What is this 'atmospheric river' heading to Lake Tahoe?

Imagine a long, narrow river in the sky about 250 to 375 miles wide, moving along with the weather. It transports water vapor with a flow several times faster than the water at the mouth of the Mississippi River. A river that, when it hits land, releases 30-50 percent of the area's annual precipitation in the form or rain or snow in just a couple of days.

This "river" is called an atmospheric river, and a powerful one can create extreme rainfall and floods and is capable of disrupting travel and causing catastrophic damage.

A well-known example of a type of strong atmospheric river that can hit the west coast is the "Pineapple Express," due to their apparent ability to bring moisture from the tropics near Hawaii to the U.S. west coast.

These rivers are a primary feature in the entire global water cycle, and are tied closely to both water supply and flood risks, particularly in the west.

One of these rivers is heading to Lake Tahoe.

Rain on top of snow was widespread throughout the Sierra on Wednesday as temperatures hung around in the mid-30s. As the current system moves through Thursday, all eyes are on the arrival of "the river" Saturday, January 7.

A Flood Watch has already been issued for the Sierra, Lake Tahoe, eastern California and western Nevada this weekend. Rising snow levels and the potential to receive 6-12 inches of rain in the Sierra will create serious flooding concerns. Anyone near creeks, streams, rivers or flood prone areas should be prepared. Up to 3 inches of rain are forecast in the lower elevations around Reno, Sparks, Carson City and Douglas County.

The rain and upper elevation snow will hang around late Saturday through Monday morning.

Expect intense rainfall with snow levels as high as 9000 feet early Sunday to Monday.

The forecast from the National Weather Service:

Thursday - Snow. High near 33. South wind 5 to 10 mph, with gusts as high as 20 mph. Chance of precipitation is 90%. New snow accumulation of 2 to 4 inches possible.

Thursday Night - Partly cloudy, with a low around 11. Southwest wind around 5 mph becoming southeast in the evening.

Friday - Increasing clouds, with a high near 35. South wind around 5 mph.

Friday Night - A 40 percent chance of snow, mainly after 10pm. Cloudy, with a low around 26. South wind around 5 mph. New snow accumulation of around an inch possible.

Saturday - Snow before 10am, then rain and snow. High near 42. Chance of precipitation is 100%. New snow accumulation of 3 to 5 inches possible.

Saturday Night- Rain. Low around 36. Chance of precipitation is 100%.

Sunday - Rain. Cloudy, with a high near 43.

Sunday Night - Rain. Cloudy, with a low around 35.

Monday - Rain before 10am, then rain and snow. Cloudy, with a high near 40. Breezy.

Monday Night - A chance of snow. Cloudy, with a low around 26. Breezy.

Tuesday- A chance of snow. Cloudy, with a high near 37.

Tuesday Night - A chance of snow. Mostly cloudy, with a low around 29. Breezy.

Wednesday - Snow. Cloudy, with a high near 39. Breezy.

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