Observing Changes in Nebraska's Climate
Key messages:
Nebraska is getting warmer and wetter.
Over recent decades, June, September, and November have all experienced pronounced warming.
Annual precipitation trends have been weakly positive since the late 19th century; however, eastern Nebraska shows negative precipitation trends in summer.
Since 2000, all locations in the state have set at least one new monthly record for the warmest average temperature and at least one record dry or record wet month.
Warming winter temperatures have led to a decrease in days with snow cover and a reduction in the number of heating degree days.
Nebraska, a landlocked state, has a continental climate with a wide range of variability. A couple of factors contribute to this variability.
a) The state has a change of 4000 feet in elevation from east to west heading toward the Rocky Mountains.
b) Access to moisture from the Gulf of Mexico is much higher in the eastern part where the elevation is lower.
c) The position in the middle of the continent makes it vulnerable to arctic air masses from the north and hot air masses coming from the southwest.
These things contribute to the climate normals, which are 30-year averages and statistics of key climate observations. The climate normals are a point of reference to compare climate conditions like temperature and precipitation to evaluate what would be normal in given locations. The current reference period is 1991-2020, and it is updated every 10 years.
Temperatures
Nebraska has cold winters, with January typically being the coldest month, and warm to hot summers, with July being the warmest. In western Nebraska, temperatures in the winter tend to be warmer than other parts of the state. It is sunnier in that region with a downslope wind from the Rocky Mountains that can elevate temperatures. Both the western and northern parts of the state can also have temperatures of -20°F or colder more regularly than other regions. In the southeast, temperatures in the summer are often warmest in the state, where higher humidity keeps minimum temperatures higher. Across the state, annual average temperatures can average from mid 40s in the northwest to lower 50s across the southeast.
Annual average temperatures have been increasing at a rate of ~0.016°F per year since 1895 and that rate of temperature increase has doubled since 1980. Historically, although there was an increasing trend, there were still years that were above and below the average temperature in the 1900s. Starting in the 1980s, temperatures showed a higher frequency being above the 20th century average. For example, four of the top 10 warmest years on record in Nebraska have occurred since 2006, with 2012 being the warmest on record for Nebraska. Precipitation has also been generally above the historical average over the past 25 years in Nebraska.
Precipitation
Annual average precipitation ranges from under 15 inches in the western Panhandle to 36 inches in the far southeast corner. Precipitation falls year-round in the state, but it peaks in May and June with the lowest in January. Precipitation can be highly variable, with periods of drought and excessive precipitation occurring with some regularity statewide. Especially in the cold season, annual precipitation can fall as snow, with a higher percentage occurring as snow in the Panhandle.
Seasonal Trends
In Nebraska, winter (considered December through February) has had the strongest warming signal of the four seasons. In the long-term, winter is warming, but in the short-term, February specifically has shown a cooling trend. The average temperatures in February spanning multiple years have been below the 20th century average, which has been linked to the increasingly erratic behavior of the polar vortex. Precipitation during the winter has increased in the short-term, while the long-term trend has remained relatively flat.
The fall months (considered September through November) has seen both long- and short-term warming. Visualizing precipitation during the fall is complicated, but generally it has seen a weakly positive trend in the long-term while having a weakly negative trend in the short-term.
In the summer (considered June to August) has seen warming, especially in the short-term. Precipitation has had a relatively flat trend in the long-term and is weakly negative trend in the short-term. Even having a slight decrease in precipitation is problematic in Nebraska where crop production and pastures are prevalent, especially if rain-fed. Less precipitation during the summer combined with a warming early summer and fall could destroy crops.
In the spring (considered March to May) has seen temperatures increasing in the long-term while having a flat trend in the short-term. In precipitation, there has been a positive trend in both short- and long-term, with there being a more weaker postive trend in the short-term.
Although the overall season is seeing a positive trend, concentrated in march, precipitation is actually negative in the short-term, which shows this time of year becoming drier than averages over the past few decades. Overall, correlating to precipitation throughout the state, there has been a trend toward fewer but more intense droughts. Regarding temperature, there have been many more new record-warm months than record-cold months occurring in the state since 2000. Statewide, there has been an increasing number of days with minimum temperatures remaining above freezing. The annual average snowfall and snow cover across the state has also shown a decreasing trend in response. Days of snow cover depends on the snowfall received and temperatures cold enough to preserve it on the ground.
HDDs and CDDs
With shifts in climate, there are changes in heating-degree days (HDDs) and cooling-degree days (CDDs). Both relate the average daily outdoor temperature to the energy used to heat or cool buildings. Historically, there were substantially more heating-degree days than cooling-degree days in a year. Nebraska has been experiencing a long-term loss of 4.8 HDDs per year and gaining about 1 CDD yearly. Moreover, the HDD decline is more pronounced in the short-term. The increase of CDDs mainly comes from the increased average temperature in months like June and September. The decrease of HDDs is spreading evenly throughout the year, but it has noticeable declines in late fall and early winter. The increasing trend of CDDs is met with the increasing averages of temperatures. If these trends continue, overall energy consumption needed to keep buildings comfortable will likely be less than it is currently in the coming years.
