Characteristics of some days involving abrupt increases in fire danger

A class of fire-weather events has been identified recently in which the normal, often diurnal, rise and fall of fire danger is interrupted by abruptly worsening conditions, or "spikes," for which fire managers may be unprepared.Frequent observations from a site in Tasmania, Australia, show that spike events are associated with the passage of negatively tilted upper-tropospheric troughs, leading to descent into the atmospheric boundary layer of dry, high-momentum air-a result that is supported by satellite water vapor imagery. Case studies from other major fire events, both in Australia and in the Northern Hemisphere, show similar characteristics. Statistically significant differences exist between the location and placement of trough and jetstreak features during spike events and normal fire-weather events, with differences in satellite water vapor imagery features also evident. The seasonality of spike events differs significantly from other fire-weather events, with their occurrence peaking from late spring to early summer in Tasmania, in contrast to broad summer primary and midspring secondary peaks for nonspike events.