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| #800954 (Received by flhurricane at: 4:35 AM 05.Sep.2015) TCDAT1 TROPICAL STORM FRED DISCUSSION NUMBER 26 NWS NATIONAL HURRICANE CENTER MIAMI FL AL062015 500 AM AST SAT SEP 05 2015 Fred continues to produce bursts of convection, although the bursts are smaller and farther from the center than they were 24 hours ago. This suggests that the ongoing 35-40 kt of westerly vertical shear is causing a gradual weakening. The initial intensity remains 35 kt based on a subjective satellite intensity estimate from TAFB, as well as objective AMSU and satellite consensus estimates from CIMSS and CIRA. The central pressure has been lowered a little based on additional observations from drifting buoy 13519, which reported a minimum pressure of 1008.4 mb as the center of Fred passed to the north. The initial motion is 290/10. Fred is approaching a large break in the subtropical ridge caused by a mid- to upper-level trough and associated surface low centered several hundred miles east of Bermuda. The track guidance is in good agreement that this baroclinic system will move eastward for the next several days, with Fred recurving to the northeast between it and the subtropical ridge. After recurvature, there is some spread in the forecast forward speed, with the new track compromising between the faster ECMWF/GFS models and the slower UKMET/Canadian models. The new track is nudged slightly to the north of the previous track from 36-96 hours, and then is nudged a little to the east of the previous track at 120 hours. The intensity forecast is low in confidence due to several possible scenarios. First, although it is not explicitly forecast, Fred could become a remnant low at any time during the next 24 to 36 hours due to continued shear and dry air entrainment, followed by regeneration to a tropical cyclone when the shear subsequently decreases. Second, the dynamical models have two scenarios for Fred as it interacts with the above-mentioned trough and surface low after recurvature. One possibility, supported by the GFS and the ECMWF, is that a relatively weak Fred gets absorbed into the baroclinic low and dissipates earlier than currently forecast. Another possibility, supported by the UKMET and Canadian models, is that Fred stays farther away from the baroclinic low and intensifies more than is currently forecast. Given the uncertainty, the new intensity forecast is similar to the previous forecast in calling for slight weakening early in the forecast period followed by slight intensification after 36 hours. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 23.2N 41.0W 35 KT 40 MPH 12H 05/1800Z 23.7N 42.1W 30 KT 35 MPH 24H 06/0600Z 24.7N 42.7W 30 KT 35 MPH 36H 06/1800Z 26.3N 42.5W 30 KT 35 MPH 48H 07/0600Z 28.2N 41.1W 35 KT 40 MPH 72H 08/0600Z 31.5N 37.5W 35 KT 40 MPH 96H 09/0600Z 35.0N 33.0W 35 KT 40 MPH 120H 10/0600Z 36.5N 30.0W 35 KT 40 MPH $$ Forecaster Beven |
