[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 281458
TCDAT4

Potential Tropical Cyclone Nine Discussion Number   1
NWS National Hurricane Center Miami FL       AL092020
1100 AM AST Tue Jul 28 2020

Shower and thunderstorm activity has increased overnight and this
morning in association with a broad area of low pressure located
several hundred miles east of the Lesser Antilles.  Recent visible
satellite imagery and ASCAT data show that the system's circulation
is quite elongated and lacks a well-defined center. Observations
from NOAA buoy 41040 and ASCAT suggest that the system is
already producing winds of 30-35 kt, and the systems's initial
intensity has been set to 35 kt. Dry air located just to the north
of the system has been hindering development over the past couple of
days, but environmental conditions are expected to be more conducive
for development over the next couple of days. Therefore, the system
is likely to become a tropical storm before it reaches the Leeward
Islands and advisories are being initiated in order to issue
Tropical Storm Warnings for a portion of the Leeward Islands, the
U.S. and British Virgin Islands, and Puerto Rico. A U.S. Air Force
Reserve reconnaissance aircraft is scheduled to investigate the
system early this afternoon, and should provide additional
information on the intensity and structure of the low pressure
area.

It cannot be stressed enough that since the system is still in the
formative stage, greater than average uncertainty exists regarding
both the short-term and longer-term track and intensity forecasts.
A subtropical ridge that extends westward from the central Atlantic
is expected to be the dominant steering mechanism over the next
several days, and the flow around this ridge should steer the low
pressure area generally west-northwestward.  However, the details in
the track forecast could change depending on exactly where within
elongated circulation the center forms.  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 24-48 hours.  After that time, a general west-
northwestward heading should continue but as mentioned before,
uncertainty exists as to how close the system tracks to
Hispaniola, Cuba, the Bahamas, and Florida. The NHC track forecast
is in best agreement with the HFIP corrected consensus model.  It
should be noted that a stronger cyclone is likely to favor a more
northern track, while a weaker system is likely to remain more
equatorward.  Users should remember that the long-term average NHC
track forecast errors at days 4 and 5 are 140 and 175 n mi,
respectively.

Given the current structure of the system, only gradual
strengthening is predicted during the next day or two, however the
system is expected to become a tropical storm when it is near the
Leeward Islands on Wednesday.  After 48 hours, possible land
interaction with the Greater Antilles, and increasing south to
southwesterly shear from an upper-level trough could temper further
strengthening. The global models generally weaken the system due to
these negative factors and the NHC forecast calls for little change
in strength at the longer range.  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 and Puerto Rico.

2. Tropical storm conditions are likely across portions of the
Leeward Islands, the Virgin Islands, and Puerto Rico Wednesday
through Thursday, and Tropical Storm Warnings are in effect.
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 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  28/1500Z 13.8N  53.7W   35 KT  40 MPH...POTENTIAL TROP CYCLONE
 12H  29/0000Z 14.8N  56.7W   35 KT  40 MPH...POTENTIAL TROP CYCLONE
 24H  29/1200Z 16.0N  60.4W   40 KT  45 MPH...TROPICAL STORM
 36H  30/0000Z 17.3N  64.0W   45 KT  50 MPH
 48H  30/1200Z 18.8N  67.6W   50 KT  60 MPH
 60H  31/0000Z 20.1N  70.7W   50 KT  60 MPH
 72H  31/1200Z 21.3N  73.5W   50 KT  60 MPH
 96H  01/1200Z 24.1N  77.9W   50 KT  60 MPH
120H  02/1200Z 26.8N  80.6W   45 KT  50 MPH...INLAND

$$
Forecaster Brown
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