Blue mold of tobacco ( Peronospora tabacina ) is a foliar disease disseminated by winds in the planetary boundary layer of the atmosphere. The N.C. Blue Mold Forecast System provided timely information on the movement of inoculum (spores) into and within the Southeast and Mid-Atlantic States. The trajectory model operated on the NOAA Nested Grid Model (NGM), a numerical weather forecast model used nationally by the U.S. Air Resource Laboratory, Silver Springs, Maryland to predict 48-hour weather. The trajectories plot the atmospheric pathway of a "packet" of air containing spores and calculates the temporal position(s) of spore clouds for the 48-hour period after a spore cloud leaves a source site or region. Sources included disease sites throughout the US, Latin America and the Carribbean. A new set of trajectories could be generated each day. Forecasts include a trajectory map, a climatology describing weather at the source and along the pathway associated with spore survival, and a risk outlook. Forecasts were issued from March through August during the seedbed and field production season. The forecasts are accessible via WWW Internet (http://www.ces.ncsu.edu/depts/pp/bluemold/).