The lowest monthly average temperature in Seattle for December is 41 degrees. July is on average the WARMEST month. The most monthly precipitation in Seattle occurs in November with 6.6 inches. The wettest month is December with an average of 150mm of rain. Seattle weather averages and climate Seattle, Washington. January is on average the COOLEST month. July is the hottest month in Seattle with an average temperature of 18°C (64°F) and the coldest is January at 5°C (41°F) with the most daily sunshine hours at 10 in July. The highest monthly average temperature in Seattle for August is 68 degrees. A climate graph showing rainfall, temperatures and normals. Get the monthly weather forecast for Seattle, WA, including daily high/low, historical averages, to help you plan ahead. The best month to swim in the sea is in July when the average sea temperature is 14°C (57°F). The air quality index in … Monthly temperature, precipitation and hours of sunshine.