[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT42 KNHC 091438
TCDAT2

Tropical Storm Ophelia Discussion Number   2
NWS National Hurricane Center Miami FL       AL172017
1100 AM AST Mon Oct 09 2017

The convective pattern of the small cyclone has continued to improve
since the previous advisory, with some thunderstorm activity having
developed near or over the center, along with an increase in curved
banding features in the eastern semicircle. The Dvorak intensity
estimate at 1200Z from TAFB was T2.0/30 kt, but given the
aforementioned improvement in the curved-band structure since that
time, the initial intensity is increased to 35 kt. This makes
Ophelia the fifteenth named storm of the 2017 Atlantic hurricane
season.

The initial motion estimate is 030/04 kt. The steering flow
surrounding Ophelia is expected to continue to be weak for the next
day or so while the cyclone remains entangled with a weak
upper-level low located just to its north and northwest. As a
result, only a slow drift toward the northeast and and east is
forecast. By 36 h, increased mid-level ridging to the northwest of
Ophelia should induce a motion toward the east-southeast and
southeast through 72 h, after which a broad mid-latitude trough is
expected to gradually accelerate the cyclone toward the northeast at
a forward speed of near 10 kt. The official forecast remains down
the middle of the guidance envelope, in close agreement with the
previous forecast track, and the HCCA and TVCX consensus models.
Ophelia should remain far away from land for the next 5 days.

Although Ophelia is forecast to remain over marginally warm SSTs of
26.5-27 deg C, much colder-than-normal temperatures aloft should
offset the relatively cool ocean temperatures and produce sufficient
instability to generate moderate to strong convection for the next
120 h. The GFS-based SHIPS model is currently assessing 25 kt of
westerly vertical wind shear, which obviously isn't occuring based
on the recent development of anticyclonic outflow over the low-level
center and across most of Ophelia's circulation. In contrast, the
UW-CIMSS shear analyses indicate lower shear values near 15 kt.
However, both shear assessments are likely too high given the large
domains that they use to calculate environmental wind shear. Based
on recent trends noted in water vapor imagery showing the shear
decreasing, along with the overall favorable upper-level wind fields
forecast by the GFS and ECMWF, especially after 36 hours, slow but
steady strengthening is expected for the next 5 days, with only
occasional intrusions of dry mid-level air being the primary
inhibiting factor to the intensification process. The official
intensity forecast is close to the NOAA HCCA consensus model, is
about midway between the more aggressive IVCN consensus model that
makes Ophelia a hurricane in about 72 h, and the more conservative
statistical SHIPS and LGEM models. However, even the SHIPS and LGEM
models have increased their peak intensity forecasts by at least 15
kt since the previous advisory.

FORECAST POSITIONS AND MAX WINDS

INIT  09/1500Z 31.4N  39.9W   35 KT  40 MPH
 12H  10/0000Z 31.7N  39.5W   40 KT  45 MPH
 24H  10/1200Z 31.6N  38.7W   45 KT  50 MPH
 36H  11/0000Z 31.3N  38.1W   45 KT  50 MPH
 48H  11/1200Z 30.7N  37.7W   50 KT  60 MPH
 72H  12/1200Z 29.8N  36.7W   55 KT  65 MPH
 96H  13/1200Z 30.5N  34.5W   65 KT  75 MPH
120H  14/1200Z 32.0N  31.0W   65 KT  75 MPH

$$
Forecaster Stewart
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