How Crowded Is Manhattan Project National Historical Park in September?

September runs about 1.2× Manhattan Project National Historical Park's average month, which makes it a busy month here — roughly 10% of the park's yearly visitors come in September.

In a typical September the park sees roughly 6,600 recreation visits, based on National Park Service monthly counts from 2016 to 2025.

That makes September the 5th-busiest of the park's twelve months, sitting between the July peak (1.7×) and the January low (0.3×).

On our Best Time to Visit score September rates 56/100 ("Fair"), versus the top-rated May at 56/100 — pleasant weather with noticeably thinner crowds (shoulder season).

Typical September weather, from 1991–2020 NOAA climate normals at the nearest station, averages a daytime high near 74°F and an overnight low around 49°F, with about 2" of precipitation.

That average of about 62°F is a comfortable range for hiking and being outdoors all day.

Because June, July and August together draw about 37% of annual visitors, choosing September instead steps outside that summer crush — the single biggest lever on how crowded the park feels.

You'll have about 12.2 hours of daylight in September to explore — between the 9.6-hour midwinter low and the 14.4-hour summer high.

Park-wide, 2025 drew 56,390 recreation visits, down 24.5% from 74,693 in 2024.

Across the past decade overall visitation has eased about 43.1%, so September may feel a touch quieter than it once did.