Hurricane OSCAR (Text)


Hurricane Oscar Discussion Number   8
NWS National Hurricane Center Miami FL       AL162018
500 PM AST Sun Oct 28 2018

Visible and infrared satellite imagery indicate that Oscar's
convective pattern has continued to improve overall today, with a
small eye having developed and been briefly evident between
1700-1800Z. Satellite intensity estimates range from T4.0/65 kt
(TAFB and SAB) to T4.4/75 kt (UW-CIMSS ADT). The initial intensity
has been increased to 65 kt, which is just below the UW-CIMSS
SATCON consensus estimate of 68 kt.

Oscar is moving westward or 270/14 kt. The compact hurricane is
expected to move around the southwestern and western periphery of a
deep-layer ridge for the next 36 hours or so, resulting in a
westward motion overnight, followed by a turn toward the
west-northwest and northwest on Monday, with a slow motion toward
the north expected by early Tuesday. By late Tuesday and continuing
into Friday, Oscar is forecast to turn northeastward and accelerate
ahead of an eastward-moving deep-layer trough that currently lies
just east of the U.S. east coast. This large synoptic-scale feature
is expected to keep Oscar away from the United States, the Bahamas,
and Bermuda. The only fly-in-the-ointment concerning the track
forecast occurs on day 5 when a strong shortwave trough is forecast
to dig southward down the west side of Oscar, possibly capturing the
small cyclone over the north Atlantic and forcing it southward
instead of allowing the small hurricane to recurve deeper into the
high-latitude westerlies. All of the global and regional models are
now indicating this interaction, with the only difference being
whether Oscar remains a separate tropical system or merges with the
shortwave trough. For now, the new official forecast follows the
trend of the previous advisory and shows Oscar remaining a separate
entity, but slowing down considerably as an extratropical cyclone on
days 4 and 5, which is similar to that depicted by the consensus
models HCCA and TVCN.

Oscar is forecast to remain within an environment of mid- to
upper-level temperatures that are colder than average by 2-3 deg C
due to the cyclone still being embedded within the original parent
larger-scale upper-level low/trough. The combination of the below-
average environmental temperatures overlying relatively warm SSTs of
26.5 deg C will result in strong instability, which will aid the
generation of deep convection, especially overnight. Add in Oscar's
small radius of maximum winds (RMW) of 10-15 nmi, and conditions
appear to be conducive for continued strengthening. The shear
directly over the inner-core region is fairly low as noted by
anticyclonic cirrus outflow now apparent in visible and water vapor
imagery, a condition that also favors continued strengthening for
the next 48 hours or so. The HWRF, HMON, and Navy COAMPS-TC models
bring Oscar to major hurricane strength by 48 hours, which isn't out
of the realm of possibilities based on the small RMW and expected
low vertical wind shear. However, the official intensity forecast
remains on the conservative side due to expected occasional
intrusions of very dry mid-level air, which could briefly interrupt
the intensification process. By 72 hours and beyond, increasing
southwesterly shear ahead of the aforementioned deep-layer trough is
expected to induce a weakening trend, which will be enhanced by
Oscar moving over sub-23 deg C SSTs shortly thereafter. The cold
SSTs are expected to aid the transition to a strong extratropical
cyclone. The new intensity forecast is similar to the previous
advisory, except to push forward the timing of peak intensity to 48
hours, and it is a little below the HCCA and IVCN intensity
consensus models due to the aforementioned dry air issues.


INIT  28/2100Z 25.7N  55.5W   65 KT  75 MPH
 12H  29/0600Z 26.1N  57.1W   70 KT  80 MPH
 24H  29/1800Z 26.9N  58.4W   75 KT  85 MPH
 36H  30/0600Z 28.2N  58.5W   80 KT  90 MPH
 48H  30/1800Z 30.7N  57.0W   85 KT 100 MPH
 72H  31/1800Z 38.0N  50.2W   80 KT  90 MPH
 96H  01/1800Z 45.0N  39.0W   65 KT  75 MPH...POST-TROP/EXTRATROP
120H  02/1800Z 45.0N  30.0W   55 KT  65 MPH...POST-TROP/EXTRATROP

Forecaster Stewart


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Page last modified: Monday, 31-Dec-2018 12:10:04 UTC