United States - weather in September

September is the first autumn month in the United States.
Temperatures are mild to pleasantly warm on the coast of the Pacific Ocean (see Seattle, San Francisco, Los Angeles), where a cold sea current flows. In the south-central part (see San Francisco), September is even the warmest month of the year.
Compared to summer, temperatures decrease especially in the north, in the Rocky Mountains (see Helena, Denver), in the Great Plains (see Fargo), in the Great Lakes region (see Chicago) and in the north-east, where the first cold nights occur, especially in the second half of the month.
At the end of September, after the first cold (but not freezing) nights, in the northernmost states (Montana, North Dakota, Maine, Vermont), the trees begin to take on autumn colors (fall foliage), which are often more intense and spectacular than in Europe. The phenomenon will be more evident in October, when it will peak in the north and spread to the rest of the country.
In the south, on the other hand, September is a hot month, still with summery characteristics.
The rains in September are still rare in the north-west (see Seattle), where, however, they begin to increase compared to the summer months, and are very rare in central-southern California, where the rainy season is narrower and corresponds to winter.
In the rest of the territory, the rains are moderate to abundant, even though they often decrease a little compared to August.
Thunderstorms can still break out in the Great Plains, where it rains little in the rest of the year, while they are still frequent in the South and in Florida. Also, in the arid southwestern states (Arizona, New Mexico, West Texas), July through September is the time of the "North American monsoon," which can bring thunderstorms from the Gulf of California.
From June to November is the season of hurricanes, which can affect the states bordering the Gulf of Mexico (Texas, Louisiana, Mississippi, Alabama and Florida) and the southernmost states of the Atlantic Ocean (Georgia, North and South Carolina). Occasionally, hurricanes can move northward and affect the other Atlantic coastal states as well.
More rarely, a hurricane from the Pacific to the west of Mexico, usually weakened, can affect the southwest (southern California, Arizona, New Mexico). In September we are in the period when hurricanes are most likely, which runs from August to October.

The sea is warm enough for swimming in the Gulf of Mexico, in Florida and in the Atlantic coast states up to Virginia.
Instead, to the west, on the Pacific Ocean, the sea is cold even in California, due to the aforementioned cold sea current.


See also: United States, the weather in August - October
The climate of United States