Irene closing in on the Outer Banks

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Recent images have begun to reveal some clues as to how the storm will deal with a progressively less storm-friendly environment in the days ahead. ( National Oceanic and Atmospheric Administration )

With cooler waters about 12-24 hours away, Irene’s encroachment into the middle latitudes -where higher wind shear and drier nontropical air abound- should result in a weakening trend. This is reflected in the National Hurricane Center’s (NHC) official forecast, which has Irene still roughly a 105 mph hurricane as it encounters the Outer Banks of North Carolina, and an 85 mph hurricane by the time it reaches the latitude of Philadelphia.

As Irene made its closest approach to the Florida coast Thursday, some 200 miles offshore, bands of showers in the storm’s outer periphery moved across the coastline. In between the rains, winds were light (often around 10 mph). But underneath the showers along the southern two-thirds of the state’s immediate coast, sustained winds in the 15-25 mph range were generally accompanied by 30-40 mph gusts. These tropical depression-like conditions were much like what we expected along the Florida shore as Irene moved through the Bahamas.

From tropical storm to hurricane. (NOAA)


(NOAA)

This eastern bias in the presentation will probably become more apparent in the coming hours as Irene moves north, owing to a gradual increase in westerly flow aloft and to the entrainment of continental air into the primary circulation. This suggests that the westward extension of “bad” weather will shrink as the storm moves north.

With offshore winds aloft along Irene’s road ahead, NHC made it clear in Thursday night’s discussion that there is realistically nowhere for the storm to go but northward or north-northeastward, especially after it moves north of the Outer Banks. Under the NHC scenario (which to their credit has remained quite consistent in recent updates), the Outer Banks of North Carolina are the locations at greatest risk to experience legitimate hurricane conditions (sustained winds of 75 mph or more). As Irene moves further poleward, the inland side of the storm should devolve pretty thoroughly, with the radius of hurricane-force winds rapidly shrinking to the west.

By the time Irene makes it to 40°N (approximately the latitude of D.C. to Philadelphia), I wouldn’t be surprised if the storm exhibits a pronounced comma-shaped presentation (in other words, a less-defined eye) in the satellite pictures. If that happens, locations as far west as Washington, D.C. are likely to receive wind conditions near or less than tropical-storm strength.

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