Climatology of cold outbreaks with snow over Tasmania

Selected Hobart upper-air data, surface observations from several sites in Tasmania and reports of cold outbreaks in Bureau of Meteorology publications are studied to produce a climatology of cold outbreaks with snow over Tasmania. This climatology is used to develop an objective definition of a cold outbreak that can be applied to Tasmanian weather conditions for the purposes of climatology, forecasting and classification of extreme weather. The parameters 1000-500 hPa thickness, 500hPa temperature, freezing level and 850 hPa temperature, which are readily measured and are available from numerical model output, can be used to differentiate between air masses associated with cold outbreaks producing snow and those relating to less extreme weather. It is found that over the thirty years from 1962 to 1991 the frequency of cold outbreaks with snow over Tasmania declined by more than 60 per cent. There may be a link between this decline and climatic change in the region as revealed by studies of ocean temperatures, air temperatures and long-term variability of Tasmanian rainfall.