- In September, changes in temperature and daylight are noticeable.
- The tropics are usually very busy when the Atlantic hurricane season reaches its peak.
- In some areas the first snow of the season can also be expected.
- In California, the Santa Ana winds often return.
September is typically a transition month between summer and fall, with weather ranging from hurricanes to snow.
Here’s a closer look at the weather changes to expect this month.
Cooling: Towards the end of the month, highs and lows begin to drop noticeably. For many, one of the clearest signs that fall is here are the cool, crisp mornings that become more frequent towards the end of September.
Average September lows range from 50 to 59 degrees Fahrenheit across much of the Lower 48, while parts of the northern and higher elevations see lows ranging from 30 to 41 degrees Fahrenheit. Across much of Florida, along parts of the Gulf Coast and the desert southwest, morning lows remain at 70 degrees Fahrenheit.
In Boston, for example, lows at the beginning of the month are around 18 degrees Celsius, but the average temperature on September 30 is 12 degrees Celsius. In Denver, the average temperature on September 1 is 13 degrees Celsius, but drops to 7 degrees Celsius by the end of the month.
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But the heat remains: Average maximum temperatures for this month are between 15 and 21 degrees Celsius in much of the northern part, while in the southern part of the USA temperatures are still around 25 degrees Celsius and in parts of Florida, Texas and the desert in the southwest temperatures even reach between 32 and 38 degrees Celsius.
For example, the average high temperature in Atlanta is 30 degrees in early September, but by the end of the month it is around 27 degrees. In Minneapolis, the average high temperature at the beginning of the month is over 25 degrees, but drops to around 18 degrees by early October.
Shrinking daylight: The number of hours of sunshine decreases in September, with the autumn equinox occurring this year on September 22. The autumn equinox occurs when the sun shines directly on the equator, meaning that both the Northern and Southern hemispheres receive equal amounts of daylight. In the United States, daylight duration at the equinox is not exactly 12 hours, but it is close in many places.
In areas further north, daylight duration changes more dramatically in September. In the northern regions of the country, September loses more than an hour of daylight from the beginning to the end of the month. In Anchorage, Alaska, daylight on September 30 is almost 2 hours and 45 minutes shorter than on September 1.
As the month progresses, sunrises rise later, but sunsets are earlier and more noticeable, signaling to many that winter is approaching.
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Peak of the hurricane season: September is typically the month with the highest number of hurricanes in the Atlantic. This is because September has the best overlap of conditions for hurricane formation over the largest area of the Atlantic Ocean.
In a typical September, four named storms form in the Atlantic, two to three of which develop into hurricanes and one of which reaches major hurricane status (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale).
The last few Septembers have been very busy. Six storms formed last September. In September 2022, two major hurricanes occurred – Fiona and then Ian. In September 2020, there were 10 named storms with four hurricanes.
Snow… really: It may sound “too early,” but there are some areas where snow can still fall in September.
It is most common at higher elevations and in Alaska, where Fairbanks receives an average of 2.5 inches of snow in September. Cheyenne, Wyoming, averages just under an inch, although measurable snowfall does not occur every September.
Snow also fell in parts of the Plains, the Great Lakes and northern New England in September. On Mount Washington in New Hampshire, the average snowfall in September is 30 mm.
In early September 2020, one of the earliest Front Range snowstorms on record brought snow from Montana to New Mexico.
Last September, a historic snowstorm in the northern Rocky Mountains broke September records, bringing up to 4 feet of snow.
Pedestrians walk down a snow-covered street lined with trees still bearing their leaves during a fall snowstorm in Helena, Montana, on Sunday, Sept. 29, 2019.
(AP Photo/Matt Volz)
Monsoon to Santa Ana: One change in the westerly wind pattern is the return of the Santa Ana winds. Strong high pressure over the west leads to an offshore wind pattern where wind from the higher elevations of the desert areas flows through canyons and passes in Southern California. Wind speeds increase as the wind is channeled through the mountain passes. The Santa Ana winds are often accompanied by high temperatures.
The combination of low humidity and strong winds can increase the risk of fire, especially in early fall when California’s dry season ends.
Meanwhile, the southwest monsoon season ends at the end of September. This is also due to a change in wind and temperature patterns in the region.
In summer, winds blow from the sea to the southwest, bringing humid weather to the region. In autumn, this changes as the wind blows from the land to the sea as the land begins to cool and the water is warmer.
Linda Lam is a senior meteorologist at weather.com and began her career at The Weather Company (formerly WSI) in 2006.
