Temperatures were topping 90 degrees not too long ago, but the National Weather Service issued its first frost advisory of the year for Monday night through Tuesday morning.

Temperatures could sink as low as 33 degrees, and widespread frost could harm sensitive outdoor vegetation, the NWS said.

Affected counties in Missouri and Illinois include St. Louis County, St. Louis city, Monroe, Jefferson, St. Charles, Bond, Madison, Fayette, Randolph, St. Clair, Ste. Genevieve, Washington, St. Francois, Clinton and Marion.

The advisory is in effect from 11 p.m. Monday through 9 a.m. Tuesday.