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Hurricane Oscar Discussion Number  11
NWS National Hurricane Center Miami FL       AL162018
1100 AM AST Mon Oct 29 2018

Oscar's convective cloud pattern has continued to improve since the
previous advisory, with a small, cloud-filled eye now apparent in
visible satellite imagery and also in a recent SSMI/S microwave
pass. In addition, cirrus outflow has been expanding in all
quadrants, especially in the eastern semicircle. The initial
intensity of 75 kt is based on a Dvorak satellite intensity estimate
of T4.5/77 kt from TAFB, a Data-T-number of T4.5/77 kt from SAB, and
an NHC objective intensity estimate of T4.4/75 kt.  It is also worth
noting that bursts of lightning activity in the eastern eyewall have
been occurring since around 1100 UTC.

The initial motion estimate is now 285/06 kt. Oscar has slowed its
forward motion significantly and has made the advertised turn toward
the west-northwest. A motion toward the northwest is expected by
late afternoon today as the hurricane rounds the southwestern
periphery of a deep-layer ridge. A turns toward the north and then
toward the north-northeast are forecast on Tuesday as Oscar moves
north of the ridge axis ahead of an eastward-moving deep-layer
trough currently approaching Bermuda. The trough is expected to
continue advancing eastward over the next couple of days,
accelerating Oscar toward the northeast at forward speeds near 25 kt
on Wednesday through Friday. Although a strong shortwave trough is
still forecast to dig southward to the west of Oscar on Wednesday,
none of the model guidance shows the hurricane being captured any
longer, and instead keep the cyclone as a separate entity that
accelerates northeastward into the mid-latitude westerlies as a
strong extratropical cyclone. The official track forecast is similar
to the previous advisory, and lies close to an average of the
corrected-consensus models HCCA and FSSE and the simple consensus
models TVCA and TVCX.

Deep-layer (850-200 mb) shear calculations by the SHIPS model and
UW-CIMSS are at least 25 kt from the northwest, which clearly is not
negatively affecting the improving cirrus outflow. This is likely
due to the large 1000-km domain that the SHIPS model uses to compute
vertical wind shear. Furthermore, most of the cloud top temperatures
within the outflow layer appear to be mostly below the 200-mb level,
and closer to the 250-mb level. The large shear values are resulting
in much less intensification forecast by the SHIPS and LGEM
statistical-dynamical intensity models. As a result, the official
intensity forecast leans more toward the HCCA and FSSE models, which
are weighing more heavily the stronger intensity forecasts provided
by the HWRF, HMON, and Navy COAMPS-TC models, which have Oscar
strengthening to just below major hurricane status in 24-36 hours.
By 48 hours and beyond, sharply decreasing SSTs along with
increasing southwesterly shear ahead of a deep-layer trough are
expected to cause Oscar to gradually weaken and transition to a
strong extratropical low in 60-72 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  29/1500Z 25.8N  58.4W   75 KT  85 MPH
 12H  30/0000Z 26.8N  58.7W   85 KT 100 MPH
 24H  30/1200Z 28.7N  58.0W   90 KT 105 MPH
 36H  31/0000Z 31.4N  56.1W   90 KT 105 MPH
 48H  31/1200Z 35.0N  52.6W   80 KT  90 MPH
 72H  01/1200Z 43.0N  43.7W   65 KT  75 MPH...POST-TROP/EXTRATROP
 96H  02/1200Z 50.0N  30.0W   60 KT  70 MPH...POST-TROP/EXTRATROP
120H  03/1200Z 55.0N  15.0W   55 KT  65 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart

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