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