Tornado outbreaks associated with land-falling tropical cyclones in the Atlantic Basin
Stephanie M. Verbout, University of Oklahoma, Norman, OK; and L. M. Leslie, H. E. Brooks, D. M. Schultz, and D. J. Karoly
Land-falling tropical cyclones (TCs) possess many dangerous features that affect life and property. Meteorologists are greatly challenged in forecasting the precise location and magnitude of high winds, flooding rains, and storm surge due to the unpredictable nature of a TC’s track into the mid-latitudes. In addition to the hazards listed above, forecasters also must prepare for possible tornadoes embedded within a land-falling TC. Generally speaking, embedded tornadoes occur within the outer rain bands of the right-front quadrant of the TC and tend to be weak F0 or F1 tornadoes. Moreover, there are tornado outbreaks that occur within TCs that further complicate the dangers related to land-falling TCs.
Defining what constitutes a tornado outbreak is a task researchers have tried to quantify since Galway’s (1977) earliest definition of 10 or more tornadoes associated with the same synoptic scale system. Over the past half century, the number of tornadoes reported in the United States has increased from roughly 600 per year in the 1950s to approximately 1200 today. The changes may not be entirely related to meteorological causes; but reporting discrepancies, increasing population and public awareness, advancements in technology, and National Weather Service vigilance all contribute to the increasing trend.
By gathering storm reports from 1954 to 2003 from the Storm Prediction Center’s Severe Plot program, a new definition of what characterizes a tornado outbreak was generated. From examining this 50-year data set, outbreak thresholds were constructed from a simple “inflation” adjustment by computing departures from a linear regression to the annual number of tornadoes. In addition to a numerical stipulation in the definition, an intensity criterion was added to ensure an outbreak contained at least three “significant” tornadoes (F2 or greater on the Fujita scale).
Focusing only on “big” outbreaks, it is shown that big outbreaks are tri-modal, with a maximum in April and secondary maximums in January and September. Speculating that the September mode might be associated with land-falling TCs during peak hurricane season, we examined all land-falling TCs in the Atlantic Basin during the period 1954-2003. Seventeen TCs were found to produce “big” tornado outbreaks once they intercepted land.
Comparisons between synoptic regimes, meteorological parameters and location of landfall will optimistically reveal common patterns amongst these cases and separate those TCs that did not produce big tornado outbreaks. Finding regimes and parameters that may distinguish this type of phenomena will aid forecasters in better preparing a coastal community, and communities further inland, of an additional danger that may threaten life and property.
Extended Abstract (396K)
Session 7, Tropical Cyclones
Thursday, 13 January 2005, 3:30 PM-5:00 PM
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