At 3 a.m. Sunday, the National Weather Service issued an updated frost advisory effective for Monday between 2 a.m. and 9 a.m. for Delaware and Philadelphia counties.

The weather service describes: “Temperatures as low as 33 degrees will cause frost to form.”

“Frost could damage sensitive outdoor vegetation. Sensitive outdoor plants may be killed if left uncovered,” the weather service says. “Take steps now to protect tender plants from the cold.”

Frost advisory: here’s what it means

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

According to the weather service, if a frost advisory is issued for your area, cover sensitive plants before sunset so that it can help retain heat near the plants, or move the plants indoors for the night , if possible.

Advance Local Weather Alerts is a service provided by United Robots, which uses machine learning to compile the latest data from the National Weather Service.