S
M
T
W
T
F
S
30
95°
63°
31
100°
65°
1
100°
70°
2
101°
73°
3
100°
73°
4
100°
73°
5
102°
74°
6
91°
73°
7
88°
62°
8
95°
59°
9
94°
70°
10
81°
69°
11
85°
66°
12
93°
65°
13
88°
67°
14
88°
62°
15
100°
69°
16
95°
73°
17
98°
71°
18
99°
72°
19
97°
70°
20
93°
70°
21
73°
64°
22
69°
59°
23
72°
59°
24
74°
60°
25
74°
56°
26
76°
60°
27
80°
61°
28
79°
60°
29
80°
58°
30
79°
58°
1
78°
59°
2
77°
55°
3
70°
61°
Temperature Graph
°F
Avg. Hi
Avg. Lo
Actual Hi
Actual Lo
Forecast Hi
Forecast Lo
September Weather in O'Fallon
O'Fallon's September 2026 forecast shows daily high temperatures ranging from 69° to 80°, with overnight lows between 55° and 64°. The average high for September is 75° with an average low of 59°. AccuWeather's monthly forecast extends further ahead than any other source, with day-by-day RealFeel® Temperatures giving a complete picture of how September weather will actually feel in O'Fallon. See also: October | November | December.
