|Hurricane Irene formed from a tropical wave near the Cape Verde Islands west of Africa, and became a named tropical system on August 20th. Shortly thereafter as she moved west, Irene became the first hurricane of the 2011 season.|
As the hurricane moved west she intensified and brushed the north coast of Hispaniola. Irene then began to turn to the northwest and became a category 3 hurricane during the time she impacted the Bahamas.
By this time, the National Hurricane Center was predicting a track that took Irene into the outer banks of North Carolina, just east of Cape Fear. Although this was several days out, this track turned out to be quite accurate and on the morning of the 27th Irene made landfall as a category 1 hurricane near Cape Lookout, NC. This spared the Cape Fear region from the worst impacts since the right front quadrant remained well east of that area.
Although the track was forecast very accurately, Irene's intensity forecast was a bit overdone. Much of the guidance several days out was predicting the potential for Irene to be a strong category 2 or even a category 3 hurricane at NC landfall. Fortunately, this did not materialize. However, with surface pressures in the eye of Irene only around 950 mb, the potential for a stronger hurricane remained possible, even as Irene continually weakened off the southeast coast.
Analysis of water vapor imagery during the day leading up to NC landfall helps explain why Irene weakened so drastically as she approached land. The two images below are water vapor satellite images at 26/1900 UTC and 26/2200 UTC respectively.
Irene is obvious off the southeast coast, but notice the very dry air across much of the eastern United States. This dry air was pushing east in response to a strengthening ridge of high pressure. As Irene continued up the coastline, she ingested a significant amount of dry air as evidenced by the change in structure and "dry slot" that is seen wrapping almost entirely around her core in the second image.|
Fortunately, this dry air was enough to keep Irene from restrengthening over what was otherwise a favorable environment. Although the category 1 hurricane still caused considerable damage from Cape Fear to the Outer Banks, it could have been much worse across eastern North Carolina.