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Tropical Depression NINE Forecast Discussion


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000
WTNT44 KNHC 300249
TCDAT4

TROPICAL DEPRESSION NINE DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
1000 PM CDT MON AUG 29 2016

Since the previous advisory, the depression's convective pattern has
improved somewhat with the development of a cluster of deep
convection with tops to -80C having developed near and also east
through south of the center. Reports from nearby ships WAHV, J8NY,
and C6CL6, along with reconnaissance data indicate that the
low-level circulation is slowly improving. The central pressure of
1003 mb is based on a recent NOAA dropsonde report of 1005 mb with
20 kt of wind just north of the center. Since no winds of tropical
storm force were sampled, the initial intensity remains at 30 kt.

Similar to this time last night, the cyclone has made a jog to the
west during the earlier convective hiatus period. However, the past
couple of dropsonde reports suggest that the depression has resumed
a longer term motion of 280/06 kt. There is little change to the
previous forecast track reasoning. Other than having to adjust the
forecast track southward slightly through 48 hours due to the more
southerly initial position, the previous advisory track remains
unchanged. The cyclone is forecast to move slowly around the western
periphery of a deep-layer subtropical ridge located over southern
Florida for the next 36 hours or so. By 48 hours, the depression is
expected to accelerate northeastward across northern Florida ahead
of a shortwave trough that is forecast to dig southeastward into the
southeastern United States and northern Gulf Mexico. The new NHC
track forecast lies between a blend of the GFS-ECMWF solutions and
the consensus model TVCN.

Data from the NOAA aircraft on its final outbound leg, along with
the latest 00Z upper-air observations indicate that mid-level
moisture north and northeast of the cyclone has increased since
yesterday. However, water vapor imagery and upper-air data still
indicate that very dry air lies just west of the cyclone across the
central and western Gulf of Mexico. The global and regional models
continue to indicate that some of that drier air will be entrained
into the western part of the cyclone's circulation by 24-36 hours,
offsetting the otherwise favorable upper-level outflow pattern and
very warm SSTs of more than 30 deg C. Therefore, only gradual
intensification is expected during the next 48 hours or so.  When
the cyclone nears the Florida Gulf coast, increasing upper-level
winds are expected to limit strengthening.  The official intensity
forecast lies close to the previous advisory and consensus model
IVCN.

Given the current forecast, a tropical storm or hurricane watch may
be required for a portion of the Florida Gulf coast by tomorrow
morning.

FORECAST POSITIONS AND MAX WINDS

INIT  30/0300Z 23.9N  85.5W   30 KT  35 MPH
 12H  30/1200Z 24.2N  86.4W   35 KT  40 MPH
 24H  31/0000Z 24.9N  87.1W   40 KT  45 MPH
 36H  31/1200Z 25.8N  87.0W   45 KT  50 MPH
 48H  01/0000Z 27.2N  85.8W   55 KT  65 MPH
 72H  02/0000Z 30.1N  82.2W   50 KT  60 MPH...INLAND
 96H  03/0000Z 33.1N  75.5W   50 KT  60 MPH...OVER WATER
120H  04/0000Z 35.8N  68.7W   50 KT  60 MPH

$$
Forecaster Stewart