Potential Tropical Cyclone Nine Forecast Discussion

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000
WTNT44 KNHC 282040
TCDAT4

Potential Tropical Cyclone Nine Discussion Number   2
NWS National Hurricane Center Miami FL       AL092020
500 PM AST Tue Jul 28 2020

The overall structure of the disturbance has not changed much since
this morning.  New clusters of convection have developed over the
northern portion of the elongated circulation and the Air Force
Reserve reconnaissance aircraft has been unable to find a
well-defined center.  The aircraft measured some SFMR winds of
30-35 kt well to the northwest of the trough axis, and these data
along with the earlier ASCAT form the basis of the 35 kt initial
intensity. The lastest dynamical model guidance still suggests that
the system will consolidate over the next 12-18 hours and the
disturbance is expected to become a tropical storm before it
reaches the Leeward Islands on Wednesday.

The particulars of the track forecast remain uncertain since
the system lacks a well-defined center.  The initial motion estimate
is a highly uncertain 285/20 kt. The overall track foreast reasoning 
remains the same, with the disturbance expected to move 
west-northwestward over the next several days to the south of a 
subtropical ridge that extends westward over the western Atlantic. 
The 12Z track guidance is in agreement on the overall scenario, but 
some differences are noted due primarily to the system's strength 
and vertical depth.  Models such as the UKMET and HWRF, which 
depict a stronger cyclone, are along the northern side of the 
guidance envelope, while the weaker solutions remain more 
equatorward.  The updated NHC track forecast is similar to the 
previous advisory and lies a little south of the various consensus 
aids.  Regardless of the exact track, the system is expected to 
bring locally heavy rainfall to much of the Lesser Antilles, and 
tropical-storm-force winds to portions of the Leeward Islands, the 
Virgin Islands, and Puerto Rico within the next 24-36 hours.

The global model guidance suggests that the system will consolidate
overnight and should acquire a better defined center.  As this
occurs, the disturbance is likely to gradually strengthen within
the low vertical wind shear environment that it is situated in.
Between 36 and 72 hours, the strength of the system will largely be
dependent on the amount of land it encounters.  If the system moves
over the Greater Antilles it is likely to be weaker than indicated
in the official forecast, but a path north or south of Hispaniola
could result in a stronger system.  Later in the period, some
southwesterly shear could also temper further strengthening.  The
GFS and ECMWF models generally weaken the system in the longer
range due to these negative factors.  The NHC intensity forecast is
unchanged from the previous advisory, but an usually high degree of
uncertainty regarding the future track and intensity of the system
remains.  Interests in Hispaniola, the Bahamas, Cuba, and Florida
should continue to monitor forecasts as changes to both track and
intensity are likely.

Key Messages:

1. Potential Tropical Cyclone Nine will produce heavy rains and
potentially life-threatening flash flooding and mudslides across
the northern Leeward Islands, the Virgin Islands, Puerto Rico, and 
the Dominican Republic.

2. Tropical storm conditions are likely across portions of the
Leeward Islands, the Virgin Islands, Puerto Rico, and portions of
the Dominican Republic beginning Wednesday and spreading westward
through Thursday.  Tropical Storm Warnings are in effect for these
areas. Do not focus on the details of the track forecast, as
rainfall and wind hazards will extend far from the center of the
system.

3. The details of the long-range track and intensity forecasts are
more uncertain than usual since the system does not have a
well-defined center and it could move over portions of the Greater
Antilles later this week. However, this system could bring some
rainfall and wind impacts to portions of Hispaniola, Cuba, the
Bahamas, and Florida by the end of the week. Interests there should
monitor its progress and updates to the forecast over the next few
days.


FORECAST POSITIONS AND MAX WINDS

INIT  28/2100Z 14.4N  55.9W   35 KT  40 MPH...POTENTIAL TROP CYCLONE
 12H  29/0600Z 15.4N  58.8W   35 KT  40 MPH
 24H  29/1800Z 16.7N  62.5W   40 KT  45 MPH
 36H  30/0600Z 18.2N  66.1W   45 KT  50 MPH
 48H  30/1800Z 19.7N  69.7W   50 KT  60 MPH
 60H  31/0600Z 20.9N  72.5W   50 KT  60 MPH
 72H  31/1800Z 22.2N  75.3W   50 KT  60 MPH
 96H  01/1800Z 24.7N  79.0W   50 KT  60 MPH
120H  02/1800Z 27.0N  81.0W   45 KT  50 MPH...INLAND

$$
Forecaster Brown



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