Hurricane Irene continues to become better organized, takes aim at Bahamas

Hurricane Irene remains a category 1 on the Saffir Simpson hurricane wind scale, and continues to be a U.S. landfall threat for anywhere from southern Florida to North Carolina. In the 5pm EDT update from the National Hurricane Center, Irene had maximum sustained winds of 80 mph. Irene is moving west-northwest at 13 mph along the northern Hispaniola coastline. Estimates from radar suggest Puerto Rico has seen anywhere from 1 to 5 inches of rain from Irene so far, with the heaviest rain probably being enhanced by topography. Satellite imagery shows a new burst of intense thunderstorm activity began near the center of the hurricane this afternoon, and despite land interaction, Irene continues to become larger and better organized, with tropical storm-force (39 mph) winds extending 160 miles away from the center on the northeast side of the hurricane. The latest Hurricane Hunter mission found maximum sustained winds just above 80 mph, with a minimum central pressure of 988 millibars. The strongest winds continue to be found on the north side of the storm. Another Hurricane Hunter mission is scheduled for this evening, and a NOAA Gulfstream is on its way to Irene as I type.


Figure 1. True-color image of Hurricane Irene taken at 11:20am EDT Monday August 22, 2011, shortly after Irene moved off the coast of Puerto Rico. At the time, Irene was a Category 1 hurricane with 80 mph winds. Image credit: NASA.

Track forecast for Irene
The models agree that the center of Irene will stay north of Hispaniola, which will allow the storm to continue to organize. The question that remains is how the trough of low pressure, which is expected to move across the Eastern U.S. on Wednesday and Thursday, will affect Hurricane Irene. The timing and strength of this trough, as well as the intensity of the hurricane, will determine just how quick Irene will turn away from the U.S. coast, if at all. This afternoon, the GFDL has finally started to inch its forecast track to the east, with a landfall in southern Florida. Both the HWRF and the UKMET models are suggesting a landfall in South Carolina, and the ECMWF and the GFS are forecasting a brush with the Outer Banks of North Carolina. The official forecast from the National Hurricane Center is a track north of Hispaniola over the next 24 hours, and through the Bahamas Wednesday and Thursday. While the center of the Hurricane Center's cone of uncertainty has Irene making landfall near the North Carolina/South Carolina border, it's important to note that the error in 4 and 5 day track forecasts remains high—around 200 miles in either direction. The official track has nudged a bit east since this morning, which reduces the threat to southern Florida, but increases the threat to the Carolinas.

Intensity forecast for Irene
The environment around Hurricane Irene remains moist, and wind shear is expected to remain relatively low (5 to 15 knots) along its forecast path over the next 5 days. Sea surface temperatures are certainly warm enough (29-30°C) to support intensification to a major hurricane (category 3+). Irene is still disorganized on its south side due to land interaction and dry air, but recent satellite imagery suggests increasing outflow at high levels to the south of the center, which is necessary for the hurricane to intensify. Both the GFS and the ECMWF are forecasting Irene to develop into a very large and intense hurricane. The National Hurricane Center expects Irene to intensify to a category 2 hurricane tomorrow evening as it moves away from Hispaniola. Beyond that, Irene will most likely intensify into a category 3 major hurricane with maximum sustained winds around 110 mph. Like Jeff said this morning, however, Irene could just as easily remain a category 2, or even reach category 4 wind speeds. In any case, Irene will be a powerful hurricane and a serious threat to the Bahamas and the East Coast of the United States.

Angela