Tracking Debby: How Delmarva was spared the worst
Tropical Storm Debby made its second landfall along the South Carolina coast early Thursday morning. The late afternoon forecast track from the National Hurricane Center had increased expectations for the potential of severe storms on Delmarva, with isolated tornadoes, damaging winds, and flash flooding, starting Friday morning and lasting into the evening hours.
Despite the forecast, much of Delmarva ended up being spared the worst. Several factors could be attributed to the deviation from the Thursday afternoon forecast, but two main factors stood out – Debby tracked further inland and moved faster than forecasted.
HEAVIEST RAINS FURTHER INLAND: Debby’s forecasted track began to shift further west several days ago. But even compared to Thursday afternoon’s forecast, the center of circulation tracked further west, especially once the storm approached the West Virginia border. This kept the heaviest and most widespread tropical rains further inland, but at the same time increased the potential for severe storms.
TORNADO THREAT DIMINISHED: The timing of Debby’s track initially lined up with an increasing unstable airmass over Delmarva Friday morning. Because the heavy shield of rain remained further inland Thursday night, some sun was able to prime the tropical layer of air at the surface by Friday morning. This set the table for potentially explosive thunderstorm development, with an expected trigger from the wind shear and increased low level flow with Debby just to the west. An inland trough to the north was set to draw Debby northward and accelerate its forward speed later in the day. This certainly ended up being the case – but at a much faster rate than anticipated. By late morning Debby was approaching the New York border, several hours earlier than forecasted. Proximity to the storm system during the heating process was critical for severe thunderstorm development and the risk for tornadoes. But since the system had moved so fast to the north (and further west as well) its influence had diminished across Delmarva by the time the window of opportunity for severe storm development had arrived.
