Climate Matters•August 30, 2023
2023 Fall Package
September 1 is the start of meteorological fall. The season has warmed across the U.S.
Since 1970, the fall season has warmed in 232 U.S. locations—by 2.4°F on average.
Unusually hot fall days now happen more often in 231 locations. And 86 locations now experience at least two more weeks of above-normal fall days than in 1970.
Fall warming is widespread across the contiguous U.S., and most intense in the Southwest.
Warmer fall temperatures prolong risks of heat-related illness, wildfire, allergies, and more.
The Atlantic hurricane season, forecast to be above average this year, runs through November 30. Climate Central’s reporting resources include:
Climate Matters: Hurricane Intensity and Impacts
Vulnerable Cities: Coastal Flood Risk in 23 cities along the Atlantic and Gulf coasts
Falling leaves, rising temperatures
Resulting shifts in risky heat, fire weather, allergies and more can affect ecosystems, human health, and the economy.
Climate Central analyzed the last 53 years (1970–2022) of fall (September, October, November) temperature data in 247 U.S. locations. Summaries of the findings below are based on 241 of the locations analyzed (see Methodology).
Fall is heating up everywhere — especially in the Southwest.
Fall temperatures have increased by 2.7°F on average across the contiguous U.S. since 1970.
Fall warming has been universal across the contiguous U.S., but the Southwest and Northern Rockies and Plains regions have experienced above-average warming.
Parts of Arizona, New Mexico, and Utah; all of Colorado; most of Wyoming; parts of southern California; northern Minnesota; northern Maine; and eastern Massachusetts have all experienced at least 3.5°F of warming since 1970.
In 232 U.S. cities, fall is warming
Since 1970, average fall temperatures have increased in 232 (96%) of the 241 locations analyzed.
The fall season warmed by 2.4°F on average across those 232 locations.
Nearly one-third (71) of those locations warmed by 3°F or more since 1970.
The top five fall warming locations were: Reno, Nev. (7.7°F); Las Vegas, Nev. (6.1°F); El Paso, Texas (5.7°F); Tucson, Ariz. (5.3°F); and Phoenix, Ariz (5.1°F).
More hotter-than-normal fall days
The warming season is also reflected in the number of additional fall days with temperatures that exceeded the 1991-2020 normal for the season and location.
Since 1970, 231 (96%) of the locations analyzed have seen an increase in the annual number of days above their 1991–2020 fall normal temperature.
More than one-third of locations analyzed (86, or 36%) now experience at least two more weeks of above-normal fall days than in 1970.
And 11 locations now experience 30 or more fall days above normal. These top locations were in Nevada, Texas, Arizona, New Mexico, Florida, and California.
Fall warming affects health, wildfire, agriculture, and more
The annual number of risky heat days has increased in 232 U.S. locations since 1970, according to Climate Central analysis. Those most at risk for heat-related illness include children, older adults, people experiencing homelessness, outdoor workers, residents of low income communities, pregnant people, athletes, and others.
An extended peak heat season brings higher cooling demand. Nationwide, fall cooling degree days have increased significantly since 1970. When air conditioners are run later into the year to adapt, energy costs and heat-trapping emissions increase.
A warming fall also contributes to longer and more intense allergy seasons for the millions of Americans suffering with allergies and asthma.
Hotter falls also mean a longer wildfire season. According to recent Climate Central analysis, wildfire seasons are getting longer and more intense — particularly in the western U.S. Many parts of the eastern U.S., which has nearly 28 million homes located in zones prone to burn, have seen smaller increases in fire weather.
Wildfire smoke combined with heat-driven stagnant air can further worsen air quality posing additional risk to human health.
Warming also allows disease-carrying insects such as mosquitoes to linger later into the fall. Fall warming can also disrupt the timing of ecologically important events like bird migrations, hibernation, and fruit ripening.
Warmer temperatures during both spring and fall also extend the growing season, which can benefit some agricultural crops — but also pests that can reduce crop yields.
Warmer growing seasons can also increase the risk of water stress in plants, especially in drought-prone regions of the West and Southwest that have experienced the most fall warming.
Conditions that trigger fall color are also shifting with climate change, disrupting the ecological and economic value linked to fall foliage. Fall plant cycles are complex, but our understanding of their response to climate change is growing. Learn more: Rising Temperatures, Falling Leaves.
POTENTIAL LOCAL STORY ANGLES
Is climate change influencing daily heat extremes in your local area?
Climate Central’s U.S. Climate Shift Index™ Map tool quantifies the influence of climate change on daily high and low temperatures across the entire U.S. The Global Climate Shift Index™ Map tool shows the influence of climate change on daily average temperatures across the entire globe.
How is heat affecting local health, and who is most vulnerable?
The CDC maps heat-related illnesses in its heat and health tracker. To identify the most vulnerable counties check out this extreme heat vulnerability mapping tool that combines NOAA projected heat events and CDC's Social Vulnerability Index.
What’s the air quality in your local area?
AirNow, a partnership of multiple government agencies, offers a wildfire and smoke tracking map, as well as interactive air quality maps, extensive resources in English and Spanish, including those focused on air quality and health. The National Allergy Bureau’s Aeroallergen Network provides station-level allergen reports across the U.S.
Is your local area a hotspot for insect-borne diseases?
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Fall (September, October, November) trends were calculated using data from the Applied Climate Information System from 1970-2022. Fall days above normal are relative to the 1991-2020 NCEI climate normal. Changes in average temperature since 1970 for U.S. climate divisions were calculated using data from NOAA/NCEI. Displayed trend lines on city analysis are based on a mathematical linear regression. Climate Central’s analyses include 247 cities, however, only 241 stations are included in reporting due to large data gaps in Dothan, Ala.; Hattiesburg, Miss.; Hazard, Ky.; Terre Haute, Ind.; Twin Falls, Idaho; and Wheeling, W.Va.