NASA/NOAA via NOAA Environmental Visualization Laboratory

According to the National Hurricane Center, Hurricane Joaquin, which is barreling towards the East Coast, may be experiencing eyewall replacement. When this happens it usually means the hurricane is weakening, according to NASA, whose imagery is showing the eyewall replacement. It could also mean the winds will cover a broader region, as explained in the news alert:

In powerful hurricanes, a new eye begins to develop around the old eye. The new eye gradually decreases in diameter and replaces the old eye. When that happens, the intensity of the hurricane usually decreases. Despite the fact that eyewall replacement can mean a weakening in a powerful hurricane, it can also spread the hurricane force winds out over a larger area.

As of Friday midday, Hurricane Joaquin, a very powerful Category 4 storm, was idling over the Bahamas. The latest forecasts predict that the hurricane is less likely to make landfall along the eastern seaboard than previously thought, however this type of dynamic storm can quickly change in force and trajectory. According to the National Weather Service, even as the threat of direct impact from the Carolinas up the mid-Atlantic decreases, strong winds and minor or moderate coastal flooding are to be expected.

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As for the next few days, the National Hurricane Center forecasts that Joaquin is expected to gradually weaken as it encounters "increasing southwesterly wind shear" but is expected "to remain a powerful hurricane for the next several days."

A little more on this phenomenon of eyewall replacement from meteorologist Jeff Haby. Haby writes on his website that basically what occurs in eyewall replacement is that "a new eye begins to develop around the old eye" and then the "new eye gradually decreases in diameter and replaces the old eye."

When an eyewall replacement cycle occurs the intensity of the hurricane usually decreases. For example, a CAT 5 hurricane could weaken to a CAT 4 hurricane. The intensity weakens due to the gradual erosion of the inner eyewall. As the outer eyewall contracts and gains organization then the storm will often increase in intensity. Because of eyewall replacement cycles, a hurricane will typically not remain a CAT 5 for a long period of time.

Although an eyewall replacement cycle tends to reduce the category of a hurricane it also spread the hurricane force winds out over a larger area. This can cause a larger region to experience the extreme damage in a hurricane.