Warm Fall Days

Más días cálidos de otoño

Oct 4, 2017

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As the calendar turns to October, many people are excited to bring out their cool weather clothes. While parts of the western U.S. have already cooled enough to see snow, much of the U.S. will continue to be warmer than normal leading into this weekend.


In this week’s analysis, we look at how the fall warming trend breaks down to individual warm fall days in cities across the U.S., as the average fall temperature across the U.S. is going up as the planet continues to warm from increasing greenhouse gases. In a balanced climate, the number of days above and below normal would not change much in the long term — translating to about 45 days (out of 91 days in the fall season), but in the large majority of cities analyzed, the number of warm fall days is increasing. That upturn is especially dramatic in some places —Miami, Phoenix, San Francisco, and the Twin Cities all have more than an additional two weeks of fall temperatures above normal compared to 1970.

Warmer fall days can lead to a later first frost, extending the growing season. While this may sound good initially, there are economic and environmental consequences. Warmer weather allows pests to survive longer which can damage crops, and fall pollen season may last longer, increasing the risk of allergy-induced asthma. Warmer average temperatures also indicate a decrease in the number of cool October nights. This decrease in cool nights can delay the start of some of the traditional cold season activities, such as snow skiing in northern states, as snow may start showing up later into the year.

METHODOLOGY: We counted the number of days above the 1981-2010 NOAA/NCEI climate normal for each month of meteorological fall (September, October, November).

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