What is this 'atmospheric river' heading to the Sierra and Carson City region?

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.

One of these rivers is heading to the Sierra and western Nevada this weekend.

Rain on top of snow was widespread throughout the Sierra on Wednesday and early Thursday as temperatures hung around in the mid-30s. As the current system moves through Thursday, having brought snow already to Carson City, all eyes are on the arrival of "the river" Saturday, Jan. 7.

A Flood Warning has been issued as of Friday for rivers in Nevada and California. A Flood Watch has already been issued for the Sierra, Lake Tahoe, eastern California and western Nevada including Carson City, Carson Valley, and central Lyon County. 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 9,000 feet early Sunday to Monday. Here's the latest forecast discussion from the weather service:

Friday will be cold, but no precipitation is expected. This is the day to prepare for the incoming atmospheric river and any associated flooding potential. If you live in a flood prone location, sandbagging is recommended. The significant amounts of snow that just fell will also block drains leading to ponding of water. Take Friday to clear drains and gutters of any snow and/or debris to help with drainage.

Moisture and warm air advection will increase across the region late Friday night into Saturday morning. Current forecast simulations are showing southwest flow aloft, while surface winds remain NE-E. With plenty of cold air trapped at the surface, this can be a classic scenario for freezing rain, especially for valleys below 5500 feet. Model sounding profiles are not quite as robust in showing the warm nose needed for freezing rain, but the overall idea is still there, so will maintain the chances in the forecast. Other locations are likely to see snow or a wintry mix as thermal profiles go isothermal before changing to rain.

Mountain locations will see 6 to 12 inches of additional snow before snow levels rapidly rise Saturday into Saturday night. Widespread travel impacts are possible due to snow and ice covered roads late Friday night into Saturday morning.

Snow levels should rise rather quickly Saturday afternoon/evening as deeper moisture and warmer air arrives. Snow levels are forecast to peak by early Sunday morning, then remain steady through Sunday night. This will coincide with the most intense precipitation.

North of I-80 we`re looking at snow levels near 8,000 feet, near 9,000 feet for Reno-Carson-Tahoe, and 9,000-10,000 feet south of US-50 including Mono County.

Liquid totals along the Sierra Crest through the foothills west of US-395 could easily reach 6 to 12 inches over the course of the event. Along US-395, rain totals could reach 3 inches with up to 2 inches possible for areas to the east. Snow totals above the snow line could easily reach 3 to 8 feet with the character of the snow being very heavy and wet. Avalanche danger is likely to increase through the weekend with backcountry activities not recommended.

Snow levels will lower during the day Monday, possibly to all valley floors before the moisture is cut off. This may once again bring travel difficulties throughout the Sierra and western Nevada Monday afternoon and evening.

As far as winds, they will be strong and gusty across ridge lines, with gusts peaking around 100 mph. Lower down, winds are not anticipated to be as strong with gusts mainly in the 20 to 30 mph range at most. However, a few periodic stronger gusts are possible, especially on Sunday.

The biggest takeaway is this: We know that it will flood on Sunday and into Monday, but the question at this point is to what extent.

In Carson City, sandbags will be made available at the city's public works yard located at 3505 Butti Way. Bring a shovel. Go here for real-time maps of the Carson City area that will show areas of flooding.
More Carson City sandbag locations beginning noon Friday include:
— Third Street parking lot off of Curry
— Fire Station 52 at College Parkway
— Ross Gold Park on Snyder Avenue
— Ormsby Boulevard and West Washington Street

Lyon County has placed sand and sandbags at the following locations:
— Lyon County Animal Services, Highway 50 east of Ramsey cutoff
— Stagecoach Community Center, 8105 Highway 50
— Dayton Utilities Keystone Property, 404 Keystone Drive
— Fire Station 35 at 231 Corral Drive
— Fire Station on Dayton Valley Road, 460 Dayton Valley Road
— Dayton Utilities Yard, 200 Lakes Boulevard
— Mound House Fire Station, 56 Red Rock Road
— Silver City Community Center, 385 High Street.

Here is the current list of public sandbag fill sites within Douglas County:
— Sheridan Fire Station, 980 Sheridan Lane. Sand is located behind the station in the upper parking area
— Genoa Fire Station, 2298 Main Street; sand is located behind the station in the upper parking area
— Jacks Valley Fire Station, 3450 Jacks Valley Road, sand is located in the front parking lot, east side of station
— Johnson Lane Fire Station, 1450 Stephanie Lane. Sand is located in parking lot, west side of station
— Fish Springs Fire Station, 2249 Fish Springs Road. Sand is located in parking lot rear of building
— Ruhenstroth Fire Station, 2008 Pinto Circle. Sand is located in lower parking lot, west side of building
— Topaz Ranch Estates Community Center, 4001 Carter Drive. Sand is located in front parking lot.
— Fire District Warehouse, 941 Mitch Drive. Sand is located front parking lot, north side
— Zephyr Cover Park, Hwy 50 and Warrior Way. Sand is located at the upper parking lot

Check the latest forecast at weather.gov/Reno and check road conditions with Caltrans and NDOT.
For the latest weather discussion go here. For weather updates for Carson City, see CarsonWeather.com.

Check back for updates.