[Amy Graff, SFGATE]
The National Weather Service Reno issued a flood and storm warning for the northern Sierra: "A strong atmospheric river will impact the region Friday-Saturday. Significant precipitation & high snow levels will bring an enhanced flooding potential for rivers, creeks, streams and urban areas. Flood preparations should be completed by Thursday afternoon."
It's a recipe for flooding.
A tropical storm pulling moisture from the South Pacific, often called an atmospheric river, will deliver mild temperatures and heavy rain to the snow-covered northern Sierra Nevada later this week.
The current forecast indicates that with temperatures in the 50s, snow levels could rise as high as 11,000 feet on Friday. The heaviest band of precipitation is expected to soak areas of the Truckee and Tahoe basins in 3 to 5 inches of precipitation.
To put that elevation in perspective, Donner Pass is located at 7,057 feet and the top of the mountain at Squaw Valley at 9,050 feet. This warm system propelled by the remnants of a typhoon will likely dump lots of rain on snow and on soil that's saturated from the wet weather in March.
Atmospheric rivers stretching across the Pacific Ocean inundate the West Coast with torrential rain and flooding, and buries the Sierra and Cascades and Pacific Northwest mountains with massive snowfall each year, so what causes them and how do they work?
"The runoff is going to be very efficient," says Scott McGuire, a meteorologist with the National Weather Service in Reno. "The bottom line here is that significant precipitation with high snow levels could bring enhanced flooding for rivers, streams, creeks and urban areas."
Ahead of the storm, the NWS issued a flood warning for the Truckee River in Nevada and Placer counties including the town of Truckee, advising those living and traveling in those areas to monitor weather information and river levels Friday through Saturday. The Carson River and flatlands around Lake Tahoe and Mammoth Lakes are also areas of concern.
The Weather Service also warns of rock and mudslides on roads.
A cold blast of air from the Northwest is expected to hit Saturday, dropping temperatures, but the precipitation will then likely be light and snowfall minimal.
State water officials measured the Sierra snowpack Monday and found it's 52 percent of average for this time of year. Runoff from the snowpack supplies about a third of the state's water. While the level is low, it's nearly double where it stood a month ago before a series of wet storms in March.
The storm on Friday and Saturday will help replenish reservoirs and ground water but is more likely to shrink the snowpack than build upon it.
