S
M
T
W
T
F
S
30
80°
61°
31
80°
64°
1
85°
62°
2
80°
56°
3
79°
55°
4
77°
56°
5
78°
57°
6
80°
57°
7
83°
60°
8
84°
64°
9
84°
59°
10
82°
55°
11
78°
62°
12
80°
60°
13
81°
53°
14
76°
52°
15
76°
51°
16
75°
53°
17
76°
59°
18
80°
52°
19
78°
59°
20
79°
58°
21
82°
58°
22
79°
59°
23
80°
59°
24
74°
58°
25
79°
58°
26
80°
57°
27
80°
57°
28
79°
56°
29
80°
56°
30
80°
55°
1
79°
55°
2
80°
47°
3
78°
46°
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 74° to 85°, with overnight lows between 46° and 64°. The average high for September is 79° with an average low of 57°. 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.
