Widespread snowfall from 6 to 18 inches is likely in the Northeast today

Millions of people in California were under flood alerts Tuesday as an atmospheric river makes its way over a region already drenched by rain.

An atmospheric river is a plume of moisture that helps carry saturated air from the tropics to higher latitudes, delivering unrelenting rain or snow. Typically 250 to 375 miles wide, atmospheric rivers can stretch more than a thousand miles long, the National Oceanic and Atmospheric Administration says.

Atmospheric rivers aren't a new phenomenon — in the western US, they account for 30% to 50% of annual precipitation, according to the National Oceanic and Atmospheric Administration. However, the eastern half of the US also experiences atmospheric rivers, with moisture pulled from the Gulf of Mexico.

While atmospheric rivers are an incredibly important source of rainfall, they can also bring flash flooding, mudslides and landslides, sometimes killing people and destroying property.

Impact of climate change: As the world warms, the atmosphere can hold more moisture – which will lead to rainier atmospheric river events.

“It’s expected that as the air temperatures increase, the air can hold more water vapor, and therefore any storms that are comprised of water vapor will have more of it,” said Jason Cordeira, associate professor of meteorology at Plymouth State University.

Atmospheric rivers will be “significantly longer and wider than the ones we observe today, leading to more frequent atmospheric river conditions in affected areas,” a NASA-led study found. The frequency of the most intense atmospheric rivers will likely double, the study found.

A ground delay remains in effect at San Francisco International Airport on Tuesday afternoon due to winds, airport spokesman Doug Yakel said.

The average delay is 155 minutes, the Federal Aviation Administration said.

The airport has been reporting sustained winds of 40 to 50 mph with gusts fairly consistently topping 60 mph over the past 3 to 4 hours, according to CNN Weather. 

The highest gust today has been a hurricane-force 74 mph, occurring just before noon local time.

CNN Weather’s Taylor Ward contributed to this report.

Military vehicles drive through floodwaters on Tuesday, March 14, in Pajaro, California. (Justin Sullivan/Getty Images)

More than 239 people have been rescued in the Pajaro area in Monterey County, California, after heavy rain created flooding and hazardous conditions on the west coast, Monterey County Sheriff Tina Nieto said.

Speaking at a news conference Tuesday, she said officials use high-water vehicles and boats to perform those rescues. More than 2,000 people have also been evacuated from the Pajaro area which was hit hard by flooding, Nieto said.

Nieto added that the sheriff’s office does not have the resources to force people to evacuate, but pleaded for residents to listen to warnings and guidance, stressing the danger of affected areas.

She cautioned people still in Pajaro not to drink the water, adding that environmental officials are working to make sure it is safe.

For people who were evacuated and displaced from their homes, some local hotels have been taking in families, said Rob O’Keefe from the Monterey Convention and Visitors Bureau.

More than 30 hotels are participating in the evacuation program, he said, but some issues are arising when the power goes out. O'Keefe said they have also reached out to hotels in neighboring counties that still have electricity to try to work around that problem.

The storm system that is impacting California has left over 250,000 customers without power on Tuesday afternoon, according to PowerOutage.US.

Santa Clara County which is south of San Francisco is experiencing the majority of the outages with more than 74,000 customers without power, the data shows. 

Residents in California are continuing to deal with yet another storm system that is bringing the risk of devastating floods across the state.

This has led various municipalities to issue either evacuation warnings or evacuation orders.

California Governor's Office of Emergency Services (Cal OES) spokesman Gustavo Ortiz told CNN that as of Tuesday morning, there were 71,678 people under evacuation warnings and 26,911 people under evacuation orders.

According to Ortiz, most of these evacuations are in Monterey County.

Warning vs. order, what's the difference?:

There are nearly 30 million people in California who are under a flood threat from the atmospheric river storm system that is battering the state.

Note: These stats are constantly changing as this is an active weather system impacting the state.

CNN’s Taylor Ward contributed to this report.

Some areas of northern Massachusetts could still see an additional 8 inches of snow by day's end, according to the National Weather Service in Boston.

The weather service warned of a messy evening commute and gusty winds through the evening.

According to snowfall reports from the weather service, some areas in higher elevations in New England have measured more than 20 inches of snow before midday Tuesday.

The National Weather Service in Albany said that the town of Piseco in the southern Adirondacks saw nearly 2.5 inches of snow in just one hour.

Officials in Santa Barbara County, California, have issued an evacuation order for several burn areas ahead of intense rain and flooding.

Anyone living in the Alisal, Cave or Thomas Fire burn areas should leave, the county said on its website. There is an evacuation center set up at the Wake Center on Turnpike Road, it said in a tweet.

"Flooding and debris flow hazards exist due to the storm" in these areas, it said in another tweet.

The county also urged people not to try to drive when it is dark or raining and encouraged those choosing not to evacuate to be prepared in case emergency responders cannot get to them.

Santa Barbara County is under a flood watch effective until Tuesday night, according to the National Weather Service.

Here's the latest forecast across the state from the Governor's Office of Emergency Services:

The Air Force Reserve hurricane hunters are flying missions in both the atmospheric river system on the Pacific Coast and the nor’easter on the Atlantic Coast to collect data to improve the forecasts for the regions impacted by these storms. 

Until the last few years, forecasters had to rely solely on satellites and forecast models for forecasting atmospheric rivers, which can become very muddy, without truly knowing what’s happening inside the storms, according to CNN meteorologist Jennifer Gray.

The University of California-San Diego, the Scripps Institution of Oceanography and the Center for Western Weather and Water Extremes have teamed up with the hurricane hunters, who are able to drop instruments called dropsondes inside an atmospheric river to relay pinpointed live weather data. The information is immediately put into weather forecast models, which improves the accuracy of the forecast dramatically. 

Paul Horgan clears his driveway for the second time this morning as more than a foot of snow has fallen in Northern Worcester County, Massachusetts, on Tuesday, March 14. (Rick Cinclair/Worcester Telegram & Gazette/AP)

Western Massachusetts and southern Vermont report over two feet of snow from the nor’easter that is impacting the region.

Surrounding areas have reported over a foot of snowfall.

Heavy snow is forecast to continue over the interior Northeast and New England on Tuesday and will taper off going into Wednesday.

Here is a list of some of the snow reports across the Northeast and New England as of 10 a.m. ET: