The New Climate Normals: 1991-2020

You may have heard recently that NOAA officially rolled out the new climate normals, but you may not have known exactly what that meant. Alternatively you may not have even heard about the new normals, but you’ve probably heard your local meteorologist refer to the weather and how it relates to normal. We at STM also relate things to climate records in both our reports for our clients and in our blogs. For example, earlier this year we talked about the anomalous cold outbreak across the United States, “With temperatures 40-50 degrees below average across the Heartland, approximately 30% of available U.S. sites set cold maximum records, and about 20% set minimum records.” When temperatures are cold (or hot!), climate normals give us a standard for how extreme (or not!) they are.

Climate normals are 30-year averages for weather and climate variables, such as temperature, precipitation, and snowfall. Normals give us information on nationwide conditions, as well as information for specific locations. These averages provide a baseline that allow us to compare a location’s current weather to the weather that is typical for that location for that specific time period. This helps us know if a particular day is hotter or colder than normal, if a particular month is wetter or drier than normal, and even if the growing season is longer than normal. Note: the growing season is the period of time between the last killing frost of spring and the first killing frost of autumn.

Climate normals are updated every ten years, hence the new rollout referenced above. The latest climate normals are an average of weather conditions from 1991-2020. Climate normals utilize approximately 15,000 federal weather stations across the United States and its territories. Thousands of citizen scientists who collect local weather observations across the country also contributed to the dataset! Though climate normals are sometimes referenced as averages, they aren’t calculated by simply summing up 30 years’ worth of weather data and dividing by 30. Some stations might be missing data, others might need updated instruments, while others might change locations. Therefore, climate normals require complex statistical calculations to ensure they consistently and accurately represent the full 30-year U.S. climate period.

These normals are helpful for us as meteorologists, whether we are forecasting for the week ahead or looking back at a past event to reconstruct the weather for a forensic case. Climate normals also help a whole host of others users as well! They are used in the United States Drought Monitor, which is used by Federal, state, local, and tribal decision makers to trigger drought responses. Farmers and gardeners also plan their crops, growing season, and production schedules based on local climate normals, which include the average first and last freeze dates. Power and utility companies monitor heating and cooling degree-days and compare weather forecasts to a location’s climate normals. Private companies and local governments also use the normal snowfall to determine how much to budget for snow plows, road salt, and personnel. You probably use climate normals without even knowing it as well! Have you ever been planning for a trip and Googled “Where is the best place to visit in April?,” “What is the average temperature in Seattle, Washington in March?,” or “What’s the rainy season in Maui?”? All of these questions are answered by climate normals!

A Recap of the New Climate Normals, 1991-2020

Since two-thirds of the data (1991–2010) in the new set overlap with the previous version, changes can be subtle, depending on the region, season, and timeframe. Nonetheless, an upward shift in temperature averages is evident. Overall, temperatures are growing warming in most areas of the United States, with a few exceptions. Most notably, in the North Central US, including parts of Montana, the Dakotas, and Minnesota, temperatures are somewhat cooler than those based on 1981–2010, especially in the late winter and spring seasons. Another unique finding is that much of the Southeast is now cooler in November than it was during the last Normals update, despite October and December being warmer. Precipitation normals show that wet areas are continuing to grow wetter, while dry areas (such as the Southwest) are growing drier.

Note that as we transition into a new set of normals, shifts in the relative frequency of above- and below-normal conditions will occur. Shifts will be most discernible in areas of the country undergoing substantial warming in the last decade, as experienced in the West and Florida. In those cases, comparisons of averages to current conditions will trigger below-normal temperature days more frequently. This does not mean that conditions are “colder” in the absolute sense; in actuality, higher averages have raised the bar for warmth.

Additionally, for the first time ever, NOAA has released a set of 15-year Climate Normals (2006-2020) to meet the needs of specific sectors (such as energy an construction) who require information for periods closer to the present. Several new climate normals will be introduced for the first time, including seasonal normals representative of different states of the El Niño/Southern Oscillation (ENSO) and High-resolution Gridded Normals, which are data that represent Climate Normals at 5 km intervals north and south across the contiguous U.S. to allow for easier calculations and mapping of climate averages and departures from normal.

Note that climate normals are useful for understanding present-day conditions, they are not used to track or define long term trends or changes in the climate. For climate monitoring, NCEI produces monthly State of the Climate reports which use twentieth-century averages (1901–2000) as benchmarks. The yearly Bulletin of the American Meteorological Society State of the Climate report also relies on longer periods of record. We use the twentieth-century averages for climate monitoring for several reasons. First, the 1901–2000 baseline offers more consistency as conditions change over time and is not subject to updates every 10 years. Secondly, the period is an easy-to-understand range when discussing long-term climate change with non-technical audiences. 

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