Silent Heavy Precipitation on Madeira on 1 October 2009

Final Considerations

The heavy precipitation event on a mountain station in Madeira Island was not "visible" in most satellite products. Although ECMWF and ALADIN models identify the existence of precipitation, they underestimate significantly not suggesting an extreme event.

ECMWF forecasts and observed estimates of stability and air mass characteristics (eg TPW, CAPE or CIN) did not together (and even less alone) determined the occurrence of heavy precipitation in the mountains of Madeira. However, in combination with the detection of small convective cells on HRV imagery they provided a more comprehensive understanding of the event.

In fact, the event occurred in an unstable environment embedded in a warm and moist air mass with wet bulb potential temperature at 850hPa of 18-20 °C and TPW close to 40 mm. The fact that precipitation occurred in the mountainous area and not in lower altitudes closer to sea level suggests that the mountain triggered convection, therefore converting high amounts of precipitable water into precipitation trough orographic lifting in an already unstable environment.