A weather forecast system, method of forecasting weather and a computer program product therefor. A forecasting computer applies a grid to a forecast area and provides a weather forecast for each grid cell. Population movement sensors sense population movement in the area. A swarm detector detects patterns in area population movement that indicate swarm activity, from which the swarm detector predicts swarm patterns. A planning module receives area weather forecasts and swarm patterns, and provides swarm path indications to the forecasting system for adjusting the grid applied to the forecast area.