Here's what happens when a mega-downpour hits the lower-lying parts of San Francisco:
Storm drains or catchment basins get clogged. Stormwater fills the sewers. And pretty soon you have inundated streets and water trying to get into local homes and businesses.
I saw a demonstration of that phenomenon in action coming in to work this morning when I got to the intersection of Harrison and 14th streets, in the northeast corner of the Mission, about 7:30 this morning.
Cars were rolling through a good half-foot of water. What's the big deal? It's rain, right?