27 Oct Sat 2018

Subtropical Storm Oscar Discussion Number 2
NWS National Hurricane Center Miami FL AL162018
500 AM AST Sat Oct 27 2018

Oscar’s convective pattern has not changed much since the previous
advisory. The surface center is displaced just to the southeast of
a burst of deep convection, and other convective elements extend
along a broken band over the eastern part of the circulation.
Although satellite classifications have not increased, a drifting
buoy (47546) near Oscar’s center recently reported a pressure of
998.8 mb, implying a significant drop in the cyclone’s central
pressure from the previous estimate. Based on pressure-wind
relationships, this low pressure would equate to maximum winds
around 45 kt, and that is set as the initial intensity for this
advisory.

Oscar is careening around the northern side of a mid- to
upper-level low, and its initial motion is now west-northwestward,
or 300/10 kt. As the circulation becomes more vertically aligned
in the coming days, the cyclone is forecast to be pushed
westward to west-southwestward by the flow on the back side of a
trough that is dropping southward over the eastern Atlantic. After
48 hours, Oscar is expected to recurve sharply and accelerate toward
the north Atlantic ahead of an approaching mid-latitude trough.
The global models have come into much better agreement on Oscar’s
eventual recurvature and acceleration, and confidence in the NHC
track forecast has increased. In fact, the 00Z guidance suite
required a significant increase in Oscar’s forecast forward speed
on days 4 and 5, and the new NHC track forecast is much faster than
the previous one at the end of the forecast period, jumping
northward by about 10 degrees of latitude. This new forecast is
close to the TVCN multi-model consensus and HCCA model on day 5,
but it’s still not as fast as the 00Z GFS and ECMWF guidance.

Gradually decreasing shear and warmer waters ahead of Oscar should
allow for a steady increase in intensity during the next few days,
and the cyclone is also expected to take on a more tropical
convective pattern in about 36 hours. The intensity models are
showing a little more intensification than before, and the new NHC
intensity forecast has been nudged upward, showing Oscar becoming a
hurricane in 3-4 days. Still, this forecast lies near the lower
bound of the intensity guidance, and additional upward adjustments
may be required in future advisories. Now that most of the models
are showing more acceleration of Oscar toward the north Atlantic
later in the forecast period, there is also more definitive
consensus that Oscar will become an extratropical low by day 5, and
that is now indicated in the official forecast.

FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 27.3N 47.0W 45 KT 50 MPH
12H 27/1800Z 27.1N 49.1W 45 KT 50 MPH
24H 28/0600Z 26.2N 52.0W 50 KT 60 MPH
36H 28/1800Z 25.7N 54.6W 55 KT 65 MPH…TROPICAL CYCLONE
48H 29/0600Z 25.9N 56.6W 60 KT 70 MPH
72H 30/0600Z 28.4N 57.4W 65 KT 75 MPH
96H 31/0600Z 35.0N 52.0W 70 KT 80 MPH
120H 01/0600Z 45.0N 39.0W 60 KT 70 MPH…POST-TROP/EXTRATROP

$$
Forecaster Berg