A moisture-laden system, which will be rated as a Category 3 atmospheric river, is expected to bring heavy rainfall starting Friday, targeting the South Coast. This will be the first of the season to impact the region. Rainfall warnings and flood watches have been issued to alert residents of prolonged heavy precipitation and strong winds through the weekend.Atmospheric rivers transport huge amounts of moisture, like rivers in the sky. This moisture can fuel heavy rains when it arrives on the coast.