Late on January 24 surface maps revealed a moisture-laden Gulf Low developing over the southern United States, while a separate and unrelated low-pressure system was present over the Upper Midwest. In about 24 hours, the merger of the subtropical jet stream (containing a wind max of 130 knots) and polar jet stream (containing a wind max of 110 knots) would lead the low-pressure system to undergo bombogenesis as it moved rapidly northward during the evening of January 25 (record low pressures were logged across parts of the South and Mid-Atlantic).[4] Bombogenesis events require a storm's central pressure to drop more than 24 millibars in 24 hours; the Great Blizzard deepened by a remarkable 40 millibars in that span of time.[4]
As the storm headed for Ohio, this resulted in a "storm of unprecedented magnitude", according to the National Weather Service, which categorized it as a rare severe blizzard, the most severe grade of winter storm. Particularly hard hit were the states of Illinois, Indiana, Kentucky, Michigan, Ohio, and southeast Wisconsin where up to 40 inches (102 cm) of snow fell. Winds gusting up to 100 miles per hour (161 km/h) caused drifts that nearly buried some homes. Wind chill values reached −60 °F (−51 °C) across much of Ohio where 51 of the total 70 storm-related deaths occurred.[5] The second lowest atmospheric pressure ever recorded in the United States, apart from a tropical system, occurred as the storm passed over Cleveland, Ohio. The barometer fell to 28.28 inches of mercury (958 mbar) on the morning of January 26. Nearby Detroit, Michigan fell to 28.34 inches of mercury (960 mbar).
The absolute low pressure with this storm was picked up at Sarnia, Ontario at around the same time, where the barometer bottomed out at 28.21 inches of mercury (955 mbar). Toronto fell to 28.40 inches, breaking the old record by 0.17. Canada did not escape the wrath of the storm as blizzard conditions were common across southwestern Ontario. London was paralyzed by 41 centimetres (16 in) of snow
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