Isabel turned maintained a westward this motion until 13 September, moving along the south side of the Azores-Bermuda High. Isabel approached a weakness in the western portion of the Azores-Bermuda High, which allowed the hurricane to turn west-northwestward on 13 September, northwestward on 15, September, and north-northwestward on 16 September. The latter motion would continue for the rest of Isabel's life as a tropical cyclone.
At 00Z on the 14th of September, there were
several features in the deep layer mean flow around Isabel. A cyclonic
circulation was located near the Carolina coastline. And anti-cyclonic
circulation was located to the south of Newfoundland Land. Just to the south of
this feature was a smaller anti-cyclonic circulation. The cyclonic
circulation over the eastern US seaboard and the anti-cyclonic circulation in
the north Atlantic would help guide Isabel to the northwest over time. In
addition, there was another cyclonic circulation to the east of Isabel, and a
short-wave trough moving into the Central United States.
Ensemble spread at 00Z on the 14th was mostly associated with Isabel. In
addition, there was a lesser amount of ensemble spread to the northwest of
Isabel, associated with the cyclonic circulation near the eastern US seaboard.
In addition, there were also regions of spread to the east of Isabel, to the
northeast of Isabel, and over the south-central United States. None of these
regions was appropriate for aircraft observations, but the area of spread
associated with the cyclonic circulation near the eastern US seaboard was
sampled.
The ET KF selection for observing sites on the 14th at 00Z concentrated on two
areas: Isabel, and the anti-cyclone to the North of Isabel. These were
considered to be the two most important and poorest observed features. While the
trough in the eastern United States was important, the ET KF did not select it
as a target area, in contrast to the ensemble spread, which did depict
uncertainty in the east coast cut-off low.
At 00Z on the 15th, the deep layer mean flow had
changed since the 14th. Now there were two primary features of interest (in
addition to Isabel) - the anticyclonic circulation to the north of Isabel, and a
trough along the eastern United States. The shortwave in the central US caught
up to the cut-off low along the east coast to form one feature. Also of
importance was a longwave trough over the central United States. Isabel was to
travel between this trough in the central United States and the anti-cyclone off
the coast of the Maritime Provinces of Canada.
Ensemble spread at 00Z on the 15th was greatest near Isabel, to the east of
Isabel, and near the short-wave trough over the eastern United States. The ET KF
gave the greatest support to sampling the storm. Secondary targets were (a) the
east coast US short wave, (b) the anti-cyclone south of the Canadian Maritimes,
and (c) an anti-cyclone to the east of Isabel. Given logistical constraints, the
anti-cyclone to the east of Isabel could not be a target area, though the flight
did go as far east as possible. The short-wave trough in the eastern US was
sampled, however, the anti-cyclone to the north of Isabel was not.
Flight of September 14th
Track of Isabel
