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Subtropical Depression ONE
ZCZC MIATCDAT1 ALL TTAA00 KNHC DDHHMM Subtropical Depression One Discussion Number 4 NWS National Hurricane Center Miami FL AL012017 500 AM AST Thu Apr 20 2017 Data from nearby drifting buoys indicate that the subtropical depression is now moving over sea-surface temperatures of 19C or less. These cooler waters have resulted in a significant weakening and erosion of the inner core convection during the past few hours. In contrast, curved outer band convection has been increasing in the eastern semicircle. Intensity estimates at 0600Z were ST1.5/25 kt from TAFB and ADT T3.1/47 kt from UW-CIMSS. Although it is possible that the system could have reached subtropical storm intensity around 0300-0500Z when a donut-ring of moderate convection completely encircled the low-level circulation center, the recent rapid erosion of the central convection argues for maintaining an intensity of 30 kt for this advisory. The cyclone is now moving a little west of due north at 12 kt. As a large extratropical cyclone located to the west of the depression continues to amplify and dig southeastward over the next 24-48 hours, the subtropical cyclone should turn toward the northwest later today, and move west-northwestward to westward tonight and on Friday as a shallow remnant low. A 24-h position has been added based on the 00Z ECMWF and GFS models now showing the depression persisting as a well-defined, shallow low pressure system into Friday before it is absorbed by the aforementioned larger extratropical low. The combination of decreasing sea-surface temperatures and increasing southwesterly vertical wind shear ahead of the large extratropical low should act to steadily erode the associated convection, resulting in the depression weakening to a remnant low pressure system by tonight...if not sooner. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 34.4N 39.3W 30 KT 35 MPH 12H 20/1800Z 36.7N 41.6W 30 KT 35 MPH 24H 21/0600Z 38.0N 46.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 21/1800Z...DISSIPATED $$ Forecaster Stewart NNNN