Jellyfish season calendar
Jellyfish blooms follow water temperature, rainfall and currents, so the season slides earlier as you head south. This grid is the typical pattern by region — the map is what is actually happening today.
| Region | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| New England | Quiet | Quiet | Quiet | Light | Moderate | Moderate | Moderate | Moderate | Moderate | Light | Quiet | Quiet |
| Mid-Atlantic | Quiet | Quiet | Quiet | Light | Light | Moderate | Peak season | Peak season | Moderate | Light | Quiet | Quiet |
| Chesapeake & Delmarva | Quiet | Quiet | Quiet | Light | Light | Moderate | Peak season | Peak season | Peak season | Light | Quiet | Quiet |
| Southeast | Light | Light | Light | Moderate | Moderate | Moderate | Peak season | Peak season | Peak season | Moderate | Moderate | Light |
| Florida Atlantic | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate |
New England
Lion's mane arrive with the cold spring water and fade by midsummer; moon jellies take over from June on. Genuine stings are uncommon north of Cape Cod.
Mid-Atlantic
Sea nettles build through July as the back bays warm past 78°F. Ocean beaches usually stay clearer than the bays behind them.
Chesapeake & Delmarva
The nettle capital of the coast. August and September in the bays are the worst of it; the surf side is often fine on the same day.
Southeast
Cannonballs dominate late summer and are harmless. The real hazard is man o' war blown ashore on winter and spring easterlies.
Florida Atlantic
Something is always in the water. Sea lice peak in spring, man o' war on winter winds, box jellies in late summer at dawn and dusk.
What moves the season
- Water temperature. Sea nettles get going once inshore water passes roughly 68°F and boom in the upper 70s and 80s.
- Rainfall and salinity. A dry spring pushes saltier water further up bays and estuaries, and nettles follow it inland. Wet years keep them near the mouth.
- Wind and tide. Onshore wind and an incoming tide pile drifting jellies onto the beach; offshore wind clears them out within hours.
- Currents. Man o' war arrive on the Gulf Stream and southeast wind, mostly in winter and spring in Florida and after storms further north.