S
M
T
W
T
F
S
30
93°
59°
31
91°
64°
1
91°
70°
2
91°
64°
3
95°
64°
4
97°
68°
5
91°
64°
6
N/A
7
97°
66°
8
94°
66°
9
86°
65°
10
81°
64°
11
81°
61°
12
80°
56°
13
80°
60°
14
81°
59°
15
83°
61°
16
77°
59°
17
76°
61°
18
75°
60°
19
74°
58°
20
75°
59°
21
75°
57°
22
76°
59°
23
76°
58°
24
75°
56°
25
76°
56°
26
76°
58°
27
75°
59°
28
75°
57°
29
74°
57°
30
74°
59°
1
73°
59°
2
74°
58°
3
74°
55°
Temperature Graph
°F
Avg. Hi
Avg. Lo
Actual Hi
Actual Lo
Forecast Hi
Forecast Lo
*Temporarily Unavailable
September Weather in Siena
Siena's September 2026 forecast shows daily high temperatures ranging from 73° to 97°, with overnight lows between 55° and 66°. The average high for September is 78° 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 Siena. See also: October | November | December.
