6 May 2011  •  Though it never reached hurricane status, 2007’s Tropical Storm Erin killed 16 people in the U.S. Southern Plains. The storm intrigued meteorologists: it weakened to a tropical depression before making landfall in Texas, then unexpectedly restrengthened over western Oklahoma more than two days later, when it generated sustained winds of 95 kilometers per hour (60 mph) and gusts of more than 130 km/hr (80 mph). Clark Evans has examined the storm in detail during his postdoctoral stint at NCAR’s Advanced Study Program, with a paper to be published this spring in Monthly Weather Review. In fall 2011, Evans will become an assistant professor at the University of Wisconsin–Milwaukee. 

What makes the case of Erin so distinctive? 

 Erin actually became stronger over Oklahoma than it did over water. There are no other cases I know of where a landfalling tropical cyclone reintensified over the Southern Plains. A couple of tropical systems did intensify over the eastern United States—David in 1979 and Danny in 1997—but those had a little bit of a kick from upper-level support.

What were the main factors that made Erin intensify?

 It appears that a number of things came together. Soil moisture across the southern Plains was very high for mid-August, in some areas twice as high as it normally would be. Part of that was due to a very wet spring and early summer, and part of it was due to rainfall from Erin itself shortly after it made landfall. Coupled with deep atmospheric moisture from the Gulf of Mexico, the rich soil moisture helped support long-lived, robust convection that aided in reintensifying the vortex. Also, vertical wind shear was very low across the southern Plains. It’s well known that if vertical wind shear is too high around a tropical cyclone, it will make the storm lopsided and prevent convection from organizing. We also hypothesize that Erin’s movement toward the axis of the low-level jet stream helped enhance convergence near the storm, helping to organize the convection around Erin’s center. We’re going to be looking further into that idea in the months to come.

What might have made Erin strengthen when it did, instead of a bit earlier or later? 

 Ultimately, one or more of the necessary factors was missing a day earlier or later, with moisture being a key factor. Prior to reintensification, Erin was heading northwest away from the low-level jet axis and away from the richer moisture in both the ground and atmosphere. After it reintensified, Erin began to accelerate northeast into the Mississippi Valley, where the wind shear was much higher and both atmospheric and soil moisture were lower, and the cyclone quickly decayed

Is there any chance of predicting a rare event like Erin?

 Heavy rains are commonly observed with tropical systems after landfall, so forecasters did expect locally heavy rains along the track both before and during reintensification. However, they didn’t necessarily anticipate the magnitude of rainfall, nor the intensification of the winds and the deepening of the surface low pressure system. There was some evidence in model guidance, particularly in the European model, suggesting the storm might be able to regain some strength. Hindsight is 20-20, but the extreme rarity of the event and lack of support from other guidance make a compelling case for not trusting those guidance products before the event occurred. With a better understanding of events such as Erin, plus improvements to our model guidance products and situational awareness, there certainly is hope for being able to better predict these events in the future. We hope that looking at Erin and other cases like it in further detail will help give us more insight into these rare yet extreme events.

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