The storm followed the same pattern as earlier systems this season, namely, a lot of moisture with mostly rain at lake level. If this trend continues, as some forecasters predict, it could produce flooding. When sea surface temperatures in the equatorial Pacific Ocean are neither warmer nor cooler than average, the risk of a so-called "Pineapple Express" weather pattern is twice as likely as normal.

The sub-tropical origin of the storms associated with the Express can produce both torrential rain and high snow levels. Then as warm rain melts the snow pack, devastating floods can wreak havoc on homes and businesses located in river floodplains. Nine out of ten floods in the Sierra Nevada are associated with Expresses.

Fifty years ago this Christmas week, Hawaiian-bred storms drenched Blue Canyon on the west slope with 45 inches of rain in 10 days and floodwater from the Truckee River inundated Reno. It was called "The Storm of the Century." Hopefully, we won't have a repeat performance.

Mark McLaughlin is a weather historian and writer who lives on the North Shore. You may reach him at mark@thestormking.com