Tropical storm forecast to dump up to 8 feet of wet, heavy 'Sierra cement' in the Tahoe Basin

An atmospheric river packed with subtropical moisture is at the Sierra Nevada's doorstep and forecast to dump snow Monday through Thursday on an already robust "This will be a marathon, not a sprint, as totals accumulate through early Thursday," the National Weather Service explains.

Snow levels will start around 4,500 to 5,500 feet Monday and slowly rise to 6,000 to 7,000 feet by Wednesday as the storm carries in some warmer air. In the Tahoe Basin, rain is likely at lake level on Wednesday, but on Thursday, snow levels will drop with snow possible around Lake Tahoe.

Snowfall totals below 7,000 feet in the Tahoe Basin are expected to be two to four feet and up to eight feet at elevations above 7,000 feet. The heaviest snowfall Monday night into Tuesday morning.

With a long vapor plume originating near Hawaii, the storm will be warmer and wetter, delivering a type snow know as "Sierra cement" through at least Wednesday.

Marvin Boyd, a meteorologist with the NWS Reno office, explains the dense snow has a higher water content. You typically need to melt about a foot of snow to get an inch of water, but with Sierra cement eight inches is equivalent to one inch of water.

Heavy snow on roofs can also trigger dangerous roof-avalanches with hundreds of pounds of snow and ice sliding off at once. Sierra cement can also be more dangerous on roadways as it's stickier and more slick, and it's not as popular with skiers.