Tornadoes in the United States
The Midwestern states are very prone to tornado activity, as they are part of "Tornado Alley." States included in the area that are hit by tornadoes the most are Oklahoma, Kansas, Missouri, Nebraska, Iowa, and South Dakota. The Midwestern States are often hit by tornadoes during the late spring and early summer months, especially the months of May and June. This is due to the aforementioned air mass moving northward into the Midwest and combining with another air mass that move southward from Canada. This creates unstable air, creating the potential for storms to strike the most during these months. The frequency begins to decrease in the middle of the summer, as the air mass moves northward into the Great Lakes states.
List of tornadoes in the tornado outbreak of May 18–21, 2013
The severe threat shifted towards damaging straight-line winds on May 21 as a large squall line developed across the southern states. Further north in Ontario, three tornadoes, including an EF2, touched down. Over the following five days, the system responsible for the outbreak moved very little across the Eastern United States. By May 24, it virtually stalled off the coast of New England, resulting in several days of heavy rain across the region. It later moved northeastward and was last noted by the Weather Prediction Center late on May 26 over southeastern Canada. During the storm's eleven-day trek across the United States, it produced at least 60 tornadoes, four of which were rated EF3 or higher. Over 1,000 reports of damaging winds were received by the Storm Prediction Center as well. The system's slow movement also resulted in record-breaking rains in North Dakota, New York, and Vermont. Additionally, during the overnight of May 25 – 26, unusually cold air behind the cyclone resulting in record-late snows across northern New England.