My edit for Seattle's wet season, which runs from October until roughly June. Everything here goes past a size L or 14, because waterproofing shouldn't stop at a medium. Umbrellas optional (and frankly a bit suspicious round here).
A reliable rain jacket that comes in proper sizes.
NORTH FACE - Antora Waterproof Rain Jacket
Wellies, but make them Chelsea boots.
HUNTER - Waterproof Chelsea Rain Boot
The belt stops you looking like a tent in the rain.
RAINS - Curve Waterproof Belted Jacket
Keeps your laptop dry on the soggiest days.
RAINS - Waterproof Backpack
Classic Blunnies, the unofficial PNW uniform.
BLUNDSTONE - Original Series Water Resistant Chelsea Boot
A long coat for proper downpours (and a little taste of home).