S
M
T
W
T
F
S
30
86°
62°
31
86°
63°
1
88°
59°
2
81°
58°
3
80°
65°
4
84°
66°
5
82°
64°
6
87°
57°
7
80°
59°
8
80°
64°
9
79°
61°
10
85°
63°
11
79°
62°
12
84°
58°
13
77°
54°
14
76°
53°
15
76°
54°
16
76°
54°
17
77°
59°
18
82°
61°
19
82°
61°
20
80°
60°
21
81°
59°
22
82°
60°
23
82°
51°
24
81°
50°
25
72°
49°
26
73°
50°
27
73°
48°
28
Hist. Avg.
75°
51°
29
Hist. Avg.
75°
51°
30
Hist. Avg.
74°
50°
1
Hist. Avg.
74°
50°
2
Hist. Avg.
74°
50°
3
Hist. Avg.
73°
49°
Temperature Graph
°F
Avg. Hi
Avg. Lo
Actual Hi
Actual Lo
Forecast Hi
Forecast Lo
*Temporarily Unavailable
September Weather in O'Fallon
O'Fallon's September 2026 forecast shows daily high temperatures ranging from 72° to 88°, with overnight lows between 48° and 66°. The average high for September is 80° with an average low of 58°. 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.
