Today's moderate to heavy rains in SF came with a surprise bout of thunder and lightning. What's up with that?

A thunderclap hit at about 1:16 p.m. Thursday, and it was cause for some surprise in San Francisco given that a) we rarely get thunderstorms in warmer months, and b) this is January. Also, there have been many reports of hail around the city, via Twitter.

As this explainer has it, the phenomenon of lightning in a winter storm is called "thundersnow," and it's fairly common in the Midwest and in strong nor'easters. "Thundersnow requires very strong upward vertical air movement within a winter storm, and is usually associated with highly baroclinic low pressure systems with a source of warmer air (not necessarily above freezing) wrapping into the storm. The warmer air ingested into the system helps to introduce instability into the cold sector of the storm, which along with the ambient upward vertical motion can produce prolific lightning activity."

So, this frigid storm blowing in from Alaska is running into some warmer air, and thus: lightning and thunder. Also, it snowed on Mount Diablo in the East Bay today.

Stay dry and warm, everybody! The sun is allegedly going to poke out by the end of the afternoon!