Fire danger increases as thunderstorms move into forecast Sunday to Monday The National Weather Service has upgraded the fire weather watch to a red flag warning for the Northern Sierra front due to the potential for dry lightning. Author: Tracy Humphrey (ABC10) Updated: 7:33 PM PDT July 17, 2021 SACRAMENTO, Calif. — Monsoonal moisture moving up from the south will bring the potential for thunderstorms with abundant lightning and little rainfall across portions of interior northern California Sunday through Monday. Thunderstorms may develop Sunday afternoon over the Sierra south of I-80, then spread northward into portions of the Coastal Range, the southern Cascades and the northern Sierra north of I-80 overnight.