Low temperatures are expected to bottom-out in the mid-30s. The advisory runs from 1-8 AM on Tuesday 5/11. weather.gov According to the National Weather service, the average date of the last 32* temperature in the Waterloo-area is between April 28-May 1. In 2020, the final day of spring to go below 32* in Waterloo was May 12, when it was 31*. weather.gov Frost can occur when the temperature falls below 36°F, especially in rural areas. Frost becomes more widespread when the temperature falls below 32°F. A hard freeze is possible when temperatures fall below 28°F. LOOK: Here are the 50 best beach towns in America