We calculated monthly soil moisture deficits from historical meteorological data from the south coast of mainland British Columbia to quantify warm and dry conditions at monthly and seasonal scales. Our main objective was to determine if climatic conditions antecedent to recorded outbreaks of western hemlock looper (Lambdina fiscellaria lugubrosa Hulst) may have triggered population growth. Periods of soil moisture deficits during the month of June were associated with the onset western hemlock looper outbreaks in the study area, as were warmer and drier conditions during the growing season two years prior to the first year of visible defoliation. Models of climate change predict that future summers in the Pacific Northwest will be warmer and potentially drier than present, and although other factors besides climate influence western hemlock looper population dynamics, the combined effects of more frequent droughts, changes in forest cover, and expanded range of western hemlock due to climate change may result in increased frequency, size and severity of outbreaks in the future.