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Warmest sea in United States in September

4,477 coastal locations in United States ranked by their typical September sea temperature. Region-wide average in September: 21.5°C. The top of the list is where to find the warmest swimming this month.

Warmest in September

Lake Pleasant

30.6°C September mean in United States

Regional September average

21.5°C

across all 4,477 coastal locations in United States

Vs annual mean

+5.3°C

above the year-round regional average

What the United States sea feels like in September

Water at this temperature is very pleasant to swim in. If you tend to wear a wetsuit when swimming, you might consider removing it now to avoid the body overheating.

United States monthly average 10.0°C to 22.6°C
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  • Warmest month August 22.6°C
  • Coldest month February 10.0°C
  • Annual mean 16.2°C
  • Months above 20°C 3 of 12

Where September fits in the year

September sits inside the regional pattern shown on the left. Pick a different month to see how the ranking shifts.

All twelve United States month rankings:

United States sea temperature map (live)

The map shows today's latest reading at every United States location. The list below is based on the typical September mean, useful for trip planning weeks or months ahead.

Zoom in to see individual locations. Click a pin for water temperatures. View list →
°C -2 0 1 3 5 7 9 11 13 15 17 19 21 24 27 30 35

Top 50 warmest United States locations in September

Sorted by the typical September mean built from a decade of daily readings at each location. Click any place for its 30-day chart, monthly profile, and full 10-year history.

# Location September mean
1 Lake Pleasant Arizona 30.6°C
2 Lake Powell Arizona 30.6°C
3 Phoenix Arizona 30.6°C
4 Denver Colorado 30.5°C
5 Las Cruces New Mexico 30.4°C
6 Sawyer Key, inside, Cudjoe Channel, Florida Florida 30.3°C
7 Sawyer Key, outside, Cudjoe Channel, Florida Florida 30.3°C
8 Sawyer Key Florida 30.3°C
9 Fleming Key, north end Florida 30.3°C
10 Key West, south side, White Street Pier, Florida Florida 30.3°C
11 Sigsbee Park, Garrison Bight Channel, Florida Florida 30.3°C
12 Smith Shoal Light Florida 30.3°C
13 Annette Key, north end, Big Spanish Channel, Florida Florida 30.2°C
14 Big Spanish Key, Florida Florida 30.2°C
15 Content Keys, Content Passage, Florida Florida 30.2°C
16 Crawl Key, Big Spanish Channel, Florida Florida 30.2°C
17 Horseshoe Keys, south end, Florida Florida 30.2°C
18 Howe Key, northwest end, Florida Florida 30.2°C
19 Johnson Keys, north end, Florida Florida 30.2°C
20 Little Pine Key, north end, Florida Florida 30.2°C
21 Little Spanish Key, Spanish Banks, Florida Florida 30.2°C
22 West Bahia Honda Key, Florida Florida 30.2°C
23 Big Coppitt Key, northeast side, Waltz Key Basin, Florida Florida 30.2°C
24 Bird Key, Similar Sound, Florida Florida 30.2°C
25 Boca Chica Channel Bridge, Florida Florida 30.2°C
Show 25 more ▸
26 Boca Chica Key, Long Point, Florida Florida 30.2°C
27 Boca Chica Key, Southwest end, Florida Florida 30.2°C
28 Boca Chica Marina Florida 30.2°C
29 Cudjoe Key, Pirates Cove, Florida Florida 30.2°C
30 Duck Key Point, Duck Key, Waltz Key Basin, Florida Florida 30.2°C
31 O'Hara Key, north end, Waltz Key Basin, Florida Florida 30.2°C
32 Rockland Key, Rockland Channel Bridge, Florida Florida 30.2°C
33 Saddlebunch Keys, Channel No. 3, Florida Florida 30.2°C
34 Saddlebunch Keys, Channel No. 4, Florida Florida 30.2°C
35 Saddlebunch Keys, Channel No. 5, Florida Florida 30.2°C
36 Saddlebunch Keys, Similar Sound, Florida Florida 30.2°C
37 Shark Key, southeast end, Similar Sound, Florida Florida 30.2°C
38 Snipe Keys, Middle Narrows, Florida Florida 30.2°C
39 Snipe Keys, Snipe Point, Florida Florida 30.2°C
40 Sugarloaf Key, east side, Tarpon Creek, Florida Florida 30.2°C
41 Sugarloaf Key, Pirates Cove, Florida Florida 30.2°C
42 Tarpon Creek, Upper Sugarloaf Sound, Florida Florida 30.2°C
43 Waltz Key, Waltz Key Basin, Florida Florida 30.2°C
44 Fort Zachary Taylor Florida 30.2°C
45 Key West Florida 30.2°C
46 Big Coppitt Key Florida 30.2°C
47 Stock Island Florida 30.2°C
48 Sugarloaf Key Florida 30.2°C
49 Cape Sable, East Cape, Florida Florida 30.2°C
50 East Bahia Honda Key, south end, Florida Bay, Florida Florida 30.2°C

Coldest 10 in United States in September

Same data the other way. Helpful if you want to know which United States coasts to avoid for swimming in September.

# Location September mean
1 Prudhoe Bay Alaska 2.6°C
2 Flaxman Island Alaska 2.7°C
3 Cross Island (Dinkum Sands) Alaska 2.9°C
4 Barter Island Alaska 3.0°C
5 Prudhoe Bay #2 Alaska 3.0°C
6 Barrow Alaska 3.3°C
7 Barrow (offshore) Alaska 3.9°C
8 Point Barrow, Alaska Alaska 3.9°C
9 Point Barrow Alaska 3.9°C
10 Maknik Lagoon entrance, St. Lawrence Island, Alaska Alaska 7.0°C

More on this region