Official Discussion issued by the National Hurricane Center

Ophelia (AL172017) DATA RELEASED: 10/9/2017 5:00:00 PM UTC

Copy of official data

Tropical Storm Ophelia Discussion Number   3
NWS National Hurricane Center Miami FL       AL172017
500 PM AST Mon Oct 09 2017

Ophelia's convective pattern has continued to steadily improve with
the formation of strong thunderstorms with cloud tops near -80C near
the center, along with the development of tighter curved banding
features in the southeastern semicircle. There has also been a
noticeable increase in lightning activity within 30 nmi of the
center. The TAFB Dvorak intensity estimate at 1800Z was T2.5/35 kt.
However, NHC objective Dvorak estimates are ranging from T3.2/49 kt
using a curved band pattern to T3.5/55 kt for a shear pattern. Based
on the aforementioned estimates and the continued improvement in
Ophelia's convective organization since the 1800Z TAFB estimate, the
initial intensity is increased to 40 kt.

The initial motion estimate is 055/03 kt. There is no significant
change to the previous forecast track or reasoning.  Ophelia is
expected to remain embedded within a weak flow regime for the next
72 hours due to the cyclone being cut off from the mid-latitude
westerlies, resulting in a slow and gradual turn toward the east and
southeast during that time frame. Thereafter, a broad mid- to
upper-level trough to the northwest, in conjunction with a building
subtropical ridge to the south, is forecast to gradually accelerate
Ophelia toward the northeast at a slightly faster forward speed of
near 10 kt. The new NHC forecast track is close to the northern edge
of the guidance envelope out of respect for the ECMWF model, but the
forward speed is slower than the ECMWF and similar to a blend of the
HCCA and TVCX consensus models.

Ophelia is expected to remain over marginally warm SSTs of 26.5-27C,
but temperatures aloft that are 2-3 deg C colder than normal should
continue to produce steep lapse rates and strong instability, which
will allow for deep convection to develop for the next 96 h or so.
Both the GFS and ECMWF models continue to forecast the vertical wind
shear to decrease sharply after 24 h, with the ECMWF forecasting
shear values to decrease to less than 10 kt from 36-96 hours. Based
on the availability of strong instability and lower shear, slow but
steady strengthening is expected for the next 3-4 days, with only
the occasional entrainment of very dry mid-level air preventing
rapid intensification from occurring, at least in the short term.
After 96 h, Ophelia will be encountering increasing southwesterly
wind shear of 15-20 kt and also be moving over 25C sea-surface
temperatures, a combination that should cap the intensification
process and possibly even induce weakening. The SHIPS and LGEM
statistical models, along with the HCCA and IVCN consensus models,
have come into much better agreement on their intensity forecasts.
Therefore, there is a higher degree of confidence in the official
intensity forecast, which basically remains unchanged from the
previous advisory. Note, it is possible that Ophelia could reach a
higher peak intensity than indicated between 72-96 hours when the
wind shear will be at its lowest value.


INIT  09/2100Z 31.5N  39.6W   40 KT  45 MPH
12H  10/0600Z 31.6N  39.1W   45 KT  50 MPH
24H  10/1800Z 31.4N  38.5W   50 KT  60 MPH
36H  11/0600Z 30.8N  37.9W   55 KT  65 MPH
48H  11/1800Z 30.1N  37.6W   60 KT  70 MPH
72H  12/1800Z 29.7N  36.1W   65 KT  75 MPH
96H  13/1800Z 31.0N  33.7W   65 KT  75 MPH
120H  14/1800Z 32.4N  30.0W   65 KT  75 MPH

Forecaster Stewart