[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 291450
TCDAT4

Potential Tropical Cyclone Nine Discussion Number   5
NWS National Hurricane Center Miami FL       AL092020
1100 AM AST Wed Jul 29 2020

Surface observations from the Lesser Antilles show that the broader
circulation of the disturbance has become slightly better defined
but a recent Air Force Reserve reconnaissance aircraft was still
unable to find a well-defined circulation.  Therefore, the system
has not yet become a tropical cyclone.  The aircraft measured peak
flight-level winds of 46 kt and several SFMR winds of around 35 kt,
so the initial intensity remains 40 kt for this advisory.

The system continues moving briskly west-northwestward at 290/20 kt.
The forecast reasoning remains unchanged as a ridge to the north of
the system is expected to steer the disturbance west-northwestward
during the next couple of days.  This motion should bring the system
near or over Hispaniola on Thursday and near eastern Cuba Thursday
night and Friday.  The ridge is forecast to weaken by the weekend
which should cause a reduction in forward speed. It still must be
stressed that since the system lacks a well-defined center and
remains in its formative stage, uncertainty in the specifics of the
track forecast remain high in both the short and longer range.  The
latest NHC track foreast has been nudged slightly south of the
previous advisory and lies between the latest ECMWF solution and the
various consensus aids.

Deep convection has consolidated and there is some evidence of
banding over the northern and western portions of the large
circulation.  As a result, the system is still expected to  become a
tropical storm later today or tonight.  Some additional
intensification is then possible before the system reaches
Hispaniola on Thursday, but weakening is likely to occur while it
interacts with land.  After that time, the system's close proximity
to eastern Cuba and an expected increase in southwesterly shear are
likely to inhibit significant re-strengthening.  The latest NHC
wind speed forecast has been lowered from the previous advisory at
72-120 h.  Given the expected land interaction and less than ideal
upper-level environment it is best to remain conservative at this
time.

Interests in Hispaniola, the Bahamas, Cuba, and Florida should
continue to monitor forecasts as changes to both the 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, and Puerto Rico today and will
spread westward to portions of the Dominican Republic, Haiti, and
the southeastern Bahamas and Turks and Caicos on Thursday and
Thursday night. 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
remain more uncertain than usual since the system does not have a
well-defined center and it is expected to move near or over portions
of the Greater Antilles later this week. However, this system could
bring some rainfall and wind impacts to portions of Cuba, the
central and northwest Bahamas, and Florida later this week and this
weekend. Interests there should monitor its progress and updates to
the forecast over the next few days.

FORECAST POSITIONS AND MAX WINDS

INIT  29/1500Z 15.8N  63.7W   40 KT  45 MPH...POTENTIAL TROP CYCLONE
 12H  30/0000Z 16.9N  66.4W   45 KT  50 MPH...TROPICAL CYCLONE
 24H  30/1200Z 18.5N  69.8W   50 KT  60 MPH
 36H  31/0000Z 19.9N  73.0W   40 KT  45 MPH...INLAND
 48H  31/1200Z 21.4N  76.0W   40 KT  45 MPH...NEAR COAST
 60H  01/0000Z 22.7N  78.3W   40 KT  45 MPH...OVER WATER
 72H  01/1200Z 23.8N  80.2W   45 KT  50 MPH
 96H  02/1200Z 26.3N  82.6W   45 KT  50 MPH
120H  03/1200Z 29.0N  83.5W   45 KT  50 MPH

$$
Forecaster Brown
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