I was eating breakfast on a Monday morning at Sears Fine Food in downtown San Francisco, casually watching the local five-day weather forecast on a television screen behind the counter. A little symbol along the bottom showed a happy-looking sun for the rest of the day. Wednesday had a friendly-looking cloud and a few raindrops, and Thursday had a dark, threatening cloud with heavier drops.

I knew Thursday’s conditions would be much rougher than the symbol conveyed. I had been studying detailed satellite data and weather models, and they indicated that a major atmospheric river (AR) was likely to hit the city. The symbol was completely inadequate for communicating the threat of the approaching storm.