The first in a new series of atmospheric rivers flowed into California on Thursday and forecasters warned that widespread heavy rain would raise the threat of flooding in a state still digging out from earlier storms. Rain spread across the north by early afternoon. But forecasters said the heart of the atmospheric river wouldn't arrive until late in the day. The heaviest downpours were expected to last into early Friday, followed by lesser precipitation. The flood threat will come from the combination of rain and the melting of lower parts of the huge snowpack built in California's mountains by nine atmospheric rivers early in the winter and later storms fuelled by a blast of arctic air. The new atmospheric river is a type known as a “Pineapple Express” because it is a deep tap of warm subtropical moisture stretching over the Pacific to Hawaii. Its greatest impacts were expected in northern and central California, with much less precipitation in the south. The snowpack at high elevations is so massive it should be able absorb the rain, forecasters said. But elevations below 4,000 feet (1,219 meters) will see melting and runoff. So much snow has fallen in the Sierra and other mountain ranges that residents are still struggling to dig out days after earlier storms. Roofs collapsed, cars were buried and roads were blocked. Gov. Gavin Newsom declared emergencies in 13 of California’s 58 counties beginning March 1. For people close to Lake Tahoe and the Nevada state line, snow and rain are both a worry. "Some of these shaded neighborhoods have probably 8-10 feet (2.4-3 meters) in a city and the county areas even more so. With all of the snow load and all the weight that's sitting on roofs, we have significant concern about roof snow load with all the rain," said South Lake Tahoe Fire Battalion Chief Kim George. People in cities including South Lake Tahoe could be seen filling sandbags on Thursday. "Our master bedroom is on the back side. The upper side. So I just wanted to have some back there in case the water gets too high," commented local resident Ashley Arcoleo.