Sea nettles, Chrysaora quinquecirrha, a sting jellyfish, seasonally infest the Chesapeake Bay and affect many activities on the Bay. Knowing where and when to expect this biotic nuisance may help to alleviate this problem. Maps of probable sea nettle presence are created by identifying locations where the current environmental conditions are favorable to sea nettles. This is accomplished using data derived from hydrodynamic computer models and NOAA satellite observations. The sea nettle is strongly constrained to live in water of salinity between 10-16 ppt and occurs only when the temperature of the water exceeds 20 C. Thus this simple habitat model has only to predict when these conditions occur to provide fairly accurate forecasts of sea-nettle occurrence. The Chesapeake Bay Operational Forecast System (CBOFS) is run daily to provide the temperature and salinity ranges. A graphic of the probability of occurrence is prepared and displayed on the Sea Nettle web page daily.   Sea nettle  (Chrysaora quinquecirrha)      Probable distribution of sea nettles in Chesapeake Bay determined by model output.