Official Discussion issued by the National Hurricane Center

Gonzalo (AL072020) DATA RELEASED: 7/23/2020 9:00:00 PM UTC

Copy of official data

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