S
M
T
W
T
F
S
30
92°
70°
31
90°
63°
1
88°
61°
2
87°
61°
3
86°
64°
4
85°
63°
5
84°
65°
6
84°
68°
7
89°
66°
8
91°
65°
9
90°
65°
10
89°
66°
11
90°
61°
12
92°
65°
13
91°
65°
14
92°
66°
15
92°
65°
16
91°
65°
17
91°
64°
18
88°
60°
19
84°
59°
20
85°
60°
21
88°
61°
22
88°
62°
23
87°
61°
24
84°
58°
25
80°
55°
26
79°
55°
27
78°
54°
28
79°
55°
29
81°
56°
30
81°
58°
1
80°
56°
2
83°
57°
3
86°
61°
Temperature Graph
°F
Avg. Hi
Avg. Lo
Actual Hi
Actual Lo
Forecast Hi
Forecast Lo
September Weather in Diamond Bar
Diamond Bar's September 2026 forecast shows daily high temperatures ranging from 78° to 92°, with overnight lows between 54° and 68°. The average high for September is 86° with an average low of 61°. 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 Diamond Bar. See also: October | November | December.
