Let's start with the science. Start with cold air -- Professor Norman Miller, a staff scientist specializing in climate science at Lawrence Berkeley National Laboratory, and Dave Reynolds, the Bay Area National Weather Service forecast office, differed some on where the cold air originated. Miller mentioned the Gulf of Alaska and Aleutian Islands while Reynolds pinpointed Northwest Canada. Either way, it's a place where cold air is as ubiquitous as ice fishing, hockey, and synonyms for snow.
In any event, the cool air up north developed into a low-pressure system to the point where, Miller notes, it "dropped down along the planetary wave weather travels on. What we're seeing here is called the 'polar jet stream.'" This is where advection comes in. The cold air reached the Bay Area not long ago -- but didn't deluge us immediately. A more temperate high-pressure system kept the icy barbarians at the gate for several days. But when the high-pressure system moved on, the cold weather descended upon us rapidly. It's a bit like teenagers crashing a bar the moment the bouncer goes out for his smoke break.
So there you go. But both Miller and Reynolds noted that it's not that cold.
"It's nothing out of the ordinary. I live in the Berkeley Hills, and in the last 15 years I've seen this amount of snow in the morning at least six times," says Miller. "We're spoiled here. Every time the weather gets outside a narrow little temperature range, all of the sudden we find ourselves shaking in our boots."
Reynolds, based in Monterey, notes that he's sure some record lows have been set for this date -- "But this is not an extreme cold outbreak. It's not an unusual situation to occur at this time here in the Bay Area."
But, yes, it's nippy. Reynolds is a born-and-raised Bay Area native and he's not used to this (and, he notes, he's very pleased it will likely warm up come Thursday).
Miller is an East Coast guy. But he's been in the Bay Area for quite some time. "Yesterday I was hesitant to do certain outdoor sports. It was too cold for me," he admitted. "I opted to sit in the hot tub instead."
Hot tub? Truly this man is a Californian through and through.