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Tropical Depression NINE Forecast Discussion


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000
WTNT44 KNHC 282055
TCDAT4

TROPICAL DEPRESSION NINE DISCUSSION NUMBER   1
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
500 PM EDT SUN AUG 28 2016

Data from a NOAA Hurricane Hunter aircraft indicate that the area of
low pressure located in the Florida Straits now has a well-defined
center.  Satellite imagery shows a significant increase in the
convective organization today, and as a result the system is now
classified as a tropical cyclone.  The initial intensity is set to
30 kt based on the highest believable wind data from the aircraft,
which reported a central pressure of 1009 mb.

The depression will be moving through a marginal environment for
intensification during the next day or so, with vertical shear of
15 to 20 kt.  As a result only slow strengthening is expected in
the short term.  Later on, the environment may improve a little as
the shear is forecast to decrease somewhat and become
southwesterly, which should allow for a little more strengthening.
However, there are mixed signals in the model guidance, with the
ECMWF now showing the cyclone dissipating in the Gulf, while the
GFS delays development until 4-5 days.  Much of the tropical
cyclone guidance is more aggressive.  Given this uncertainty, the
NHC intensity forecast is quite conservative and shows the system
peaking at 45 kt, below all the explicit intensity guidance in
consideration of the negative signal from the ECMWF.  Needless to
say, the confidence in the intensity forecast is even lower than
usual for this system.

The initial motion estimate is an uncertain 270/09 given the recent
formation of the center.  The cyclone will be steered in the short
range by a mid-level ridge centered over the southeastern United
States.  This ridge will weaken in 2-3 days, which will cause the
cyclone to slow down and turn northward during this time.  Late in
the period a northeastward acceleration is expected ahead of an
approaching mid-latitude trough.  There is reasonable agreement in
the track of the cyclone in the global model guidance, although
there is a fair bit of along-track spread late in the period.  The
NHC forecast is close to a consensus of the GFS and ECMWF through
day 4 and is a little faster than the GFS and GEFS ensemble mean at
day 5.

FORECAST POSITIONS AND MAX WINDS

INIT  28/2100Z 23.7N  81.7W   30 KT  35 MPH
 12H  29/0600Z 23.9N  83.6W   30 KT  35 MPH
 24H  29/1800Z 24.3N  85.4W   35 KT  40 MPH
 36H  30/0600Z 24.6N  86.8W   35 KT  40 MPH
 48H  30/1800Z 25.1N  87.6W   40 KT  45 MPH
 72H  31/1800Z 26.8N  87.4W   45 KT  50 MPH
 96H  01/1800Z 29.0N  84.5W   45 KT  50 MPH
120H  02/1800Z 31.0N  80.0W   45 KT  50 MPH

$$
Forecaster Brennan