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

In the aftermath of last year's record-breaking thunderstormlosses exceeding $26 billion in the U.S., risk modelers arescratching their heads to figure out if the country's stormexposure profile is increasing due to historically inadequate modeling methods orfrom other factors such as climate change.

"Severe thunderstorms are a little different from other modeledcatastrophes such as hurricanes, since the overall losses from [an]event is an aggregate of a number of other small thunderstormoccurrences, referred to as 'micro-events'," said AIR PrincipalScientist Tim Doggett in a June webinar about the nation's current and future trends in stormmodeling. 

He continued, "While it's rare for any individual thunderstormto result in mass losses, when we get a large collection of thesemicro events, the total losses for the outbreaks are what we referto as 'macro events'. From a modeling perspective, micro eventdetails are very important in determining losses at the locallevel."

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

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

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

Firstly, severe thunderstorm reports rely on human reportsinstead of standardized instrumentation, and reporting frequencyhas grown along with the U.S. population. Furthermore, internetcommunication, weather RADAR and storm spotters have impacted thedata collection process over time.

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

In addition, some building materials bend more easily undercertain wind speeds. Concrete and steel, for example, stand upagainst all but the strongest tornadoes, and high-rise buildingsare 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 evennatural variables in local meteorological conditions. 

Significant climate factors may also be altering severethunderstorm risk. If overall low level temperature gradientsbetween equator and poles are weakening, they are causing aweakening of vertical wind shear and reducing the probability ofsevere thunderstorms, AIR says. However, if warming at the earth'ssurface is increasing vertical instability and increases low-levelmoisture, the probability that severe thunderstorms will occur goesup.

Unfortunately, research on the relationship between climatechange and thunderstorms only began in 2007, and reports from theIntergovernmental Panel on Climate Change and other researchersdivulges that current studies and climate models are too limitedand thus insufficient to project future changes in tornadoactivity.

Besides long-established weather patterns and recentobservations of climate change, there are other factors swayingloss calculations. Severe storm activity in recent years has struckincreasingly urbanized and highly populated areas, hiking upexposure values in those areas and regions that were once open andrural. Differences in regional roofing requirements may make someareas more disaster-prone as well.

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