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| #872428 (Received by flhurricane at: 10:37 AM 18.Aug.2016) TCDAT1 TROPICAL STORM FIONA DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL062016 1100 AM AST THU AUG 18 2016 Deep convection has decreased since the previous advisory due to Fiona moving into the daytime convective minimum period, plus likely entrainment of dry mid-level air. Satellite classifications remain T3.0/45 kt and T2.0/30 kt from TAFB and SAB, respectively, but UW-CIMSS ADT values have increased to T3.2/49 kt. However, the recent erosion of the inner-core convection argues for maintaining the initial intensity at 40 kt despite the higher ADT value. The forward speed has decreased sharply since the previous advisory, and Fiona is now moving at 300/07 kt. The recent decrease in forward speed has been well advertised by the NHC track guidance for the past couple of days. The latest model guidance is in much better agreement on Fiona moving west-northwestward to northwestward toward a break in the subtropical ridge between 50W-55W throughout the forecast period, and is now converging tightly around the previous forecast track. Therefore, the new official forecast track is just an update of the previous advisory track, and lies a little to the left of consensus track model, TVCN, due to a strong right bias caused by the much stronger and vertically deeper GFDL model. The intensity forecast is a little less straightforward than the track forecast due to mixed dynamic and thermodynamic signals. On one hand, shear conditions are expected to increase to more than 20 kt in 48-72 hours while the cyclone is moving into a much drier air airmass consisting of near 40 percent mid-level humidity values. This combination of negative parameters generally supports significant weakening. However, Fiona will also be moving over warmer SSTs of 28-29C and into a region of much cooler upper-level temperatures, which will act to generate greater instability and likely more vigorous and persistent convection despite the drier mid-level environment. Given these mixed signals, the new NHC forecast is an average of the various intensity models, which at 96 hours ranges from hurricane strength in the GFDL model to a 25-kt remnant low in the Navy-CTCI and ECMWF models. FORECAST POSITIONS AND MAX WINDS INIT 18/1500Z 16.4N 40.5W 40 KT 45 MPH 12H 19/0000Z 17.2N 41.7W 45 KT 50 MPH 24H 19/1200Z 18.1N 43.3W 45 KT 50 MPH 36H 20/0000Z 19.1N 45.1W 40 KT 45 MPH 48H 20/1200Z 20.3N 47.0W 35 KT 40 MPH 72H 21/1200Z 22.6N 50.6W 35 KT 40 MPH 96H 22/1200Z 24.4N 53.7W 35 KT 40 MPH 120H 23/1200Z 26.4N 56.0W 35 KT 40 MPH $$ Forecaster Stewart |
