All Weather News

Rapidly Intensifying Tropical Systems – Why Does It Happen in Some Storms?

6 Dec 2017, 10:18 am

The future strength of a tropical system is the most challenging aspect for forecasters to predict. Rapidly intensifying tropical systems are the most worrisome for safety and preparations, as seen during the tropical season this year with Hurricane Harvey, Irma, Jose, and Maria. Rapid intensification is roughly defined by the National Hurricane Center as an increase in the maximum sustained winds of a tropical cyclone of at least 35 mph in a 24-hour period, often resulting in a major hurricane (category 3 or higher).

The main model that forecasters use to predict what a hurricane will do is the Hurricane Weather Research and Forecast (HWRF) model. Because we cannot exactly measure what is currently happening in the hurricane, the model updated is many times at once with different measurements to get an idea of the different forecasts that are possible. This set of model runs is called an ensemble and the individual runs are called members. In a recent study published with the American Meteorological Society’s Journal of Atmospheric Sciences, forecasts from an HWRF ensemble for Hurricane Edouard (2014) were examined to learn the differences between hurricanes that intensify rapidly and storms that do not (NI).

[Hurricane Edouard from 2014]

Important Conclusions:

  • Hurricanes are made up of thunderstorms, what meteorologists call convection. The strongest thunderstorms in the hurricane are called convective bursts. In hurricanes undergoing rapid intensification, convective bursts move around the hurricane center, but they stay in the same place for the NI storms.
  • If the wind around the top but outside the hurricane moves the same way the hurricane wind moves (for example, counter-clockwise north of the Equator), this allows the convective bursts to move around the hurricane center and undergo rapid intensification. If that wind blows in the opposite direction, the hurricane will not undergo rapid intensification.
  • The center of the hurricane is not in exactly the same place from the surface to the top as you go up in height. This is called tilt. Because of this tilt, strong winds in the middle and top of the hurricane can flow over the center at the surface. This wind can push strongly spinning wind over the surface center and increase the spinning wind and reduce the tilt in hurricanes undergoing rapid intensification. In contrast, in NI members, this does not happen. Therefore, this increase in the spinning wind is a key ingredient for rapid intensification.
  • Shear is the difference in wind direction between low and high levels of the atmosphere around the tropical cyclone. The direction this shear points toward what we call the downshear part of the hurricane. The rapid intensification member have more moisture in the part of the storm to the right of the downshear part than the NI members, so this is important for the formation of convective burts in the rapid intensification members.

Information from NOAA HRD

Edited for WeatherNation by Meteorologist Mace Michaels

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