San Bernardino and Riverside County Valleys-The Inland Empire under a frost advisory until Monday morning, according to the NWS

Frost advisories are issued from May to October (but can be extended if necessary) when temperatures, winds, and sky cover are favorable for frost development. This is most likely to happen when the temperature is 36 degrees or less. In some cases, the frost is severe enough to end the growing season and is then referred to as a 'killing frost'.