Hurricane Isabel, September 2003
(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.
(Click the image 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.
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.
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.
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