Last week, the states surrounding the Great Lakes in the USA saw a deep area of low pressure barrel northwards from Ohio to Michigan, whipping up 70mph wind gusts and large waves. The storm system underwent rapid intensification in its track, with the surface pressure falling 27 millibars in 24 hours. This is meteorologically labelled as “explosive cyclogenesis”, or more informally, “a weather bomb”. For this to merit its title, the surface pressure must drop at least 24mb in 24 hours.
In its wake however, the jet stream nosedived southwards into northern America and winds swung northerly, pulling in cold air from Canada. As cold air continued to invade from the north towards the end of last week, parts of the Midwest saw their first flakes of the season as rain turned to snow. Across Minneapolis-Saint Paul, the Twin Cities of Minnesota, it is relatively rare to see snow in October, with this being the first snowfall at this time in several years; daytime temperatures barely exceeded 0C (32F) in places.
Conversely, western parts of New York, Michigan and Ontario are no strangers to high snowfall at this time of the year. These regions are particularly susceptible to the phenomenon of lake-effect snow, whereby cold air intrudes from the north-west, moves over the warm body of lake water and picks up moisture; the air rises, cools and freezes, to deposit snow on leeward slopes.
Former Super Typhoon Lan, which battered Japan last week, can also be held partially responsible for the extent of this cold snap, alongside an already weak polar vortex. As the ex-typhoon recurved into the Northern Pacific, it jolted the jet stream further southwards and helped drive cold air into parts of northern America.
