Abstract : N.2
Heuristic forecasting of orographic precipitation
The prediction of the location, the onset and the let-up time of stationary heavy orographic precipitation embedded in frontal passages is a difficult task, especially in an operational weather centre where high precision forecasts are needed and fast decisions have to be taken. We are currently developing a probabilistic heuristic nowcasting system which makes use of the strong orographic forcing existing in the Alpine environment. In a first step, we look for possible predictors of the heavy rain, such as the direction and the strength of the flow on different vertical levels. Estimates of rain amount and winds are obtained from the observations collected by the Swiss weather radars, which are available every few minutes with high spatial resolution, and then are suitable for nowcasting purposes. First results from a preliminary analysis of a number of past events that occurred in the Lago Maggiore region in the Southern Alps suggest a strong relation between stationarity and intensity of orographic precipitation and direction and intensity of low-level inflow, cross-barrier flow and upper-level flow. This relation can be exploited for operational nowcasting. In a next step, the stability of the upstream airmass will be investigated, and other sources of data will be considered, such as weather stations and soundings.