The case depicted in fig. 3 shows a typical pre-frontal situation over the Swiss Alps. A south westerly flow precedes an approaching cold front and causes some prefrontal convective developments (visible on the infrared image provided by MSG), but, as confirmed by the COALITION and TRT, during the considered period just one convective cell has developed to a severe thunderstorm (see in the center of the images). This situation is usually very favorable for the development of well-structured and long lasting severe thunderstorms, crossing the northern part of Switzerland along the pre-alpine region. Such storms very often produce hail, strong wind gusts and, depending on their velocity and extension, large amounts of rain, increasing the danger of flash floods, especially in the valleys. Sometimes such cells show supercellar features.
The COALITION outputs probabilities in four different classes and colors (fig. 11). For the considered case, at 12:00 UTC COALITION indicates the detected cell to have a probability between 50% and 75% to become severe within the next 15 minutes. This information is confirmed even in the next forecast and therefore can be considered to be robust. As shown in the reference (TRT), the considered storm cell was firstly detected at 12:05 UTC and classified as a weak thunderstorm until 12:20 UTC. During this period, the thunderstorm cell did not show any significant increase of intensity. This is confirmed by the analysis of the surrounding environment included in COALITION: most of the environmental parameters did not show favorable conditions supporting further developments. At 12:25 UTC the cell started an intensification process and at 12:40 was classified as severe.
The analysis of 80 different thunderstorm cells with different intensities demonstrated that for the cases where COALITION provides a probability greater than 25% for more than three consecutive time steps, the probability to increase its intensity and to reach the severe stage is very high.
Diagrams in fig. 4 show a first global validation of the algorithm, where the module's results are merged by fuzzy logic system . During the first 20 minutes lead time, the POD decreases from 79% to 60%, whereas the FAR increases from 26% to 39%. These first results could already be considered as useful for automatic nowcasting of heavy thunderstorms. Forecasts for lead times longer than 20 minutes, however the skill decreases often rapidly.