There has been a lot of talk and forecasting leading up to this parade of storms that were expected to hit California. And after all of the talk and prognosticating, the two storms that have already occurred have brought pretty much par-for-the-course Winter storm conditions.

Normally when a somewhat strong storm hits California during the Winter months, it brings copious amounts of rainfall to the coastal hills from the Oregon border through the hills north of San Francisco. Then, there is is a fairly tight rainfall gradient to the south, where south of the San Francisco Bay Area receives significantly less rainfall than the rest of the state. The one exception to this is in the Santa Cruz mountains. These mountains receive some of the heaviest annual rainfall in the entire state.

This is what has happened for the first two storms. A lot of rain fell north of San Francisco, with some of the higher elevations receiving upwards of 10 inches of rain on Thursday/Friday. There was so much rainfall that the typically flood-prone Russian and Navarro Rivers were threatening to burst their banks once again.

Above is the flood stage graph for the Navarro River at Navarro, CA. Note that the river went into a minor flood on Friday before quickly receding below flood stage. There is another people in that graph, and it is considerably higher. This is because another major storm is on its way and could produce some significant flooding.

Likewise, during major storms, the Russian River at Guerneville often floods. As you can see from the above graph, the river did not exceed flood stage Friday, but it is forecast to reach minor flood stage Sunday and into Monday.

Next storm set for Sunday

Saturday will be far from dry as showery weather is likely for many areas of Northern California. However, the main event will hit California Sunday with more heavy rain and possibly another 7+ inches in the higher elevations. This rain is what could put flood-prone rivers above flood stage.

You can see the HPC precipitation forecast for Sunday morning through Monday morning shows a bulls eye of rain along the western Slopes of the Sierra Nevadas. This brings up a very important story. In a normal Winter storm that hits California, rain turns to snow in the Sierra Nevadas. This snowfall represents a very important source of drinking and irrigation water for the Golden State. But, it is important that this precipitation falls as snow, staying in the mountains as a natural reservoir until the Spring, when it melts and flows into the various state’s manmade reservoirs. However, this storm will dump nearly all of this precipitation as rain. That liquid water immediately flows into the mountain rivers and down into the Central Valley. Thus, the biggest concern I personally have with this upcoming storm is the potential for flooding along the Truckee River at Truckee, CA. A Flood Warning has been issued in anticipation of flooding. The stage could reach 7.7 feet by late Sunday morning, with the potential to cause major flooding to bridges and homes along Highway 89 in Truckee.

from:    http://www.wunderground.com/blog/shauntanner/show.html?entrynum=222