TROPICAL DEPRESSION NINE DISCUSSION NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092015
1100 AM AST WED SEP 16 2015
Organized convection associated with the area of low pressure in the
central tropical Atlantic has now increased to the point that the
system is considered a tropical cyclone. The initial intensity is 25
kt based on the latest Dvorak classifications and in agreement with
earlier ASCAT wind data. Some southwesterly shear is already
affecting the cyclone, with most of the convection displaced east
and north of the estimated center position. The environment only
becomes less favorable from this point forward, with the shear
forecast to quickly increase in 12 to 24 hours and remain high
through the remainder of the forecast period. In addition, the
southwesterly flow aloft will bring dry mid- to upper-level air over
the cyclone. As a result, only slight strengthening is shown in the
NHC forecast following the trend of most of the intensity guidance.
The cyclone is expected to weaken to a remnant low by 72 hours.
After that time, the GFS and ECMWF models show the system weakening
and perhaps opening up into a trough in about 5 days, so the
official forecast shows dissipation at that time.
The initial motion estimate is 340/07, as the depression is moving
into a weakness in the Atlantic subtropical ridge. This north-
northwestward to northwestward motion should continue for the next
2 to 3 days, with a turn toward the west-northwest forecast by 96
hours as the shallow cyclone comes under the steering influence of
the low-level trade wind flow. The NHC track forecast is close to
the middle of the guidance envelope and near a blend of the GFS and
ECMWF forecasts.
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