SAN FRANCISCO (KRON) -- The first notable atmospheric river of the season is due to impact Northern California this week and could bring heavy rain and flood risk to the Bay Area, according to the National Weather Service. The weather system is due to arrive mid-week with the heaviest impact expected in the North Bay. Flood watch issued for parts of Bay Area There are still "lots of unknowns," according to the NWS but a flood watch has been issued beginning at 4 a.m. on Wednesday and lasting through 4 a.m. Thursday. The flood watch will be in effect in the North Bay with Sonoma, Napa and Marin counties affected. 1 dead, 4 hospitalized as another boat capsizes near Bodega Bay Rainfall will be much lighter further south, according to KRON4 Meteorologist John Shrable. Potential impacts of the storm include rises on creeks, rivers and streams, flooding of creeks, streams and urban areas, and ponding on roadways and low-lying areas. The NWS has also issued frost advisories for Monday night and Tuesday morning for the interior East Bay and South Bay. Frost and freezing conditions that are potentially hazardous to unsheltered populations are expected. The NWS is advising residents to take measures to protect people, pets, plants, and pipes form the cold. What is an atmospheric river? An atmospheric river is a narrow, flowing column of condensed water vapor in the atmosphere that's responsible for producing significant levels of rain and snow, according to the NWS. When ARs move inland and sweep over mountains, the water vapor rises and cools to create heavy precipitation. Most ARs are weak systems that provide beneficial rain or snow. However, larger, more powerful ARs, like those experienced in the Bay Area over the winter of 2022-23, can create "extreme rainfall and floods," the NWS said.