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| #873220 (Received by flhurricane at: 10:56 PM 28.Aug.2016) TCDAT4 TROPICAL DEPRESSION NINE DISCUSSION NUMBER 2 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 1100 PM EDT SUN AUG 28 2016 Flight-level wind data from an earlier NOAA reconnaissance mission along with WSR-88D Doppler radar data from Key West indicate that the depression had been moving southwestward between 1800-0000 UTC. However, the most recent radar data and nearby surface observations suggest that the cyclone has now turned toward the west. The last reliable wind data from the NOAA WP-3 recon aircraft supported an intensity of 30 kt, and that intensity is being maintained for this advisory given that the radar and satellite signatures haven't improved. The central pressure of 1007 mb is based on a reliable observation from ship WMKN, located just north of the center. The initial motion estimate is 270/08 kt. Now that deep convection has waned, the system has turned westward and this motion is expected to continue for the next 24 hours or so. This short term motion is supported by NOAA recon dropsonde data on the return leg home, which indicated that 500 mb heights were 10-20 meters higher over the southeastern Gulf of Mexico than what the global models have been forecasting. After that time, the global and regional models are in surprisingly good agreement on the cyclone slowing down and turning toward the west-northwest and then northward in the 36- to 48-hour periods as the depression moves around the western periphery of a narrow subtropical ridge that is expected to be located over the Bahamas and South Florida at that time. By 72 hours and beyond, the tropical cyclone is forecast to lift out and accelerate to the northeast as a shortwave trough over the western Great Lakes digs southeastward and captures the depression. The new NHC forecast track has been shifted to the right of the previous advisory track mainly due to the more southerly initial position, and lies a little to the left of the consensus model TVCN. Strong vertical shear that has been plaguing this system for the past week is expected to gradually subside to less than 10 kt in 18-24 hours, which should allow for more organized deep convection to develop. However, the southerly low-level inflow will still be disrupted by the terrain of western Cuba until the cyclone moves west of 85W longitude, which will then provide a straight trajectory across the Yucatan Channel and into the low-level center. By 36 hours and beyond, the depression will moving over SSTs greater than 30C and the light vertical wind shear is expected to back around from a northerly to a southwesterly direction, which usually favors more significant intensification. However, dry air in the mid-/upper-levels noted in the recent 0000 UTC soundings from Key West northward to the Gulf coast is expected be entrained into the northwestern semicircle of the cyclone's circulation by 48 hours and beyond, and this appears to be the main inhibiting factor to strengthening by the global models. Given these mixed signals, the NHC intensity forecast remains conservative and closely follows the intensity model IVCN. The confidence in the intensity forecast remains lower than usual for this system. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 23.4N 82.7W 30 KT 35 MPH 12H 29/1200Z 23.5N 84.0W 30 KT 35 MPH 24H 30/0000Z 23.8N 85.4W 35 KT 40 MPH 36H 30/1200Z 24.4N 86.8W 35 KT 40 MPH 48H 31/0000Z 25.1N 87.1W 40 KT 45 MPH 72H 01/0000Z 27.0N 86.3W 45 KT 50 MPH 96H 02/0000Z 29.1N 82.9W 50 KT 60 MPH 120H 03/0000Z 31.2N 78.3W 50 KT 60 MPH $$ Forecaster Stewart |
