Heavy convective rain events over Québec: a forecasting tool

Heavy convective rainfall events, generally leading to flash floods, account for almost half of summer severe convective reports over Québec. Depending on soil moisture content and terrain, it has been observed over the province that as little as 25mm in one hour could cause local flash floods. Although events of more than 100mm are not uncommon over the province, MCSs and Maddox type synoptic patterns barely take place over the area.

In order to provide a forecasting tool for events over the province, all 34 convective heavy rainfall events over Québec from 2002 and 2003 have been studied and then compared to a few events prior to 2002. Results from this study intend to serve as a guide for forecasting such events. This forecasting tool lists particular operational ingredients (e.g. low-level jet, precipitable water, convergence line, etc), their favourable threshold values and positions next to each other as indicated by the operational version of the GEM model, as well as preferred sounding profiles and threshold values for related ingredients. Precipitation amount and areal extent categories have been set accordingly. Events have been split in diurnal types and nocturnal types, each having common features and threshold values leading to three forecast categories.