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Weather in Seattle
Seattle's climate is usually described as Oceanic or Marine west coast, with fairly mild, wet winters and very warm, dry summers. Like much of the Pacific Northwest, according to the Köppen climate classification it falls within a cool, dry-summer subtropical zone (Csb), with 'cool'-summer Mediterranean characteristics. Other climate classification systems, such as Trewartha, place it firmly in the Oceanic zone (Do). Temperature extremes are moderated by adjacent Puget Sound, the greater Pacific Ocean, and Lake Washington. The region is largely denied Pacific storms by the Olympic Mountains and Arctic air by the Cascade Range. Despite being on the margin of the rain shadow of the Olympic Mountains, the city has a misleading reputation for frequent rain. This reputation comes from the frequency of precipitation in the winter. In an average year, more than 0.01 in/0.3 mm of precipitation falls on 150 days. It is cloudy 201 days and partly cloudy 93 days. The location of official weather and climatic records, the Seattle-Tacoma (nicknamed Sea-Tac) International Airport, is located about 12 miles south of downtown and records more cloudy days and fewer partly cloudy days per year. At 944mm (37.17 in.), in reality, the city receives less precipitation annually than New York City (1201 mm, 47.28 in.), Atlanta (1290 mm, 50.79 in.), Boston (1055 mm, 41.53 in.), Baltimore (1038 mm, 40.87 in.), Portland, Maine (1128 mm, 44.41 in.), Jacksonville, Florida (1304 mm, 51.34 in.), and most cities on the Eastern Seaboard of the U.S. Seattle was also not listed in a study that revealed the 10 rainiest cities in the continental United States. This is due largely to Seattle's dry summers, which result in statistically moderate annual accumulations. Seattle receives the largest amount of rainfall of any major (pop > 250,000) U.S. city in November, and is in the top 10 through Winter, but is in the lower half of all cities June-September. Thunderstorms are rare. Seattle reports thunder on just seven days per year For comparison, Fort Myers, Florida reports thunder on 93 days per year Kansas City 52, and New York City 25.
There are occasional downpours. One downpour 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. People blamed heavy rain and strong winds for several power outages and at least four deaths. Interstate 5 at Chehalis, Washington was flooded and closed for almost two days. Precipitation totals exceeded 14 inches (356 mm) in some areas with winds topping out at 130 mph (209 km/hr) along coastal Oregon. It became the second wettest event in Seattle history when a little over 5 inches (130 mm) of rain fell on Seattle in a 24 hour period. People claim the rain indirectly led to five deaths and widespread flooding and damage. Early spring, late fall, and winter usually have many days when it does not rain. Winters are cool and wet with average lows in the mid 30s °F (2–4 °C) on winter nights. Colder weather does sometimes occur. Summers are very dry by comparison and warm, with average daytime highs around near 75 °F (24 °C). Hotter weather occurs during some summer days. Seattle's hottest official recorded temperature was 103 °F (39 °C) on July 29, 2009; the coldest recorded temperature was 0 °F (−18 °C) on January 31, 1950. Eastern suburbs of Seattle, such as Bellevue and Issaquah, are typically even hotter when the temperature soars above 80 °F (27 °C), due to their location closer to downslope winds from the Cascade Mountains and further from Puget Sound; on Seattle's recorded hottest day of July 29, 2009, parts of south Bellevue, Renton and Issaquah peaked at 110 °F (43 °C). Eighty miles (130 km) to the west, the Hoh Rain Forest in Olympic National Park on the western flank of the Olympic Mountains receives an annual average rainfall of 142 inches (361 cm). Sixty miles to the south of Seattle, the state capital Olympia, which is out of the rain shadow, receives an annual average rainfall of 52 inches (132 cm). Seattle typically receives some snowfall on an annual basis but heavy snow is rare. Average annual snowfall, as measured at Sea-Tac Airport, is 8.1 inches (21 cm). Single calendar-day snowfall of six inches or greater has occurred on only 15 days since 1948, and only once since February 17, 1990, when 6.8 inches 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. 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 inches even before noon. Another moderate snow event occurred from December 12–25, 2008, when over one foot of snow fell and stuck on much of the roads, causing widespread difficulties in a city not equipped for clearing snow. Seattle's daily record snowfall is 20 inches (51 cm) on January 13, 1950. The largest snowstorm on record 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 inches (74 cm) of snow on the ground by the time the event was over. A very sunny and dry climate typically dominates from May to late September. An average of 0.8 inches (20 mm) of rain falls in July and 1.0 inch (25 mm) in August. Summer thunderstorms are rare. 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 can occur north and south of town, but Seattle itself rarely receives more weather 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), not caused by the Puget Sound Convergence Zone. 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. 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.



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