S
M
T
W
T
F
S
30
95°
65°
31
97°
64°
1
99°
73°
2
99°
73°
3
100°
74°
4
98°
72°
5
96°
71°
6
96°
69°
7
92°
70°
8
89°
66°
9
90°
68°
10
86°
64°
11
84°
58°
12
82°
59°
13
84°
60°
14
81°
57°
15
83°
63°
16
84°
62°
17
84°
62°
18
83°
61°
19
83°
63°
20
84°
62°
21
83°
62°
22
83°
61°
23
82°
60°
24
83°
60°
25
82°
59°
26
83°
60°
27
82°
59°
28
82°
59°
29
81°
58°
30
81°
58°
1
80°
57°
2
80°
58°
3
79°
58°
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 79° to 100°, with overnight lows between 57° and 74°. The average high for September is 86° with an average low of 63°. 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.
