WebGIS is a kind of distributed information system which holds the potential to make geographic information available worldwide. It is cost effective and provides an easy way of disseminating geospatial data. This paper outlines the design and development of a WebGIS based Decision Support System (DSS) for disseminating Nowcasting of Extreme Orographic Rain events generated at regular intervals from (NETRA) model. Dissemination of events include heavy rainfall alerts all over India and cloudburst alerts over Western Himalayan Region every half an hour. In India, natural calamities like flood and cloudburst results in lot of causalities. If any early Heavy rain alerts dissemination system is developed then it will protect several lives and mitigate damage of property or infrastructure in affected areas. The development of such WebGIS based decision support system originates from this concept. Objective of this paper is to describe the near real time WebGIS based Decision support System developed for disseminating rainfall alerts to the general public and administrators about heavy rain (all over India) and cloud burst (over Western Himalayan region) using interactive maps. Users can also get non spatial information like number of affected cities and their names, district level population (census 2011), forecast date and time, Radius of influence etc. This WebGIS based decision support system can help government agencies, NGO’s and general public in planning to save lives, properties and can be used for decision making to reduce economic and material loss from the resulting floods. This paper also illustrates use of open source technologies for developing such WebGIS -DSS at low cost. The principal development component includes: GeoServer, Java, PostgreSQL, OpenLayers, and GeoExt. The framework of the system can be divided into two categories:(1) Dissemination system which includes visualization of centroid and precise locations of Heavy Rain all over India and clo