Event Summary
     National Weather Service, Raleigh NC


Hurricane Isabel, September 2003



Satellite Imagery of Hurricane Isabel on 2003/09/18 Z - Click to enlarge
(Click the image to enlarge.)



Hurricane Isabel Track


Hurricane Isabel's track was rather typical for tropical cyclones that are generated from tropical waves emerging off the west coast of Africa. These so-called Cape Verde storms are often subject to long westerly tracks steered by a deep easterly flow associated with high pressure to the north.

In the case of Isabel, high pressure extended well westward toward the east coast of the Untied States delaying a more northerly component of motion that might have prevented landfall on the mainland. A more northerly turn in the motion and big declines in intensity began by September 15th. These changes were associated with an increase in shear from an upper level trough pushing eastward in the westerlies. Behind the trough was considerable dry air which subsequently was entrained into Isabel's circulation. Isabel made landfall just east of Cape Lookout. Its northwesterly track maximized sound flooding in the far western portions of North Carolina's sounds.

Hurricane Isabel Track - Click to enlarge
(Click the image to enlarge.)



Hurricane Isabel Track - Click to enlarge
(Click the image to enlarge.)



Total Precipitation from Hurricane Isabel


Storm total precipitation from Hurricane Isabel shows the maximum amounts located along and just to the east of Isabel's track. Amounts decreased sharply just to the west of the track. This pattern of rainfall distribution is typical for tropical cyclones subject to upper level wind shear. In general as the shear increases in the 850 - 300 mb level, the distance of the axis of maximum rainfall increases eastward of the track. In the case of Isabel, the shear was on the order of 30 knots resulting in the maximum rainfall being located along and just eastward of the storm's track.

The sharp drop in rainfall amounts west of the storm track was in part due to the extensive dry air that was in place over North Carolina prior to Isabel's landfall. A secondary maximum of rainfall seen well west of the Isabel's track was not expected. The first rainfall band well ahead of Isabel's center moved rapidly westward into the western Piedmont. There the rainfall band persisted while other bands rotated around Isabel's circulation enforcing the first outer band. Factors accounting for the secondary maximum likely included Isabel's massive size, low level convergence, and enhanced upper level divergence associated with a nearby small scale upper level trough.


Total Precipitation from Hurricane Isabel



Maximum Wind Gusts from Hurricane Isabel


Maximum wind gusts from Hurricane Isabel shows the highest wind speeds along and just east of the storm's track. Tropical storm force wind gusts extended well westward into the western Piedmont. This far western extension of tropical storm forces winds was due in part to Isabel's massive size and a strong outer rain band that persisted for several hours.


Maximum wind gusts from Hurricane Isabel



Wind Speeds from Hurricane Isabel in the Raleigh - Durham Area
(Jordan Hall data courtesy of Dr. Alan Riordan, North Carolina State University)

Wind data from instruments atop Jordan Hall (seven stories high) on the campus of North Carolina State University show both sustained winds and max gusts on Thursday September 18th 2003 as Hurricane Isabel moved inland on a northwesterly course from landfall near Cape Lookout.

The data shows that the highest sustained winds and wind gusts occurred during Thursday afternoon when sustained winds were typically between 30 and 45 mph while max gusts were generally between 50 and 65 mph. During this period the maximum surface wind gust reported at the Raleigh-Durham Airport ( 9 miles west of Jordan Hall) was 45 mph. This disparity in wind speeds just above ground and those measured at the 10m level support the hypothesis that cool stable air at the surface was impeding the downward mixing of strong winds. A surface ridge of high pressure advected relatively cool and dry air into the Raleigh area preceding the arrival of Isabel whose center passed some 100 miles due east of Raleigh at around 4 pm.


Total Precipitation from Hurricane Isabel




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