[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 221439
TCDAT2

Tropical Storm Gonzalo Discussion Number   4
NWS National Hurricane Center Miami FL       AL072020
1100 AM AST Wed Jul 22 2020

Satellite imagery indicates that the cyclone has become
significantly better organized since the last advisory.  Visible
imagery shows a well-defined central dense overcast with a hint of
an eye and an outer convective band in the western semicircle, while
a 09Z GMI microwave overpass showed a well-defined inner convective
ring feature.  Various subjective and objective satellite intensity
estimates range from 35-55 kt, so the initial intensity is raised to
a possibly conservative 45 kt.

The initial motion is now 270/10.  Gonzalo is on the south side of
a low- to mid-level subtropical ridge, and this feature should
steer the storm generally westward at a faster forward speed for
the next 3 days or so.  After that time, a motion toward the
west-northwest is expected.  The new NHC forecast track is little
changed from the previous track, and it lies very close to the
consensus models.

The intensity forecast remains very problematic and of low
confidence. On one side, the cyclone has been strengthening quickly
and the good organization suggests additional, and possibly rapid,
strengthening should occur.  In addition, the SHIPS-based guidance
and the HWRF make the system a hurricane and keep that intensity
through 120 h.  On the other side, the GFS, UKMET, ECMWF, and
Canadian models are not big fans of this system, as they all
forecast it to either be a weak low or dissipated by 120 h, possibly
due to dry air entrainment and large-scale subsidence.  The NHC
intensity forecast again compromises between these extremes, showing
Gonzalo peaking as a hurricane in 36-48 h, followed by weakening in
deference to the global models.  The new intensity forecast has
significantly higher intensities than the previous forecast for most
of the forecast period based on the recent intensification.  As
noted before, the small size of this system makes it susceptible to
significant fluctuations in intensity, both upward and downward.

Interests in the southern Windward Islands should monitor the
progress of this system.

Gonzalo is the earliest 7th named storm on record in the Atlantic
basin, beating Gert of 2005 by 2 days.


Key Messages

1. Gonzalo is expected to move near or over the southern Windward
Islands this weekend, and could bring direct impacts from winds and
heavy rainfall. While it is too soon to determine the magnitude and
timing of those impacts, interests in the southern Windward Islands
should monitor the progress of Gonzalo.

FORECAST POSITIONS AND MAX WINDS

INIT  22/1500Z  9.9N  43.6W   45 KT  50 MPH
 12H  23/0000Z 10.0N  45.3W   55 KT  65 MPH
 24H  23/1200Z 10.0N  47.7W   65 KT  75 MPH
 36H  24/0000Z 10.1N  50.2W   70 KT  80 MPH
 48H  24/1200Z 10.4N  52.8W   70 KT  80 MPH
 60H  25/0000Z 10.7N  55.6W   65 KT  75 MPH
 72H  25/1200Z 11.4N  58.8W   60 KT  70 MPH
 96H  26/1200Z 13.0N  66.0W   50 KT  60 MPH
120H  27/1200Z 15.0N  71.5W   40 KT  45 MPH

$$
Forecaster Beven
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