Now We Can Predict Where And When Extreme Weather Is Likely To Hit Up To Two Months In Advance

Extreme weather, like the insane Colorado flooding in September and the tornadoes that ripped through Illinois in November, is rocking the Midwest.

Normally we don't have much advanced warning for storms like these, but a new tool from researchers at Utah State University could help predict when this kind of destructive extreme weather is more likely.

Their research was published in the Journal of Geophysical Research on Oct. 15.

This weather-predicting index was created by monitoring a specific weather pattern - a low-level jet stream that interacts with another circumglobal stream - that makes strong storms and tornadoes form in the Midwest.

Before any storm can form the water in the air has to rise and condense, said study researcher Robert Davies. Davies hopes the index can be used with climate models to predict which areas should be expecting some extreme weather up to 60 days before they happen.

The researchers analyzed precipitation data from the past 32 years to create the index.

Davies explained that "extreme" precipitation is a relative measurement since it's different for every area. Extreme precipitation is compared to the average precipitation an area gets over time. That time period is usually around 30 years in the climatology field. The abnormally high rainfalls that occur in that time period are classified as the extreme ones.

How this index can help us prepare

The index can predict extreme weather up to 60 days in advance, but it also identifies the regions that should expect to see the biggest increase in extreme rainfall over time. Storm water systems are built based on the average precipitation a city will get. But if extreme rainfall is becoming more regular, the current storm-water systems won't be able to handle the job. That's where the index comes in. If a city is located in a region that the index predicts is going to see a big increase in rainfall, it should update its storm-water system to handle more water.

While the index cannot pinpoint a precise date, it could be used to warn communities when the threat of a storm is elevated. For example, the index could not predict the exact dates of the tornadoes that swept through Illinois this month, but it could have warned the people in Illinois they were at a higher risk than normal for tornadoes in November. Most tornadoes in the area come during the Spring, so many people were unprepared.

Davies said the same kind of analysis could be used to predict which areas will become more at risk for tropical storms like hurricanes.

Davies said if the index had been operating earlier this year it could have warned Coloradans that they should have expected some elevated chances of extreme precipitation in September. The index is not precise enough that it would have been able to identify the city of Boulder as the specific city that experienced the flooding, but at least the greater state of Colorado would have had some warning.

How much more rainfall?

The map below shows the northern hemisphere's spring precipitation from 1979-2010. The red dots show areas where extreme precipitation has increased significantly over time for that month, while the blue dots show where it has actually decreased.

By analyzing trends over time, the researchers identified the areas that have seen big increases in rainfall. Climate change means it's almost certain extreme rainfall events will continue to increase in these areas.

Thunderstorms produce tornadoes, so the same analysis was used to pinpoint areas that should expect to see an increase in tornadoes. The scientists think that the "extremeness of the April 2011 tornado outbreaks may be part of a long-term trend."

The image to the left shows that tornadoes will become more regular, especially in the Midwest.

Why this predictive power is important

The index has shown that the definition of extreme weather is changing in many areas. What counts as extreme now might become the new average. The researchers think climate change is largely responsible. Climate change is a problem because the warmer the atmosphere gets, the more water it can hold, and that means a lot more rainfall.

The National Research Center released a report this year warning that a dangerous "tipping point" is coming. So far global climate change and the consequences that come with it, have been gradual.

We've had time to build better infrastructures to guard against increasingly powerful storms and create laws to protect species like polar bears threatened by habitat loss.

But once we reach this tipping point the researchers warn that the environment will change too quickly for us to keep up with. The changes may come "faster than expected, planned, or budgeted for, forcing more reactive, rather than proactive, modes of behavior," according to the report.

A predictive weather index like this one is an important first step to preparing for these changes.