11am DISCUSSION Number 42

[ericinmia]

a NOAA hurricane hunter aircraft reported that the central pressure
continues to slowly rise and the eye has become poorly defined on a
radar image transmitted from the aircraft. Peak flight-level winds
were 105 kt in the northwest quadrant from 7000 ft...which would
support surface winds of about 90 kt. This makes Isabel a category
two hurricane on the Saffir-Simpson scale. Water vapor imagery
continues to show dry air in the western part of the circulation...
and the convection has weakened markedly over past few hours.
Recent images...however...show an improvement in the outflow
pattern northwest of the center.
The initial motion is 330/7. There has been no significant change to
the track forecast. 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 westward as the shortwave lifts out...
which should cause Isabel to move in a general north- northwesterly
direction through 72 hr.
With the deterioration of the central core...additional weakening
seems likely over the next 24 hours. Most of the model guidance
continues to show increasing anticyclonic outflow over Isabel as a
result of a digging and negatively-tilted upper-level trough that
should interact with the hurricane in the 24 hours prior to
landfall. For this reason...the official forecast allows for some
restrengthening. It is possible...however...that the circulation
could become so disrupted over the next day or so that Isabel would
be unable to respond to the more favorable upper-level forcing.
Forecaster Franklin

forecast positions and Max winds

initial 16/1500z 27.4n 71.2w 90 kt
12hr VT 17/0000z 28.4n 71.8w 85 kt
24hr VT 17/1200z 29.9n 72.6w 85 kt
36hr VT 18/0000z 31.7n 73.9w 90 kt
48hr VT 18/1200z 33.8n 75.6w 95 kt
72hr VT 19/1200z 39.0n 79.0w 45 kt...inland
96hr VT 20/1200z 47.0n 78.5w 30 kt...extratropical
120hr VT 21/1200z 54.0n 71.0w 30 kt...extratropical