Fire danger anomalies associated with foehn-like winds in southeastern Australia

In southeastern Australia the synoptic archetypical severe fire weather day is that of the dry “cool change”, or coastally modified cold front. Less well known, however, are synoptic cases that occur in connection with the topography of the eastern Australian mainland and Tasmania, which can also lead to abrupt spatiotemporal changes in fire weather variables that ultimately result in locally elevated fire danger rating. Some of these cases are associated with cross-mountain flows, mountain wind waves and foehn-like occurrences, the latter of which are characterised by warm, dry winds on the lee side of mountain ranges. In this paper we consider two foehn-like occurrences over parts of the southeast Australian mainland. In particular we focus on the Gippsland region of Victoria and the south coast of New South Wales. The events of interest are characterised by regional warming and drying at times when surface winds place the regions in the lee of significant topography. The warming, drying and strong and/or gusty winds lead to regional, statistically significant, anomalies in fire danger rating. The events presented here all resulted in McArthur Forest Fire Danger Rating at or above the 95 percentile, for the particular region and time of year. The events considered in the present paper occurred on 19 and 20 September 2008, and 27 October 2008. While these events may not have caused the extremes of fire danger rating that have been experienced in southeastern Australia, they are worthy of examination due to the magnitude and rapidity of the changes in fire weather they caused and the fact that they produced fire danger ratings that were statistically anomalous when compared to climatological values. On 20 September 2008, the rapid onset of foehn-like conditions resulted in a transition in fire danger rating from ‘Low’ to ‘Very High’ within 25 minutes, while on 27 October 2008, the onset of foehn-like conditions resulted in a transition in fire danger rating from ‘Low’ to ‘Very High’ within 36 minutes, with fire danger rating eventually reaching an ‘Extreme’ value of 62. The foehn-like occurrences are analysed based on observational data and the results of a numerical weather prediction (NWP) model. The numerical weather model suggests that the foehn-like events occurred in connection with partial orographic blocking of relatively moist lower-level winds and large-scale mountain waves. As such, the two events presented qualify as significant foehn occurrences. The observational analyses provide an initial step in discerning regions that are prone to foehn occurrence, while the numerical modelling provides more detailed information on the processes involved in the manifestation of foehn conditions at the surface. Knowledge of the extent and frequency of foehn occurrences in southeastern Australia, in addition to knowledge of the physical mechanisms that cause them, will assist in the development of more complete bushfire risk management models.