Hurricane Isabel Discussion Number 41
Statement as of 5:00 am EDT on September 16, 2003
Satellite imagery and Air Force reserve hurricane hunter data
indicate that Isabel has become quite disorganized during the past
6-12 hr. Center fixes from IR imagery are 20-25 nm east of the
fixes from the Air Force reserve hurricane hunter aircraft and
microwave data...suggesting westerly shear is affecting the system.
The aircraft reports that the wind field has spread out...with an
inner radius of maximum wind of 40 nm and several maxima present
outside of that. The maximum flight-level winds have been 101
kt...and the latest central pressure is 956 mb. The maximum
winds are reduced to 100 kt...and this is likely generous.
The initial motion is now 325/6. The track forecast is reasonably
straight forward. Isabel is on the west side of a deep-layer ridge
extending from the Canadian Maritimes south-southeastward to east
of the hurricane. A broad area of westerly flow is over the
central and eastern United States...with a shortwave trough lifting
northeastward through the Great Lakes. Large-scale models indicate
that the ridge should build somewhat westward as the shortwave
lifts out...which should cause Isabel to move in a general
northwest or north-northwesterly direction through 72 hr. After
that...the storm should turn northward and north-northeastward as
it merges with the next strong shortwave trough moving eastward
across the northern United States. All dynamical models agree with
this scenario...as does the official forecast which is mostly an
update of the previous package. It should be noted that while the
forecast currently calls for landfall in North Carolina in about 60
hr...it would only take a 10 degree change to the right for Isabel
to make landfall on the Delmarva Peninsula as shown in the Canadian
model.
The intensity forecast is very problematic. Large-scale models
suggest that the current shear could continue for another 12-24 hr.
Beyond that time...the models all forecast the development of a
negatively-tilted shortwave trough over the southeastern United
States. While this trough may not reduce the shear...it could
provide a better outflow pattern and dynamical forcing to sustain
or strengthen the storm. On the other hand...Isabel's broad wind
field suggests that the storm might be slow to respond to a more
favorable environment...and thus might move ashore before
significant strengthening could occur. There are three possible
scenarios. First...continued weakening due to shear as forecast by
the SHIPS model. Second...continued short-term weakening follwed
by re-intensification as Isabel approaches the coast. Third...a
relatively steady-state hurricane until landfall. The intensity
forecast will go with the third scenario at this time.
Forecaster Beven
