Tornado mother. 2008's Hurricane Ike spawned 33 tornadoes.


Larger Hurricanes Kick Up More Twisters

Larger, more intense hurricanes are bringing more tornadoes to the U.S. Gulf Coast, according to a new study. The analysis also identifies four key factors that determine how many tornadoes a given hurricane will generate in the region, which could lead to more reliable warning systems.

Meteorologists have known for some time that hurricanes tend to generate tornadoes when they make landfall. The far-reaching storms, which often measure hundreds of kilometers wide, disturb atmospheric conditions so much that they can create tornadoes just about anywhere within their sphere of influence. Tornadoes are particularly common in the so-called right-front quadrant (RFQ) of hurricanes, which pack the most intense winds. Up to now, however, little has been known about exactly how many tornadoes a particular type of hurricane can generate.

A team of researchers from the Georgia Institute of Technology in Atlanta focused on Gulf Coast hurricanes, which tend to present their RFQs for a longer time, and over a larger area, than do those along the U.S. Atlantic Coast when they make landfall. About 85% of hurricane-generated tornadoes strike the Gulf Coast, and the region has seen an uptake in hurricane frequency over the past 4 decades, compared with the previous peak period of 1948 to 1964. James Belanger, a Ph.D. candidate in atmospheric dynamics, and colleagues manually combed through all available records on the 127 hurricanes that have hit the Gulf Coast since 1948. They also added data, which was less complete and readily available, from Gulf Coast hurricanes as far back as 1920.

The results, the team reports this month in Geophysical Research Letters, show that the current Gulf Coast hurricane pattern is generating an average of 15 tornadoes per storm versus six for the period of 1948 to 1964. That may be because recent Gulf Coast hurricanes have been about one-third larger than in the past, an increase other researchers have attributed to climate change. In all, four factors seem to create more hurricane-generated tornadoes: larger storms, higher winds, a Gulf Coast versus Atlantic Coast trajectory, and drier air at mid-altitudes surrounding the storm.

The team also built a statistical model that could be used to predict the frequency of hurricane-generated tornadoes. When the researchers applied the model to two recent, major Gulf Coast hurricanes--Ike in 2008 and Katrina in 2005--it predicted that the storms would spawn 33 and 56 tornadoes, respectively. The actual totals were 33 and 58. Belanger says the model turned out so well that it can be used not only as a real-time predictor of hurricane-related tornado activity but also to reconstruct the historical record "with reasonable confidence."

The paper is "exciting" because "it offers a fresh perspective on the predictability of [hurricane-generated] tornadoes," says research meteorologist J. Marshall Shepherd of the University of Georgia, Athens. And the statistical model could help clarify how climate change is affecting tropical storm hazards, he says.