Weather in Key West in September

Shoulder season. September sits in between. You will get good days and you will lose some. You should expect rain on 23 days of the month, which is more days wet than dry.

September ranks 11 of 12 — near the bottom of Key West's year. January is the month that works, and the swing between the two is 39 points, which is a different holiday rather than a slightly better one.

Rain on 23 of the month's 30 days, 4 of them heavy. At that frequency a wet-weather backup is not a contingency, it is the itinerary — book the indoor things first and treat the clear days as the bonus. Pack for 29.4 °C days and 28.5 °C nights; the spread matters more here than any single number.

What the averages hide

Averages flatten a month, and this one hides a few things: 5 days top 30 °C — hot spells arrive, but they break; the day barely swings — 0.9 °C between the overnight low and the afternoon high. Across the twenty years measured, the hottest September day reached 30.9 °C and the coldest night fell to 24.5 °C, and the wettest single day dropped 466 mm — roughly 263% of the month's rain in one day.

This is the volatile part. September rainfall has ranged from 20 mm to 705 mm across the twenty years measured — a 685 mm spread around an average of 177 mm. The average is real but almost no individual September looks like it, so treat the figure as the middle of a wide range rather than a forecast.

Wind is light at 14.4 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 77%, high enough to notice on the warm days without dominating them.

How September sits in the year

Temperatures hold steady either side: 29.9 °C in August, 29.4 here, 28.1 in October. Nothing about the timing within the month matters much. Rain is the thing that moves: October brings 7 days less of it, so pushing the dates later buys drier weather.

Daylight in September

12.3 hours of daylight, losing 0.7 hours across the month. That is between Key West's extremes of 13.6 and 10.6 hours, so days are long enough not to constrain the plan.

Sea temperature

The sea sits at about 29.4 °C in September — warm enough that the water offers no relief from the heat. This is the warmest the water gets all year.

How busy it gets

Off season. The genuine off season: cheapest by a distance, with the caveat that some tours and restaurants close entirely.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Key West versus nearby

If Key West is one option among several, the nearby comparison is worth making. Miami is close enough to be a coin toss — within a degree on temperature, 3 days more rain. Cape Coral is close enough to be a coin toss — within a degree on temperature, 2 days more rain.

So: September is a compromise. January is 37 points better and is what Key West is actually like at its best — worth moving the trip for if that is possible, and worth adjusting expectations for if it is not.

Every month in Key West

See all twelve months in Key West, or where else to go in September.