[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 290253
TCDAT4

Potential Tropical Cyclone Nine Discussion Number   3
NWS National Hurricane Center Miami FL       AL092020
1100 PM AST Tue Jul 28 2020

Earlier wind data from an Air Force reconnaissance aircraft and a
recent 0038Z ASCAT-A overpass indicate that the disturbance has not
become any better organized since the previous advisory. There were
indications of a circulation center located near the position
estimate used in this advisory. However, there was a sharp cusp
noted in the ASCAT wind field, and that was used for positioning
the disturbance since it lies closest to the strong convective band
and best upper-level divergence. The initial intensity of 35 kt is
based on several ASCAT wind vectors of 35-36 kt located well north
of the center. The 35-kt intensity is also consistent with a 0000Z
TAFB Dvorak satellite intensity estimate of T2.5/35 kt.

The initial motion estimate is an uncertain 295/22 kt. The ridge to
the north of the disturbance is forecast to remain strong for the
next 36-48 h, which keeps the system moving in a general
west-northwestward direction across the Lesser Antilles tonight and
Wednesday, and near or over the Greater Antilles Wednesday night and
Thursday. For such a loosely organized system at this time, the
models are in fairly good agreement on the large disturbance
slowing down significantly after 48 h, reaching forward speeds of
only 10-12 kt when it reaches the very warm waters of the Straits
of Florida in 72-96 h. On days 4 and 5, the system is expected to
slow even further and turn northward into a break in the subtropical
ridge that is expected to develop across Florida and the Bahamas.
The new NHC track forecast is similar to but a little south of the
previous advisory track, mainly due to the more westward initial
position, and lies along the southern portion of the guidance
envelope near the middle of the consensus models.  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 during the next 24 hours.

The intensity forecast remains problematic for primarily two
reasons: 1) the lack of a well-defined center and inner-core wind
field and 2) likely land interaction to some degree. In the
short-term, a bonafide center could develop tonight in response to
the expected development of intense convection caused by orographic
forcing by the mountainous islands of the central and northern
Leeward Islands. Once a center closes off, which has likely been
inhibited from doing so due to the disturbance's fast forward speed
in excess of 20 kt, the low-level convergence will improve and
convection will become more organized and symmetrical, allowing for
strengthening to occur. The main question is how much land
interaction with Puerto Rico and Hispaniola will disrupt the
circulation in the 36-48-hour period. Assuming the system remains
intact after emerging off the coast of Hispaniola, the slow track
over the very warm waters of the Straits of Florida would result in
more strengthening, assuming the system doesn't interact with the
Cuban landmass. Although the GFS-and ECMWF-based SHIPS intensity
guidance shows considerable southwesterly vertical wind shear of
20-30 kt in the 72-96 h period when the disturbance is over the
Straits, the global model fields show that this is self-induced
shear caused by the SHIPS model incorporating the system's
impressive upper-level outflow winds in its shear calculations. As a
result, this is not being considered a negative intensity factor
compared to land interaction. Due to aforementioned uncertainties,
the new NHC intensity forecast remains on the conservative side, and
lies between the slightly weaker IVCN and stronger NOAA-HCCA
consensus models. 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, 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  29/0300Z 14.6N  59.4W   35 KT  40 MPH...POTENTIAL TROP CYCLONE
 12H  29/1200Z 15.7N  62.3W   40 KT  45 MPH
 24H  30/0000Z 17.4N  65.9W   45 KT  50 MPH
 36H  30/1200Z 18.7N  69.3W   45 KT  50 MPH...INLAND
 48H  31/0000Z 20.1N  72.6W   45 KT  50 MPH...OVER WATER
 60H  31/1200Z 21.5N  75.5W   50 KT  60 MPH
 72H  01/0000Z 22.8N  77.7W   55 KT  65 MPH
 96H  02/0000Z 25.7N  80.7W   55 KT  65 MPH...INLAND
120H  03/0000Z 27.9N  82.0W   45 KT  50 MPH...INLAND

$$
Forecaster Stewart
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