Of course no one really knows for certain but the odds are very high that this was indeed Tom Thomson's last work. He didn't like painting with biting insects any more than I do. Blood will drip off your nose when the little flies get up under the brim of your hat. The black flies emerged on May 24th in 1917. Tom certainly did not paint after that date.
These oils are a blur of weather energy. There was certainly a lot of thundering and crashing as well. The weather can be full of awe although some think these conditions are awful. Tom was awed. Anyone who has survived a May supercellular thunderstorm will have that memory etched into their brain.
Tom wanted to record this aftermath of a severe convective storm. Tom was looking east to northeast at the back side of the retreating supercell thunderstorm. Thunderstorms typically move west to east given the prevailing wind direction. May is also the ideal month for severe supercell thunderstorms across southern Ontario. In fact La Nina years are the also best for Ontario supercells!
Research I completed in the 1990's found a strong correlation between La Nina events over the equatorial Pacific Ocean and supercell thunderstorms over Ontario. The meteorological linkage is the jet stream and the wind shear that is required for supercells to form. It is not surprising that 1917 ranks very, very high in La Nina events between 1895 and 2015. I would have very much liked to pursue this research but alas, my career was more into training, satellite and radar meteorology and operational prediction.
Note that 1974 ranked just above 1917 in the La Nina ranking - not surprising! The Super Outbreak of April 3 to April 4, 1974 was the second-largest tornado outbreak for a single 24 hour period only marginally surpassed by the 2011 thunderstorm outbreak. At least 148 tornadoes were confirmed across the U.S. states and Ontario. The associated tornadoes were especially violent with thirty F4 and F5 tornadoes confirmed. I think my research had a lot of merit... but I digress.