Weather in Philadelphia in September

Prime time. If you get to pick your dates, September is the one. Highs average 26.5 °C, lows 15.1 °C, and rain falls on 9 days — nothing in the month is working against you.

September is the highest-scoring month of the twelve. The nearest rival is October, and the month at the other end of the year is January.

Rain on 9 days, 3 of them heavy. Enough to need a plan B, not enough to need a plan around it. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

Averages flatten a month, and this one hides a few things: 6 days top 30 °C — hot spells arrive, but they break. Across the twenty years measured, the hottest September day reached 36.7 °C and the coldest night fell to 5.2 °C, and the wettest single day dropped 77 mm — roughly 75% of the month's rain in one day.

September is moderately variable: 19 mm in the driest year against 196 mm in the wettest, around an average of 102 mm. Roughly two Septembers in three land near that average, and the third does something else entirely.

Wind is light at 5.8 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 74%.

How September sits in the year

The month is on the way down: 3.1 °C cooler than August, with October dropping another 7 °C. The first week and the last are not the same trip.

Daylight in September

12.5 hours of daylight, losing 1.3 hours across the month. That is between Philadelphia's extremes of 15 and 9.4 hours, so days are long enough not to constrain the plan.

An inland climate

With no coast nearby, the year swings a fair way — 34.9 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

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

September in Philadelphia versus nearby

If Philadelphia is one option among several, the nearby comparison is worth making. New York City is close enough to be a coin toss — 3.3 °C cooler, the same number of rain days. Washington is close enough to be a coin toss — within a degree on temperature, 1 day more rain.

So: September is the answer for Philadelphia. If it is booked out or priced out, October is the next best thing at 3 points behind, and January is the month to keep off the shortlist.

Every month in Philadelphia

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