When Do The First Freeze And First Frost Events Typically Occur In Kansas?

By definition, meteorological fall runs from September 1 through November 30. During this 3-month period, the weather changes dramatically. For example, in Manhattan the average low temperature falls from 62°F to 27°F, and Goodland’s normal lows drop from 56°F to 21°F. Somewhere in between the start and end of meteorological fall we expect both a first frost and a first freeze, the latter of which puts an end to the growing season. The timing varies from year to year, but when on average do these events occur in Kansas?  In this report, we take a look at when we typically experience our first frost and first freeze across the state.

Data for 20 sites in Kansas were examined, using the full period of record at each location. For the purposes of this study, the first frost date is defined as the first occurrence of a temperature at or below 36°, and first freeze is defined as the first occurrence of 32° or colder. The dates that the first frost occurred each year were sorted from earliest to latest, and then the dates on which 10%, 20%, 30%, etc., of the first frosts occurred on or before were identified. This methodology was repeated for the first freeze data. In the absence of short-term forecasts, the percentages can be considered probabilities of event occurrence on or before each given date for each location.

First Frost Probabilities

Looking at the first frost data (Table 1), the median date is in October, except for the far west and northwest areas of Kansas where it is in September. The median date is when there is a 50% chance of the first frost having already occurred. The earliest median date is in Oberlin (September 26), and the latest is in Wichita (October 18). Except for Wichita, there is a 10% probability of the first frost occurring before the first of October at all locations, and a 90% probability that the first frost has occurred by the end of October.

Table 1. Probabilities of first fall frost occurring before the given dates.

First Freeze Probabilities

There is a 60% or greater chance that the first freeze occurs by the end of October at all locations (Table 2), but the probabilities are higher for earlier dates in the west and north. As was the case with the first frost data, Oberlin has the earliest median date (October 3) and Wichita has the latest date (October 28). September freezes do happen on occasion, and occasionally it’s November before the first freeze happens (as was the case for many locations in Fall of 2021). However, October is on average the month when the growing season ends (Figure 1). You can track the current length of the growing season and when the first freeze occurs real-time at http://mesonet.k-state.edu/airtemp/min/hoursbelow/#mtIndex=7&tab=table-tab. You can also determine how long your area was below certain temperatures thresholds of interest to agriculture in the fall and through the winter.

Table 2. Probabilities of first freeze occurring before the given dates.

Figure 1. Average first Fall freeze in Kansas (Kansas Weather Data Library).

October is also the month when the normal lows each day decrease the fastest (Table 3); the daily normal temperatures drop by 11 to 14 degrees. By month’s end, Goodland, Oberlin and Tribune all have normal lows below freezing. Only locations in eastern and southern Kansas have normal lows in the 40s by October 31; all other locations have normal lows in the 30s.

Table 3. Normal low temperatures for selected dates in October across Kansas. Normals are based on the period 1991-2020.

As October draws near, monthly and 8 to 14-day outlooks will better define the chances for the first frost and freeze this year, but until then, understanding the range of possibilities helps to plan for the inevitable return of shorter days, colder nights, and sub-freezing temperatures. Until then, let’s enjoy the warmer temperatures while they last, because they won’t for much longer.