This paper outlines the problems associated with sudden onset hail events on the Transport Infrastructure Ireland road network which have led to a number of road traffic accidents in recent years. In an attempt to minimise these accidents a dense network of weather observing sensors is defined and explained which enable rapid identification of events as they happen in order to warn drivers approaching the event area. The paper sets out the problem in detail and also identifies the sensors to be used in the deployed network. This incudes the novel use of IoT based sensors as supplementary to a grid of reference grade present weather detectors. The methodology for the network design is explained and the primary network is shown with reference to the need to detect hail events that can be of relatively small footprint area, hence explaining the need for a dense observing network to detect these events in real time.