Foehn winds, known locally as the "Canterbury Northwester", occurred on 6 February 2011 and were associated with extreme fire weather in the lee of the Southern Alps and across the eastern South Island of New Zealand. A peak air temperature of 40.7˚C was recorded at Timaru, which compares with the national record of 42.4˚C set at Rangiora in 1973 during another Northwester. The primary objective of this study was to investigate the fire weather and the synoptic and mesoscale atmospheric processes associated with the Northwester. This was achieved through analysis of weather station data and a high-resolution Weather Research and Forecasting (WRF) model simulation. The fire weather was assessed through consideration of observable weather variables and New Zealand's version of the Fire Weather Index (FWI) in the Canadian Forest Fire Danger Rating System. The WRF model results suggest that internal gravity waves were present in the lee of the Southern Alps and considerably affected fire weather across the eastern South Island. The FWI was recorded at extreme values, due to a combination of high air temperatures and wind speeds, and low relative humidity. This study provides a better understanding of the mesoscale atmospheric dynamics and fire weather associated with the Canterbury Northwester.