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Tropical Storm FIONA Forecast Discussion Number 8
2016-08-18 22:37:47| National Hurricane Center (Atlantic)
Issued at 500 PM AST THU AUG 18 2016 000 WTNT41 KNHC 182037 TCDAT1 TROPICAL STORM FIONA DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL AL062016 500 PM AST THU AUG 18 2016 After a brief convective hiatus, deep convection with cloud tops of -70C to -75C has redeveloped into a small CDO feature directly over the previously exposed low-level circulation center. Satellite classifications are T3.0/45 kt and T2.5/35 kt from TAFB and SAB, respectively, and UW-CIMSS ADT values have decreased to T3.0/45 kt. A blend of these values support an initial intensity of 40 kt. Fiona is moving a little faster toward the northwest, or 300/09 kt. Other than minor fluctuations in the forward speed of the cyclone due to intermittent periods of convective re-organization like the most recent episode, the latest model guidance continues to be in strong agreement on Fiona moving west-northwestward to northwestward toward a break in the Bermuda-Azores High for the next 120 hours. Given the tightly packed model guidance about the previous few forecast tracks, the new NHC track forecast is just an extension of the previous advisory, and lies close to but a little slower than the consensus model, TVCN. There is no significant change to the previous intensity forecast or rationale. Despite the earlier sharp decrease in deep convection, the inner-core wind field of the compact cyclone has remained quite robust based on the lack of no arc cloud lines or outflow boundaries seen emanating outward from the center in visible satellite today. As result, Fiona should be able to generate additional convection in the short term and strengthen some during the next 12-24 hours. After that time, the global and regional models remain in good agreement on the cyclone moving through a band of strong southwesterly vertical wind shear of 20-25 kt from 36-72 hours, which is expected to induce weakening. However, the amount of weakening remains uncertain due to continued mixed dynamic and thermodynamic conditions. Although Fiona will be propagating through significant shear and into a drier airmass, the small cyclone will also be moving over warmer SSTs of 28C-29C and into a region of much cooler upper-tropospheric temperatures, which will produce greater instability and generate fairly strong convection that could help offset the unfavorable shear conditions. Given these mixed signals, the official intensity forecast remains an average of the various intensity models, which at 72 hours still ranges from hurricane strength in the GFDL model to a 25-kt remnant low in the ECMWF and Navy-CTCI models. FORECAST POSITIONS AND MAX WINDS INIT 18/2100Z 17.0N 41.3W 40 KT 45 MPH 12H 19/0600Z 17.7N 42.5W 45 KT 50 MPH 24H 19/1800Z 18.5N 44.3W 45 KT 50 MPH 36H 20/0600Z 19.6N 46.1W 35 KT 40 MPH 48H 20/1800Z 20.8N 48.0W 35 KT 40 MPH 72H 21/1800Z 23.0N 51.7W 35 KT 40 MPH 96H 22/1800Z 25.0N 55.0W 35 KT 40 MPH 120H 23/1800Z 27.2N 56.6W 35 KT 40 MPH $$ Forecaster Stewart
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