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Tropical Storm FIONA Forecast Discussion Number 4
2016-08-17 22:42:23| National Hurricane Center (Atlantic)
Issued at 500 PM AST WED AUG 17 2016 000 WTNT41 KNHC 172042 TCDAT1 TROPICAL STORM FIONA DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL062016 500 PM AST WED AUG 17 2016 During the past 6 hours, the tropical cyclone has made a transition from a large outer banding pattern to more of a CDO-type feature. Also, a 1635Z SSMI overpass confirmed the tight inner-core structure noted in the two earlier ASCAT passes. Dvorak satellite intensity estimates are T2.5/35 kt from TAFB and SAB, and a 1225Z ASCAT-A overpass indicated winds of near 35 kt at that time. Based on these data, the initial intensity is increased to 35 kt and the cyclone is upgraded to Tropical Storm Fiona. The initial motion estimate is 305/14 kt. The latest model guidance has come into much better agreement with the previous track forecast, with the GFS model shifting a little farther south and the ECMWF making a significant shift to the north due to that model not weakening Fiona as much now as compared to previous forecast cycles. These model changes have resulted in a northward shift in the consensus models TVCN and GFEX. Given the much better agreement in the NHC model guidance, the new track forecast was only shifted slightly southward of the previous advisory track after 72 hours in order to move closer to the consensus models. There are no significant changes to the previous intensity forecast. Fiona is a compact tropical cyclone with a radius of maximum winds of 15 nmi or less, which makes the storm susceptible to sharp fluctuations in intensity. The aforementioned SSMI satellite pass also indicated that the small inner-core region of Fiona now appears to be insulated from the very dry mid-level air that had earlier eroded the cyclone's convection. That, along with low shear conditions and marginal SSTs near 27C, should allow for some gradual strengthening for the next 36-48 hours, assuming that the cyclone can mix out any occasional intrusions of dry air. By 72 hours, the vertical shear is forecast to increase to around 20 kt from the southwest, which should act to arrest the intensification process and perhaps even induce some slight weakening. However, by 96 hours and beyond, the shear is forecast to steadily decrease when the cyclone will be moving over SSTs greater than 28C. These conditions could allow for the compact cyclone to maintain a steady intensity despite being embedded in mid-level humidity values of near 50 percent. The official intensity forecast is similar to the previous advisory and closely follows the intensity consensus model IVCN through 48 hours, and then lies just below the Decay-SHIPS model at 72-120 hours. FORECAST POSITIONS AND MAX WINDS INIT 17/2100Z 15.1N 37.8W 35 KT 40 MPH 12H 18/0600Z 16.0N 39.0W 40 KT 45 MPH 24H 18/1800Z 17.0N 40.6W 45 KT 50 MPH 36H 19/0600Z 17.8N 42.1W 50 KT 60 MPH 48H 19/1800Z 18.7N 43.8W 50 KT 60 MPH 72H 20/1800Z 20.3N 47.4W 45 KT 50 MPH 96H 21/1800Z 22.6N 51.3W 45 KT 50 MPH 120H 22/1800Z 24.6N 55.0W 45 KT 50 MPH $$ Forecaster Stewart
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