The pump that lifts water out when the ground won't take it — and the one piece of a wet-basement system most likely to fail at the worst moment.
A drainage system only removes water if there's somewhere lower for it to go. On a sloped lot with a downhill exit, gravity handles it. But plenty of Seattle homes sit on flat ground, or the basement floor is below the nearest storm connection — there's no downhill. A sump pump solves that: collected water from an interior or footing drain gathers in a basin, and the pump lifts it up and out to daylight or the storm system. Without it, the drain just fills and backs up.
Because it's mechanical, the sump is also the part that fails — and in Seattle it tends to fail on exactly the night you need it.
Seattle's heaviest rain often rides in on the same Pacific systems that bring the wind — and the wind takes the power out. A standard sump pump is electric, so an outage during a downpour means the pump stops precisely when the water is rising fastest. Every winter, basements flood not because the pump was bad but because it had no power.
That's why a backup is close to essential in this climate, not a luxury add-on:
A proper install is more than dropping a pump in a hole: a sealed basin sized to the inflow, a check valve so lifted water can't drain back, a discharge line run far enough from the foundation that it doesn't recycle straight back into the drain, and freeze-resistant routing for the discharge. Sizing to your actual water volume is the part cheap installs skip — an undersized pump short-cycles itself to an early death.
Most Seattle sump pumps last around 7–10 years; if yours predates that and you can't remember it being replaced, the wet season is not the time to find out. Get it checked before the first big storm.