S
M
T
W
T
F
S
30
71°
55°
31
76°
54°
1
74°
58°
2
70°
53°
3
69°
52°
4
69°
53°
5
74°
52°
6
73°
53°
7
70°
55°
8
68°
53°
9
72°
54°
10
70°
53°
11
72°
53°
12
72°
55°
13
71°
52°
14
70°
53°
15
72°
54°
16
66°
53°
17
65°
54°
18
65°
50°
19
66°
54°
20
64°
50°
21
64°
47°
22
65°
47°
23
63°
48°
24
62°
46°
25
63°
46°
26
67°
48°
27
68°
54°
28
63°
52°
29
62°
47°
30
69°
51°
1
66°
53°
2
62°
53°
3
63°
50°
Temperature Graph
°F
Avg. Hi
Avg. Lo
Actual Hi
Actual Lo
Forecast Hi
Forecast Lo
September Weather in Mercer Island
Mercer Island's September 2026 forecast shows daily high temperatures ranging from 62° to 74°, with overnight lows between 46° and 58°. The average high for September is 68° with an average low of 52°. 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 Mercer Island. See also: October | November | December.
