TROPICAL STORM IDA DISCUSSION NUMBER 15
NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015
1100 PM AST MON SEP 21 2015
Ida's cloud pattern has become increasingly disorganized. Satellite
data indicate that the large separation between the cyclone's low-
to mid-level centers persists as a result of a strong west-
northwesterly shear. The nearest deep convection, coincident with
the mid-level center, remains well removed to the southeast. The
initial wind speed is conservatively lowered to 40 kt and is above
the latest Dvorak intensity estimates.
The tail of a large mid- to upper-level trough over the eastern
Atlantic is forecast to sweep southward over Ida during the next 24
hours. Even stronger deep-layer northwesterly shear and confluence
aloft associated with this feature should adversely affect the storm
during the next few days, and cause weakening. One could envision
Ida not surviving such a greatly inhospitable environment. However,
global models depict Ida remaining a coherent feature and eventually
re-strengthening, though Ida's forecast intensification occurs in
different ways. The ECMWF has consistently shown Ida re-
strengthening, seemingly as a consequence of baroclinic forcing from
the trough interaction. Such an interaction could potentially
result in Ida's taking on the characteristics of a hybrid cyclone.
The GFS shows Ida becoming disentangled from the trough, and
re-intensifying after the trough lifts out. Regardless of which
scenario occurs, any re-strengthening of Ida as a tropical cyclone
would likely be tempered by substantially drier air on the backside
of the trough being entrained by the cyclone. The new intensity
forecast is lower than the previous one in the short term due to the
overall harsh environment affecting the storm. A modest increase in
strength is shown at the end of the forecast, similar to the
previous forecast.
The definition of Ida's center has been deteriorating, and it has
thus become harder to track. It appears though the the cyclone's
forward speed has been decreasing, and the initial motion estimate
is an uncertain 340/04. The deep layer west-northwesterly to
northwesterly flow, associated with the trough overtaking Ida, could
impart a slow eastward or east-southeastward motion for a couple of
days. After that, the bulk of the track guidance shows Ida caught
between two mid-level ridges, which should induce a north-
northwestward and then northward motion at a gradually increasing
forward speed. Only the GFS shows Ida encountering a blocking ridge
with a track much farther to the west, a solution very different
than its ensemble mean. The new track has been adjusted somewhat to
the east overall through 72 hours and is a little faster than the
previous one, in good agreement with a model consensus without the
GFS.
TROPICAL STORM IDA DISCUSSION NUMBER 3
NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015
1100 PM AST FRI SEP 18 2015
The depression's cloud pattern has been increasing in organization.
A primarily convective band over the eastern half of the circulation
has been increasing in curvature and the cloud tops are cooling
significantly. The initial wind speed is increased to 35 kt in
agreement with the latest CI numbers from TAFB and SAB and UW-CIMSS
ADT value.
Westerly shear associated with an upper-level trough, extending
from the eastern subtropical Atlantic into the eastern Caribbean,
could increase over Ida during the next day or so and slow the
cyclone's rate of intensification. Global models agree that a
relative reduction in the shear should occur in 2 to 3 days when a
piece of the upper-level trough cuts off near the Greater Antilles,
which could allow Ida to strengthen, perhaps even more than
forecast. Late in the forecast period, the uncertainty in the
intensity forecast increases considerably, with the ECMWF calling
for more northwesterly shear and less favorable thermodynamic
variables in the storm's environment than in the GFS. The new
intensity forecast is only slightly higher than the previous one
through 72 hours. The forecast after that time is of low confidence
and essentially levels Ida's intensity off in agreement with the
SHIPS model output.
Recent fixes suggest that the cyclone's motion has been more
northwesterly (or 305/06) since earlier today. Ida is being steered
by east-southeasterly to southeasterly flow underneath a subtropical
ridge migrating westward with the cyclone, and some increase in
forward speed is predicted by the models during the next day or so.
In about 48 hours, a deep-layer trough amplifying southwestward
across the eastern Atlantic will cause steering currents surrounding
Ida to collapse, which should result in the cyclone's forward motion
coming to a halt. With increasing northerly or northwesterly flow
aloft, Ida should begin to meander in about 4 days and even
drift southward or southeastward by day 5. The new track forecast
is adjusted to the right some throughout the period, primarily due
to the more northwesterly initial motion but also in the direction
of the multi-model consensus.
TROPICAL DEPRESSION TEN DISCUSSION NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015
1100 AM AST FRI SEP 18 2015
Satellite data indicate that the area of low pressure over the
tropical eastern Atlantic has acquired sufficient organization to be
classified as a tropical depression. The low-level center appears
embedded beneath a relatively small cluster of deep convection, but
convective banding is also increasing to the west and north. The
initial intensity is set at 25 kt based on Dvorak estimates from
TAFB and SAB and recent ASCAT data.
The depression is located near the southwestern edge of a mid-level
anticyclone, giving the cyclone an initial motion of 285/8 kt. The
anticyclone is expected to move westward in tandem with the
depression over the next couple of days, keeping it on a general
west-northwestward track for much of the forecast period. By days 4
and 5, however, there is significant uncertainty in the track
forecast. The GFS and GFDL show the cyclone feeling the influence
of an amplifying deep-layer trough over the eastern Atlantic, which
turns the depression northward. The ECMWF and UKMET, on the other
hand, keep the ridge as the dominant influence and continue a
west-northwestward motion. Due to the large spread in the guidance,
the official forecast shows the cyclone slowing down considerably by
days 4 and 5, and is a little west of the model consensus aids at
those times.
The environment looks conducive for at least gradual strengthening
during the forecast period. Sea surface temperatures are warm,
vertical shear should remain generally low, and environmental
moisture patterns appear favorable for at least the next two days.
Beyond 48 hours, most of the global models show the cyclone
threading the needle beneath a narrow upper-level ridge, with
mid-level dry air possibly getting into the western part of the
cyclone's circulation. This pattern makes the intensity forecast
complicated because hostile upper-level winds won't be too far away,
and the ultimate strength of the cyclone will depend heavily on its
eventual track. For now, the NHC official forecast shows only
gradual strengthening through day 4, more or less in line with the
SHIPS and LGEM guidance.
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