The weather report says it's supposed to rain A LOT tomorrow, in a small amount of time (check out that Total Precipitable Water graph: nice Pineapple Express jet, huh? It's aimed pretty much directly at where I live. I get unhealthily interested in being at weather epicenters)
University of Wisconsin - go take a look at the animated one, it's neat.
They're predicting 3-6 inches of rain in the city in the next 24 hours, along with heavy winds. They've already canceled school in SF, Oakland, and Marin (which is supposed to get even more).
Now, to be fair, 6 inches of rain in a short amount of time creates big problems in a place that:
(1) already has saturated wet ground, so the soil can't soak up any more. This is kind of hard to explain to anyone who doesn't live in a desert, but the rain doesn't soak in, it RUNS OFF. (Or even more fun, it feeds underground springs and creeks. See also the sinkhole that opened up last week). Back in Portland the ground could absorb enormous amounts of water before it just got waterlogged and couldn't take any more and suddenly you had a pond not a backyard. The soil here is totally different.
Other cities that are used to flash floods have infrastructure to contain them. Albuquerque, New Mexico has an extensive system of storm drain arroyos to funnel off all that massive precipitation so that it doesn't just run down streets, e.g. the path of least resistance, like it will here tomorrow.
(2) Also there are a bunch of plants and trees suffering from the drought, so many of them are either dead or parts of them are dead, leading to:
(3) expected high winds at the same time == trees falling down and bringing down power lines.
So if the worst happens, it'll be fairly dangerous driving around, with pouring rain, downed trees, and possibly power outages and downed live wires.
But still, it makes me wince a little inside to think of school called for rain. Oh, California.