【Belgium】

California has suffered through a seemingly perpetual drought that has lasted more than half a decade now. There were high hopes that the drought fever would break last winter,Belgium thanks to an El Niño-related weather pattern. But alas, those rains and mountain snows never materialized.

Enter this winter.

There is no El Niño, which would favor a stormy West Coast weather pattern, but a parade of storms is marching across the Pacific anyway. One will hit California from Tuesday through Wednesday, while another gathers steam offshore, ready to hit the state beginning during the weekend.

Both these storms have the potential to cause significant flooding, wind damage and other hazards. They are are tied to narrow highways of moisture in the sky known as atmospheric rivers. These phenomena transport huge amounts of water vapor — the gas form of liquid water — toward the West Coast, where it will be wrung out in the form of rain and snow.

By the time these are over with, the Sierra Nevada Mountains may see up to 15 feetof new snow, although warm air flooding into the region at times will alter snow levels and limit totals in some areas. Regardless, such heavy snows will not only be a boon for previously parched ski resorts, such as Squaw Valley and Northstar, but will also mean good news for water resource managers who keep close tabs on the liquid water content in the snowpack.

The winter snow turns into spring runoff, which fills the state's reservoirs to quench the thirst of the densely populated southern part of the state.

If recent model forecasts even close to correct, much of California will face significant flood threat over next 10 days. #CAwx #CAdrought pic.twitter.com/EoSCnI5Lq8

— Daniel Swain (@Weather_West) January 3, 2017

The storms will bring heavy rain to lower elevations, including San Francisco, where several inches of rain may fall through Thursday, and again from Sunday to Tuesday of next week. The second storm may also bring heavy rain to southwest California, including California.

Even with these atmospheric river events, California’s drought won’t be over. It would take several more such events to make a more significant dent in the long-term precipitation deficit the state has accrued.

Spotter at Kingvale (elevation: 6035 ft) reports 23" ❄️in last 24 hours and 32" storm total. 😱 Much more to come! #cawx #letitsnow

— NWS Sacramento (@NWSSacramento) January 3, 2017

Atmospheric Rivers

Both storm systems are tapping into a pipeline of deep moisture that extends southwest into the tropics.

The storm that is slated to hit this coming weekend appears to be particularly potent, according to computer model guidance, aiming a firehose of water vapor at the Golden State.

Several feet of snowfall expected across the higher terrain of #NorCal thru mid-week. Mountain travel is discouraged! #CAwx pic.twitter.com/HkcrfWpayJ

— NWS Sacramento (@NWSSacramento) January 3, 2017

The air containing this moisture will hit the Sierra Nevadas and be forced to rise, cool and condense via a process known as orographic lifting. This will enhance precipitation totals there.

The National Weather Service is predicting snowfall totals of 5 feet or more through Thursday alone, with the potential for 5 to 10 feet from the next storm. It is too early to pinpoint precise 5-to-7-day snowfall totals from these events, but computer model projections are showing the potential for 15 feet in some parts of the Sierra Nevadas.

Extreme amounts of rainfall are projected for lower elevations as well, with the potential for at least 7 inches of rain through Monday in central California.

Such amounts are not unheard of in atmospheric river events, but they would be remarkable in the context of the state's worst drought on record, which began in 2012.

El Niño years tend to be more efficient at producing atmospheric rivers, since moisture and atmospheric heat are heightened above parts of the tropical Pacific Ocean at such times. 

According to Marty Ralph, a Scripps Institution of Oceanography researcher who is working to design an atmospheric river forecast system, told Mashablein 2015 that about a half dozen to a dozen storms per year typically contribute nearly half of California's annual precipitation.

While other parts of the country receive precipitation from various types of storms, such as everything from low pressure systems and warm fronts to hurricanes along the East Coast, California is highly dependent on atmospheric rivers for its annual precipitation.

Some research shows that extremely prolific atmospheric rivers could become far more common — perhaps twice as common as they are now — in parts of California by the end of the century, though this is still a matter of scientific debate. If this occurs, water managers will have to adjust to accommodate more precipitation extremes and avoid severe floods.