When hurricanes form: the season month by month
In the Atlantic, September is the month hurricanes form: since 1966, 154 of 396 Atlantic hurricanes first reached hurricane strength in September, 39% of the total, and 82% did so between August and October. The eastern and central Pacific peaks earlier and more broadly, with August the busiest month and 70% of its hurricanes forming from July through September.
Why count from 1966
The full Atlantic file goes back to 1851, and its month counts look much the same, but early storms that formed far out at sea and never met a ship are missing, and those are not spread evenly across the year. Using seasons from 1966, when satellite pictures became routine, compares like with like. The tables below use that period. For the full record the shape holds: 377 of 979 Atlantic hurricanes since 1851 reached hurricane strength in September.
May hurricanes are a Pacific speciality. The Pacific file holds 16 storms that reached hurricane strength in May, the most recent Agatha (2022); the Atlantic file holds 4 over a record a century longer.
Month by month
| Month | Atlantic | Share | Pacific | Share |
|---|---|---|---|---|
| May | 1 | 0% | 15 | 3% |
| June | 10 | 3% | 59 | 11% |
| July | 29 | 7% | 113 | 21% |
| August | 99 | 25% | 142 | 26% |
| September | 154 | 39% | 123 | 23% |
| October | 71 | 18% | 75 | 14% |
| November | 29 | 7% | 10 | 2% |
| December | 2 | 1% | 2 | 0% |
The two seasons compared
The eastern Pacific season officially opens on May 15, two weeks before the Atlantic's June 1, and both close on November 30. The data shows why. Pacific hurricanes come early: 74 formed in May or June since 1966, against 11 in the Atlantic. The Atlantic catches up and overtakes in late summer. Its September total of 154 is the largest single-month count in either basin, and October still brings 71 Atlantic hurricanes against 75 in the Pacific.
The difference comes from where the storms form. Eastern Pacific hurricanes grow in warm water off southern Mexico that is warm early in the year. Atlantic hurricanes need the main development region between Africa and the Caribbean to warm through the summer, and the long tracks of the Cape Verde storms belong to August and September.
Landfall months
Formation and landfall do not follow the same calendar. In the eastern Pacific, where hurricanes form most often in August, landfall records in Mexico peak later: 46 of the 125 Mexican landfall records from Pacific storms are dated September and 39 October, against 15 in August.
In the Atlantic the two calendars line up more closely. Of the 291 storms with a United States hurricane landfall, 107 made their first one in September, 73 in August and 57 in October. The table puts the two side by side; the Pacific column counts landfall records, kept almost only from 1989, and the Atlantic column counts storms over the whole record.
| Month | US hurricane landfalls (storms) | Mexico, Pacific landfall records |
|---|---|---|
| May | 1 | 2 |
| June | 19 | 16 |
| July | 29 | 4 |
| August | 73 | 15 |
| September | 107 | 46 |
| October | 57 | 39 |
| November | 5 | 3 |
The edges of the United States season
The earliest hurricane landfall in the calendar year on a United States coast belongs to Storm 2 of 1908, which reached North Carolina on May 29. Next comes Alma (1966), in Florida on June 9. Both are rare: only 20 of the 291 storms made their first United States hurricane landfall before July.
The latest is Kate (1985), in Florida on November 21, after Lenny (1999) on November 17. November landfalls are rare: 5 storms made their first United States hurricane landfall in November.
All storms, not only hurricanes
Counting every storm that reached at least tropical-storm strength, by the month of its first record, gives the same picture with softer edges. Since 1966, 32% of the 793 Atlantic storms formed in September and 25% in August. In the Pacific, August leads with 25% of 1004 storms, and May and June together hold 16%, against 9% in the Atlantic. At the far end, 52 Atlantic storms formed in November or December, against 31 in the Pacific.
Outside the season
Hurricanes outside the official seasons are rare but real. Over the whole record, 12 Atlantic hurricanes reached hurricane strength outside June to November, and 4 Pacific hurricanes outside May to November. The earliest in the calendar year in the Atlantic file is Storm 1 of 1938, a hurricane on January 4; Alex (2016) followed on January 14. In the Pacific, Pali (2016) reached hurricane strength on January 12. At the other end, Alice (1954) became a hurricane on December 31 and Paka (1997) on December 10.
Using this
These are counts of formation, not of landfall. A September hurricane may spend a week at sea before it reaches a coast. Each season page has a calendar showing when every storm was active and how strong it was day by day, and each landfall page shows the months when storms came ashore there.
Sources
NOAA National Hurricane Center, HURDAT2 Atlantic 1851–2025 and Northeast and North Central Pacific 1949–2025. Landsea, C. W. and J. L. Franklin, 2013, Monthly Weather Review 141, 3576–3592. Counts, ranks and totals worked out on this site; see how to read the record.