NU Online News Service, July 6, 2:04 p.m. EDT

In the aftermath of last year's record-breaking thunderstorm losses exceeding $26 billion in the U.S., risk modelers are scratching their heads to figure out if the country's storm exposure profile is increasing due to historically inadequate modeling methods or from other factors such as climate change.

"Severe thunderstorms are a little different from other modeled catastrophes such as hurricanes, since the overall losses from [an] event is an aggregate of a number of other small thunderstorm occurrences, referred to as 'micro-events'," said AIR Principal Scientist Tim Doggett in a June webinar about the nation's current and future trends in storm modeling.

He continued, "While it's rare for any individual thunderstorm to result in mass losses, when we get a large collection of these micro events, the total losses for the outbreaks are what we refer to as 'macro events'. From a modeling perspective, micro event details are very important in determining losses at the local level."

The news-breaking events of 2011, including the tragedies in St. Louis and Joplin, Mo., and Tuscaloosa, Ala., were actually part of more than 596 tornado, 1,600 hail and 1,305 wind events reported throughout the country.

Severe thunderstorm losses, while volatile, stayed at a mean of about $15 billion since 1990. Traditionally, Texas has been the hardest hit state in the past decade, with average annual losses topping $1 billion from 1991 to 2011.

However, says AIR, historical event analysis is no longer sufficient for managing severe storm risk due to the volatility of historical loss data and the limited nature of claims data.

Firstly, severe thunderstorm reports rely on human reports instead of standardized instrumentation, and reporting frequency has grown along with the U.S. population. Furthermore, internet communication, weather RADAR and storm spotters have impacted the data collection process over time.

Secondly, not all winds are the same in the way they damage buildings. Straight-line winds tend to impact garage corners, roofs and trees, while tornado damage intensity depends on the path's length and width, wind speeds, degree of torsion stress and whether it kicked up wind-borne debris.

In addition, some building materials bend more easily under certain wind speeds. Concrete and steel, for example, stand up against all but the strongest tornadoes, and high-rise buildings are not as vulnerable to winds as are low-rise structures.

On top of the soft management of thunderstorm statistics, long-term climate factors such as the El Niño-Southern Oscillation (ENSO) phenomenon, North Atlantic Oscillation (NAO), and even natural variables in local meteorological conditions.

Significant climate factors may also be altering severe thunderstorm risk. If overall low level temperature gradients between equator and poles are weakening, they are causing a weakening of vertical wind shear and reducing the probability of severe thunderstorms, AIR says. However, if warming at the earth's surface is increasing vertical instability and increases low-level moisture, the probability that severe thunderstorms will occur goes up.

Unfortunately, research on the relationship between climate change and thunderstorms only began in 2007, and reports from the Intergovernmental Panel on Climate Change and other researchers divulges that current studies and climate models are too limited and thus insufficient to project future changes in tornado activity.

Besides long-established weather patterns and recent observations of climate change, there are other factors swaying loss calculations. Severe storm activity in recent years has struck increasingly urbanized and highly populated areas, hiking up exposure values in those areas and regions that were once open and rural. Differences in regional roofing requirements may make some areas more disaster-prone as well.

"The current state-of-the-science is inconclusive in terms of the potential impacts of climate change on severe thunderstorm risk, but there is little evidence to support the idea that the current risk is changing…," summarized AIR's Doggett in his presentation notes. "A measured approach and a measure of a wait and see attitude have proven to be very prudent in developing the view of risk."

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