Probability of precipitation type

Based on the IFS atmospheric model ensemble with 1 control and 50 perturbed forecasts, the probability of precipitation type is calculated from the precipitation type variable which has 6 different precipitation categories: rain, freezing rain, snow, wet snow, sleet and ice pellets. The precipitation rate variable was used to classify the probabilities in different precipitation intensities.

Description & Technical Information


The probability of precipitation type is displayed combined with the instantaneous total precipitation rate (another IFS variable) to provide for example an indication of potential freezing rain events and their likely severity, heavy snowfalls, etc. In this aspect, each precipitation type is also divided in 3 different categories depending on the precipitation rate, from one minimum value to 0.2 mm/h (low intensity), from 0.2 to 1 mm/h (medium intensity) and greater to 1 mm/h (high intensity).

To define the minimum precipitation rate for each precipitation type to classify dry from precipitating, we use a daily basis verification tool that compare present weather from SYNOP observations and for each type of precipitation the frequency bias is enforced to 1 (the same number of real events than forecasted events). Six values of precipitation rate threshold are applied in order to calculate the bias for each precipitation type: 0.02, 0.05, 0.07, 0.1 and 0.12 mm/h. According to this technique, the minimum precipitation rate threshold applied for each precipitation type is: rain>0.12 mm/h, sleet>0.1 mm/h and all other types>0.05 mm/h.

Technical information

Hazard ,
References Gascon, E., Hewson, T. and Haiden, T. Probability and Most Probable Precipitation Type: Description and Verification of Two ECMWF New Products, Weather Forecast. (in preparation)
Developer ECMWF