Sunday, May 21, 2017

Melting Snowpack Poses Dangers in the Sierra Nevada This Week, Including Major Flooding Near Nevada Towns

Chris Dolce
Published: May 21,2017

The Sierra Nevada snowpack will rapidly melt this week as very warm temperatures grip the region, raising the risk of several dangers, including the potential for major flooding near a couple of Nevada towns.
(MORE: Crews Clear Winter's Epic Snowfall in the West)

Satellite image of the snow-covered Sierra Nevada on May 19, 2017.
Snowpack in the Sierra from this past winter was still well above average to begin May. The SNOTEL network estimated water content of the snowpack in the Sierra was almost three times the early-May average.
With well-above-average temperatures in place to start this week, melting of that snowpack will be accelerated.
Rapidly increasing flows are likely on rivers and streams in the region. Flows on some waterways could be two to three times greater than normal, the National Weather Service (NWS) says.
The Walker River in western Nevada poses the greatest risk of property damage, as it's forecast to experience major to record flooding by late week near the towns of Mason and Yerington.
A crest of more than 12 feet is currently forecast for the Walker River near Mason, which could result in the inundation of some homes, businesses and farmland. Here are some of the projected impacts, according to the NWS, if the river rises over 12 feet or 12.5 feet:
  • At 12 feet: Major flooding in the Mason Valley-Yerington area. Many homes, businesses, structures, roads, bridges and well over a thousand acres of farmland and pasture near the river flood. Some portions of Yerington flood.
  • At 12.5 feet: Near-record flooding in the Mason Valley-Yerington area. Homes, businesses, farmland, pasture, roads, bridges and structures in the floodplain are flooded. At about 3,700 cfs – about like the record flood of Jan. 4, 1997 (est. 3,900 cfs), which flooded over 3,000 acres in the Yerington area – with over 500 businesses and homes affected. Central Yerington and airport were not flooded in 1997. USGS estimates that this much flow has about a one in 25 chance of being exceeded any year.
The NWS added that flooding could be much more widespread and severe in Yerington should the Walker River breach its banks near and upstream from the town.
Preparations for the potential flooding are being encouraged. Sandbagging efforts are underway and crews are working to build up levees in Yerington, according to posts on the Lyon County, Nevada, Facebook page.
Latest river status (blue line) and forecast rise (purple line) for the Walker River near Mason, Nevada.
Elsewhere, the NWS is encouraging any visitors in the larger Sierra Nevada region to avoid camping in and around rivers or streams given the projected rises and higher flows. Peak flows from snowmelt typically occur in the evening and overnight, posing a danger to campers that set up near a waterway for the night.
Another danger to consider is that the water will be very cold. That could raise the risk of hypothermia for those that spend even a small amount of time in the water.
MORE: California Flood Impacts 2017

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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