When will the first freeze be? The history of first freezes in Iowa


Freezing weather

Despite the forecast for above-average temperatures this week, freezing temperatures will make their way to Iowa at some point this fall.

First of all, what is a fall freeze defined as?
The fall freeze represents the first time during the fall season when surface temperatures drop to 32° or colder. The extremity of the freeze is determined by the actual temperature. 29-32° would be considered a light freeze, while temperatures between 25 and 28° indicate a moderate freeze, and anything at or below 24° is considered a severe freeze. 

The average date of the first freeze in Iowa is getting later. 
From 1893-1990 the average first freeze for Des Moines, Algona, Fort Dodge, Marshalltown, Osceola, and Lamoni ranged from Oct. 5 to Oct. 17. Between 1990-2020 those same places saw an average first freeze between Oct. 7 and Oct. 20. 

Average first Freeze 1893-1990 vs 1990-2020. Maps courtesy of the Iowa Environmental Mesonet

First freeze over the past 5 years in Des Moines, Algona, and Lamoni

Latest freeze on record
The latest occurrence for the first freeze of the cold season in Des Moines is Nov. 13, which was set in 2015. Algona, among other cities, did experience a November first freeze during the early 1900s, but Nov. 8, 1900 is the latest Algona has ever experienced the first freeze of the season. Lamoni’s record for the latest first freeze was set on Nov. 12, 2016.

Will we see a freeze anytime soon?
The forecast for the next 7 days keeps highs in the 70s and 80s and lows in the 50s and 60s. The trend beyond that also looks warmer. The Climate Prediction Center’s 8-14 day outlook for Oct. 12-18 shows that temperatures will likely be above average. On Oct. 12 the average high in Des Moines is 66°, while the average low is 45°. By Oct. 18, the average high is 62° and the average low is 42°. While the overall temperature is expected to be above average, that doesn’t preclude us from not seeing a couple days with lows in the 30s. However, the likelihood of dropping all the way into the lower 30s remains very low. 

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