Seattle, where it is

In Seattle, the climate is sub-oceanic, with quite mild, rainy winters and mild, fairly sunny summers.
The city is located in northwestern United States, in the state of Washington, of which it is the largest city. Although it is surrounded by water, since it lies between the Puget Sound and Lake Washington, Seattle is still 150 kilometers (95 miles) away from the Pacific coast, and it is partially protected by the mountains of the Coastal Chain which lie to the west (where the Olympic National Park is located), so it is less humid than the coastal area (for example, in Forks, a short distance from the Pacific coast, 3,000 millimeters or 120 inches of rain fall per year).
And although it is famous in the United States for being damp and cloudy, in summer, when it is protected by the high pressure system of the Pacific, there are many sunny days. However, the rains in the cold half-year are frequent, and often occur in the form of drizzle.
The average temperature of December (which in western United States is often the coldest month) is 5 °C (40.5 °F), that of August is 19 °C (66 °F). Here are the average temperatures at the Tacoma airport.
Seattle - Average temperatures
Min (°C)3346911131311842
Max (°C)8101215182124252115118
Min (°F)373739434852555552463936
Max (°F)465054596470757770595246

Although winter is mild enough, there can sometimes be cold waves, sometimes even in November, with lows going down to -10 °C (14 °F) or even below. For example, the temperature dropped to -16 °C (3 °F) in January 1950, to -15 °C (5 °F) in December 1990, and to -13 °C (8.5 °F) in November 1955, January 1957, November 1985, and February 1990.
While rainfall in Seattle is frequent, snow is rare: on average, 17 centimeters (6.8 in) of snow fall per year. However, occasionally there is a moderate snowfall. In December 2008, snow remained on the ground for more than 10 days in a row. Two particularly snowy months were then February 2019, with 50 cm (20 in) in total, and January 1969, with as much as 127 cm (50 in).
In summer, daytime temperatures are generally mild or pleasantly warm. However, there may also be heat waves, which last a few days, with highs around 35 °C (95 °F), and sometimes more. The temperature reached 40.5 °C (105 °F) in July 2009, 38 °C (100 °F) in August 1981, and 37 °C (99 °F) in July 1994.
Precipitation amounts to 950 mm (37.5 in) per year. It is more abundant from November to January, and in any case, it is frequent from October to May. However, as we have said, in summer it is reduced: the summer, in July and August, is pleasant, with cool nights, and quite sunny, with some morning fogs. Here is the average rainfall.
Seattle - Average precipitation

About 2,100 hours of sunshine per year are recorded in Seattle. As mentioned, even though the sky is often cloudy for many months a year, in summer (and even in late spring) the sunshine amount becomes good. Here are the average sunshine hours per day.
Seattle - Sunshine

In Seattle, the sea is cold all year round, as can be seen in the following table.
Seattle - Sea temperature
Temp (°C)8881012131414131198
Temp (°F)464646505455575755524846

Seattle, with Mount Rainier in the background

Mount Rainier

About 100 km (62 mi) south-east of Seattle, we find Mount Rainier, a volcano, 4,392 meters (14,411 feet) high. It is the snowiest place in the United States, or perhaps in the world (among those where snow is measured regularly): at the Paradise Ranger Station, 1,650 meters (5,413 ft) above sea level, 17 meters (55.7 feet) of snow fall per year! The volcano is covered with snow and ice from an altitude of about 2,000 meters (6,500 feet).
Here are the average temperatures at the Paradise Ranger Station.
Paradise Station - Average temperatures
Min (°C)-5-6-5-3026740-4-6
Max (°C)22361013171814931
Min (°F)232123273236434539322521
Max (°F)363637435055636457483734

Precipitation amounts to 3,000 mm (118 in) per year, of which, as mentioned, the quantity falling in the form of snow amounts to 17 meters (55.7 feet). Snow normally falls from mid-October to mid-May. Here is the average precipitation.
Paradise Station - Average precipitation

On top of Mount Rainier, at 4,392 meters (14,411 feet), the climate is polar. Here are the average temperatures.
Mount Rainier - Average temperatures
Min (°C)-19-19-19-17-14-12-9-10-11-14-17-19
Max (°C)-14-13-12-8-6-310-2-7-10-13
Min (°F)-2-2-2171016141271-2
Max (°F)791018212734322819149

Also north-east of Seattle, the Coastal Chain (see for example the Mount Baker Ski Area) is very wet, and snowy in the winter months.

When to go

The best time to visit Seattle is summer, especially in July and August, being the mildest, driest and sunniest period of the year. May, June and September are also acceptable months.