[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 232041
TCDAT2

Tropical Storm Gonzalo Discussion Number   9
NWS National Hurricane Center Miami FL       AL072020
500 PM AST Thu Jul 23 2020

Scatterometer data received just after the release of the previous
advisory only showed winds of 40-45 kt, but given Gonzalo's small
size, it's unlikely that the resolution of the instrument was able
to capture the maximum winds.  Even correcting for that, Gonzalo's
winds were lowered to 50 kt on the 2 PM intermediate advisory, and
that still looks good now with satellite intensity estimates
ranging from 35 kt to 55 kt.  Structure-wise, deep convection has
redeveloped on top of Gonzalo's center, so the storm is not ready
to give up its fight quite yet.

There is still greater-than-normal uncertainty in Gonzalo's
forecast intensity due to its small size and how it will behave in
an environment of relatively light shear and warm sea surface
temperatures counterbalanced by a lot of dry air.  The  statistical
and dynamical hurricane models continue to show strengthening, while
several of the global models, in particular the ECMWF and UKMET,
still show Gonzalo weakening and opening up into a trough near or
after it passes the Windward Islands.  Based on the lower initial
intensity and the latest guidance, the new official forecast
has been lowered slightly, but it still brings Gonzalo to the
hurricane threshold in 36-48 hours.  Either way, Gonzalo's small
size makes it susceptible to short-term changes in intensity that
cannot be reflected in the official forecast.  There is more
confidence that weakening will occur after Gonzalo moves over the
Caribbean Sea, with even the GFS showing the system becoming a
trough.  Given that, dissipation is now shown in the forecast at day
5.

Gonzalo's initial motion is westward, or 275/11 kt.  There has been
no change in the track forecast reasoning, with Gonzalo expected to
accelerate toward the west and west-northwest through the forecast
period.  The track guidance is fairly tightly clustered, showing
Gonzalo's center crossing somewhere through the Windward Islands
between 48-60 hours.  Some of the models shifted northward a bit on
this cycle, and the NHC track forecast has therefore been nudged in
that direction close to the multi-model consensus aids, HCCA, and
the Florida State Superensemble.  However, users should not focus
on these relatively small shifts in the forecast track from cycle
to cycle, and for the Windward Islands in particular, consider that
48-60 hour forecast points can be off by an average of 60-80 nm.

Key Messages

1. The risk of wind and rain impacts from Gonzalo in portions of
the southern Windward Islands this weekend continues to increase,
however there is significant uncertainty in how strong Gonzalo will
be when it moves across the islands.

2. Despite the uncertainty in Gonzalo's future intensity, hurricane
or tropical storm conditions are possible across portions of the
southern Windward Islands Saturday and Saturday night.  Hurricane
and Tropical Storm Watches are currently in effect some of
the islands, and additional watches or warnings could be required
tonight or early Friday.  Interests in the southern Windward Islands
should monitor the progress of Gonzalo and follow any advice given
by local officials.

FORECAST POSITIONS AND MAX WINDS

INIT  23/2100Z  9.8N  49.4W   50 KT  60 MPH
 12H  24/0600Z 10.0N  51.2W   55 KT  65 MPH
 24H  24/1800Z 10.5N  53.8W   60 KT  70 MPH
 36H  25/0600Z 11.1N  56.6W   65 KT  75 MPH
 48H  25/1800Z 12.0N  59.7W   65 KT  75 MPH
 60H  26/0600Z 13.0N  62.6W   60 KT  70 MPH
 72H  26/1800Z 13.8N  65.6W   55 KT  65 MPH
 96H  27/1800Z 14.5N  72.0W   40 KT  45 MPH
120H  28/1800Z...DISSIPATED

$$
Forecaster Berg
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