[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 290846
TCDAT4

Potential Tropical Cyclone Nine Discussion Number   4
NWS National Hurricane Center Miami FL       AL092020
500 AM AST Wed Jul 29 2020

Satellite and surface data indicate that the system remains a
trough of low pressure, elongated from SSW to NNE, with almost all
of the strong winds far north of the center position.  The most
significant curvature in the low-level wind field and on radar are
near Dominica now, which has good continuity from the previous
advisory, so this feature will continue to be used as the center.
A combination of unflagged SFMR winds from an Air Force plane and
earlier scatterometer winds support 40 kt as the initial wind speed.

The initial motion estimate is about the same as before or 295/20
kt. The ridge to the north of the disturbance is forecast to remain
strong for the next 36 h, which keeps the system moving speedily in
a general west-northwestward direction just south of the Leeward
Islands today, and near or over the Greater Antilles on Thursday.
The ridge is forecast to weaken after that time, which should cause
the cyclone to slow down, and potentially gain more latitude over
the southwestern Atlantic.  The model guidance is generally showing
a narrow ridge persisting for a bit longer, however, causing a
small south and west shift in the new NHC forecast at long range.
However, it should be emphasized that this forecast track is highly
uncertain until a true center forms.

Satellite images indicate that a large burst of convection is
occuring near the poorly defined center, which will likely lead to
the system becoming a tropical storm later today.  Further
intensification is possible before landfall in the Dominican
Republic on Thursday, assuming the structure continues to improve,
and the wind speed forecast is adjusted slightly higher in the near
term.  The cyclone will probably take some time to recover after
moving over the very high mountains of Hispaniola, and some of the
guidance after that time shows an increase in southwesterly shear
over the Straits of Florida that could limit the potential of the
cyclone.  Simply put, there are a lot of hurdles in the system's
way, so it is best to stay on the conservative side at the moment
and continue to stress the large uncertainty after it leaves the
Caribbean. Little change was made to the forecast intensity,
although the guidance has come down at longer range for many of the
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 and Puerto Rico today and
spreading westward to portions of the Dominican Republic and Haiti
on 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 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  29/0900Z 15.3N  61.3W   40 KT  45 MPH...POTENTIAL TROP CYCLONE
 12H  29/1800Z 16.3N  64.1W   45 KT  50 MPH...TROPICAL STORM
 24H  30/0600Z 17.8N  67.6W   50 KT  60 MPH
 36H  30/1800Z 19.3N  71.0W   40 KT  45 MPH...INLAND
 48H  31/0600Z 20.6N  74.0W   40 KT  45 MPH...OVER WATER
 60H  31/1800Z 22.1N  76.7W   45 KT  50 MPH
 72H  01/0600Z 23.3N  78.7W   50 KT  60 MPH
 96H  02/0600Z 25.5N  81.5W   55 KT  65 MPH
120H  03/0600Z 28.5N  83.0W   45 KT  50 MPH

$$
Forecaster Blake
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