As far as I can tell, they are only trying to predict the type of rain showers which originate from thunderclouds, ie. Cumulonimbus clouds. These clouds are cau

As far as I can tell, they are only trying to predict the type of rain showers which originate from thunderclouds, ie. Cumulonimbus clouds. These clouds are caused by a chain reaction starting in the lower atmosphere, where a huge mass of warm, moist air is sucked up to an altitude of 10000-60000 feet, depending on temperature, humidity and latitude. The moisture (gaseous water) in the rising air condenses at altitude due to low temperatures, and hence frees additional heat which increases the upwards vertical motion of the airmass. This causes the cloud to grow. At some point, there is too much condensed water in the cloud and the water falls back down - a rain shower. The development of Cumulonimbus clouds can be observed on weather radars. I'm guessing that they have an algorithm which, given the known conditions of temperature and humidity computes the time and size a Cb cloud has to be in order for a rain shower to start. The growth rate of a Cb cloud can be inferred given the vertical temperature distribution in the atmosphere. It could also be that things are much simpler and that precipitation can be predicted from knowing the density of water in a cloud, which is what a weather radar measures. I am not a meteorologist.