Terrain

Seattle  gets 35" = 91 cm annually

Tim Karle

How wet is Seattle?

When comparing precipitation amounts with other cities, Seattle's reputation as a wet place seems erroneous. Officially the weather bureau lists Seattle's annual precipitation at 35 inches (90 cm).

At first glance it would appear that San Francisco and Seattle should have the same weather. Both are along the Pacific Ocean, surrounded by large bodies of water, and affected by the same large weather oscillation patterns. Yet San Francisco is drier by 13 inches.

Both cities are quite hilly, but it's the surrounding mountains that really cause the difference in precipitation. To confuse things even more, just 10 miles east, west, south, or north of downtown Seattle annual precipitation totals can easily be 10 to 20 inches more.

Precipitation totals for: Atlanta 50, Boston 43, Buffalo 41, Cleveland 38,Houston 54, Minneapolis 29, Nashville 48, New York City 48, Philadelphia 42, Portland 37, Salt Lake City 16, San Francisco 22, St Louis 38 and Washington DC 39

 

Normally,wind patterns in the Seattle area come in from the west off the Pacific Ocean. Before they reach the city, however, the Olympic mountains, just 50 miles west of Seattle, intercept most of this moist airflow. Rainfall in the Olympics approaches 200 inches a year, resulting in a temperate zone rainforest. Hilly areas surrounding Seattle intercept additional moisture so that by the time Pacific air masses reach reach Seattle most of the moisture has been removed.

But the story doesn't end here.

Directly east of Seattle the Cascade mountain range stretches for almost 800 miles from northern California to British Columbia. As prevailing southwest and west winds head inland into the Cascade more moisture is squeezed out. Annual precipitation amounts to some 60 inches, often falling as snow. The Cascades are some of the snowiest mountains in the world with annual snowfall totals sometimes measuring in the hundreds of inches.

 

Convergence Zone for Northwest wind

 

SW wind convergence zone