Rod Donavon, National Weather Service; William Gallus, Department of Geological and Atmospheric Sciences, Iowa State University


Making use of historical reports of severe weather, the months of November and December were reanalyzed using multiple definitions of severe weather outbreaks and how to classify them. Over one hundred different dates over more than 60 years were examined to look for a trend in the number of reported severe weather events; tornadoes, strong damaging winds, and hail, and their severity. These were then compared to their corresponding 850 millibar (mb) temperature maps to identify the location of the -5°C line and its association with the outbreak area.

The individual severe weather events; tornadoes, hail, strong damaging winds, were all looked at independently and overlap was looked at, and a strong tornado outbreak took precedence as the outbreak of the day. This reasoning was used in prior research that took outbreak criteria the same way and outlined that a date where tornado reports made up a certain percentage, then that day was used for tornado outbreak research. Categories were used in favor of percentages in this research.

Early results are mixed in their outcomes showing variability in the nature of severe weather. These results are not just mixed in number, but it varies by the intensity and even the type of event that occurred. These early results, despite variability, suggest a positive correlation between the retreating -5°C line and outbreaks. The implications of the research outline new scenarios and concerns for climate studies and the impact of warming on the planet.

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Leyton S. Briol


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