Meteorological assessment of wildfire risk has traditionally involved identification of several synoptic weather types empirically determined to influence wildfire spread. Such weather types are characterized by identifiable synoptic-scale structures and processes. Schroeder et al. (1964) report that nearly half of all Great Lakes wildfires from April to October are observed in conjunction with northwesterly flow at mid- and upper-tropospheric levels. Such synoptic-scale regimes are often associated with the development of upper-level frontal zones and their attendant intrusions of stratospheric air into the troposphere.

In this paper we employ a fine-scale numerical simulation of the atmospheric conditions associated with the Mack Lake Fire of May 1980, performed using the PSU/NCAR MM5, in order to advance the theory that the development of an upper-level front provided the high momentum, low mixing ratio air that contributed to the fire spread in this case. The simulation demonstrates the presence of a vigorous upper-level front and an accompanying intrusion of stratospheric air into the lower troposphere (to nearly 800 hPa) in the vicinity of the Mack Lake Fire. It is suggested that the high ozone mixing ratios often observed in the wake of such wildfires may be the result of similar stratospheric intrusions. The results from this case study are used to formulate a Stratospheric Intrusion Index designed to provide fire managers with a diagnostic tool for assessing the likely influences of upper-frontal activity in wildfire growth.