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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

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