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Tropical Storm Debby Discussion Number   4
NWS National Hurricane Center Miami FL       AL042018
500 AM AST Wed Aug 08 2018

Over the past several hours, deep convection with cloud tops of -55
to -60 deg C has developed in the southeastern semicircle, with
some of the convective tops covering the previously exposed
low-level circulation center. In addition, outer banding
features have dissipated, and an elongated upper-level anticyclone
has developed over the cyclone. These convective- and synoptic-scale
features indicate that Debby has made the transition from a
subtropical to a tropical cyclone. The initial intensity of 40 kt
is based on a blend of UW-CIMSS ADT and SATCON intensity estimates
of 39 kt and 42 kt, respectively. Furthermore, the 34-kt wind radii
and radius of maximum winds (RMW) were decreased significantly on
this advisory based on ASCAT wind data.

The initial motion estimate is now 015/08 kt.  Debby has made the
forecast turn toward the north-northeast, and a further turn toward
the northeast is expected later today as the cyclone moves around
the northwestern periphery of a deep-layer ridge and ahead of an
approaching mid-level trough. The latest model guidance remains in
good agreement on this developing track scenario, and the new NHC
forecast track is similar to the previous advisory, and lies close
to a blend of the track consensus models HCCA and FSSE.

Debby will be moving along a tight sea-surface temperature (SST)
gradient for the next 24-36 hours or so, with the northwestern half
the circulation being over sub-25C SSTs and the southeastern
semicircle being over warmer waters where convection could continue
to develop. Given the relatively low vertical wind shear regime
that the cyclone will be moving through, along with the possibility
of deep convection persisting near the center, the intensity
forecast calls for little change in strength today, followed by
only slight weakening tonight and early Thursday. By Thursday night
or early Friday, SSTs beneath the cyclone are expected to decrease
to near 20 deg C and the shear is forecast to increase to more than
20 kt, an unfavorable combination that should result in Debby
dissipating over the far north Atlantic by 48 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  08/0900Z 40.8N  48.8W   40 KT  45 MPH
 12H  08/1800Z 41.8N  48.2W   40 KT  45 MPH
 24H  09/0600Z 43.2N  46.1W   35 KT  40 MPH
 36H  09/1800Z 44.5N  43.4W   30 KT  35 MPH
 48H  10/0600Z...DISSIPATED

$$
Forecaster Stewart

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