Jefferson County Climate Profile

Jefferson County is located in the south-central part of Wisconsin. The land cover of Jefferson County is primarily agricultural, along with woodlands, wetlands, and grasslands/pasture. There is also a larger amount of urban land cover in the county, with multiple communities of more than 10,000 residents (Watertown, Whitewater, and Fort Atkinson). Jefferson County is home to a few larger lakes, including Lake Koshkonong and Rock Lake. Jefferson County has a humid continental climate, characterized by precipitation year-round and summers that are warm to hot.

Records are computed using all available station data, some of which goes back to 1895 (Source: ACIS).

Averages for freeze dates are computed for the most recent 30 years (1995-2024; Source: ACIS).

Temperatures have been increasing since 1950 in Jefferson County on the annual and seasonal time scales, which is the case for most of Wisconsin. Since 1950, annual average temperatures in Jefferson County have increased at a rate of 0.4°F/decade, which translates to an increase of 1°F every 25 years. This increase in temperature has been most pronounced in the winter months and less pronounced in the other seasons. When annual average temperatures are split into daily high and low temperatures, low temperatures have been warming at a faster rate compared to high temperatures. In particular, winter minimum temperatures have been increasing by 1°F every 14 years since 1950.

With temperatures warming, this does not mean that every year will be warmer than the last, or that more recent years will all have above-average (1895-2025) temperatures. However, Jefferson County is experiencing more years that are above average than it did in the past. For example, since 2000, Jefferson County has experienced only four years with below-average temperatures. The average for 1895-2025 is 46.4°F. Similarly, if we rank the top ten warmest years in Jefferson County since 1895 (black circles on the above chart), six have occurred since 2000, with 1998 also in the top ten. The top ten coldest years (black squares) are a bit more clustered towards the earlier years in this time period, with just one of the top ten years occurring since 2000.

On the annual timescale, precipitation has been increasing at a rate of 0.85″ per decade since 1950, which means an increase of one inch every 12 years. At the seasonal level, spring is the season of largest increase in precipitation in terms of inches per decade (0.31”/decade) and in terms of the percentage increase from average annual precipitation in the early 1950s (28%).

Annual precipitation shows a lot of year-to-year variability, which is normal within a long-term trend. However, there are two notable trends that come from this dataset. First, precipitation is increasing at a rate of 0.55 inches per decade from 1895 to 2025. That’s an increase of one inch approximately every 18 years. Second, if we look at the top ten wettest years since 1895 (black circles), seven have occurred since 2000. By contrast, all of the top ten driest years (black squares) occurred prior to 2000. These trends are quite similar to what has been observed in Jefferson County with temperatures since 1895.

Jefferson County is no stranger to sweltering summer heat, where daytime temperatures can top 90°F or even 100°F. But when are these hot days most likely to occur in Jefferson County? The State Climatology Office analyzed the likelihood of a “hot day” (daily high temperature ≥ 90°F) or “warm night” (daily low temperature ≥ 70°F) occurring between two calendar dates using historical measured temperature data from NOAA stations in Jefferson County. Based on this analysis, hot days have occurred most frequently between July 21 to 31. Warm nights have been most common from July 11 to 31. Over the past 20 years (2006-2025), Jefferson County experienced, on average, 18 hot days per year and 11 warm nights per year.

Have Jefferson County summers been getting hotter? Since 1950, summertime average temperatures have warmed by about 1.5°F, but have not warmed as much compared to other seasons in Jefferson County. Summertime low temperatures have warmed by about 2°F since 1950. By comparison, summer high temperatures in Jefferson County have not warmed as much (less than 1°F). You can read more about trends in summertime temperatures in Wisconsin in this blog post.

Jefferson County can get quite chilly once winter rolls around! Some of our coldest days of the year in Wisconsin are when the overnight low temperatures dip below 0°F. The State Climatology Office performed an analysis on the likelihood of a “very cold day” (daily high temperature ≤ 10°F) or “very cold night” (daily low temperature ≤ 0°F) occurring between two calendar dates using historical measured temperature data from NOAA stations in Jefferson County. Based on this analysis, very cold days and nights have occurred most frequently between January 21 to 31. Over the past 20 years (2006-2025), Jefferson County experienced, on average, eight very cold days per year and 22 very cold nights per year.

Across all four seasons, winter has warmed the most in Jefferson County since 1950, a trend that holds true across most of Wisconsin. Since 1950, average winter temperatures in Jefferson County have warmed by 4-5°F. Over this same time period, low temperatures in winter have warmed a bit more than the average temperatures (5-6°F). The coldest temperature of the year in Jefferson County has also shown a warming trend since 1950.

Heavy precipitation days, which for this publication refers to a one-day liquid precipitation total of two inches or more, can impact Jefferson County at all times of the year. Extreme precipitation in the winter is measured by the amount of liquid that would be present if you melted down all of the snow that fell that day. The vast majority of these heavy precipitation days (75 percent) have occurred from June through September. August is the month that has historically had the most heavy precipitation days in Jefferson County. In general, summer is the wettest season in Jefferson County, with winter being the driest season. Over the past 20 years (2006-2025), Jefferson County experienced, on average, 16 heavy precipitation days per decade.

Future climate projections from the Wisconsin Initiative on Climate Change Impacts (WICCI) indicate that by mid-century (2041-2060), under a moderate greenhouse gas emissions scenario, average annual temperatures in Jefferson County will continue to increase by approximately 4°F compared to recent averages. Winter is projected to continue to be the season of the largest temperature increase (about 5°F). The number of nights where temperatures drop below 0°F is expected to decrease as winters warm (about 5 per year), and the number of nights in summer with temperatures staying above 70°F is expected to increase. The frequency of days that top 90°F is projected to increase with a warming climate to 35-40 days per year.

Annual precipitation is expected to increase by around five percent by mid-century, according to WICCI projections. Winter and spring are projected to be the seasons of greatest precipitation increase in Jefferson County (about 10%), with summers projected to be 5% drier by mid-century. Despite projections for drier summers, the frequency of heavy precipitation days (two inches or more) is projected to increase by mid-century. 

To explore more climate projections from WICCI and learn more about future climate modeling, please visit this website.

The data in this publication are a snapshot of the historical climate data for Jefferson County. If you have questions about this publication or would like more data on a topic, please contact the Wisconsin State Climatology Office. 

For more information about how climate affects your farm, community, health, and livelihood, please check out resources from the Division of Extension and the Wisconsin Initiative on Climate Change Impacts.