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Hurricane Oscar Forecast Discussion Number 8
2018-10-28 21:57:11| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sun Oct 28 2018 000 WTNT41 KNHC 282057 TCDAT1 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. FORECAST POSITIONS AND MAX WINDS 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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