high was set. The average window for freezing temperatures is November 16 through March 10, allowing a growing season of 250 days.
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. A single calendar-day snowfall of six inches (15 cm) or greater has occurred on only 17 days since 1948, and only three times since February 17, 1990; of snow officially fell at Sea-Tac airport on January 18, 2012. This 2012 moderate snow event was officially the 12th snowiest calendar day at the airport since 1948 and snowiest since November 1985. Locations to the south of Seattle received more, with Olympia and Chehalis receiving . Much of the city of Seattle proper received somewhat lesser snowfall accumulations. 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 , causing widespread difficulties in a city not equipped for clearing snow. In February 2019, Seattle experienced its snowiest month in 50 years (since January 1969), with 20.2 inches of snow, all from February 3–11, with 6.4 inches on Feb 8 and 6.1 more inches on Feb. 11. The largest documented snowstorm occurred from January 5–9, 1880, with snow drifting to in places at the end of the snow event. From January 31 to February 2, 1916, another heavy snow event occurred with of snow on the ground by the time the event was over. With official records dating to 1948, the largest single-day snowfall is on January 13, 1950. Seasonal snowfall has ranged from zero in 1991–92 to in 1968–69, with trace amounts having occurred as recently as 2009–10. The month of January 1950 was particularly severe, bringing of snow, the most of any month along with the aforementioned record cold.
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. 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 , 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 less than the usual precipitation falls in the Puget Sound area. 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.