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