Official Discussion issued by the National Hurricane Center

Gonzalo (AL072020) DATA RELEASED: 7/23/2020 3:00:00 PM UTC

Copy of official data

Tropical Storm Gonzalo Discussion Number   8
NWS National Hurricane Center Miami FL       AL072020
1100 AM AST Thu Jul 23 2020

Recently-obtained GCOM and WindSat microwave data from overnight 
shows that Gonzalo's center is a little farther south than 
previously estimated.  In addition, the storm's structure has 
become a little disheveled since yesterday, with the deep 
convection losing some organization.  SAB's data-T number responded 
to this by falling to 2.5, but overall the CI numbers and SATCON 
support maintaining 55 kt for now.

There is still an incredible amount of uncertainty in Gonzalo's 
intensity forecast.  Sea surface temperatures to the east of the 
Windward Islands are warm--close to 29C--and the storm is likely to 
be moving through an environment of relatively light shear at least 
for the next 48 hours or so.  The ambient environment is not 
particularly moist, however, with mid-level relative humidity 
generally around 50 percent, and it already appears that this dry 
air is affecting Gonzalo.  Small cyclones like Gonzalo tend to 
succumb to any type of adverse environmental conditions quite 
easily, and it's possible that the system could struggle during the 
next couple of days.  This is the solution shown by some of the 
global models, particularly the ECMWF and UKMET.  On the other 
hand, the hurricane statistical and dynamical models, as well as 
the GFS, continue to show Gonzalo strengthening to a hurricane 
before it reaches the Windward Islands.  Out of an abundance of 
caution, the official forecast continues to show Gonzalo becoming a 
hurricane in about 24 hours, but the uncertainty in this scenario 
cannot be stressed enough.  There is a higher degree of certainty 
that Gonzalo would weaken once it moves over the eastern and 
central Caribbean Sea, where even the GFS shows it opening up into 
a wave.

Even with the southward adjustment of the initial position, Gonzalo 
still appears to be moving westward, or 270/12 kt.  The subtropical 
ridge to the north is forecast to push Gonzalo toward the west or 
west-northwest, with an increase in forward speed, for the entire 
5-day forecast period.  Much of the track uncertainty hinges on 
exactly how strong Gonzalo gets, with the stronger model 
representations showing the storm making more poleward process.  
Models such as the ECMWF and UKMET, which keep Gonzalo weak or open 
it up into a trough, are along the southern side of the guidance 
envelope.  The updated NHC track forecast has been shifted 
southward some, to account for the new initial position and to be a 
little closer to the intensity consensus, HCCA, and the Florida 
State Superensemble.


Key Messages

1. There is an increasing risk of wind and rain impacts from
Gonzalo in portions of the southern Windward Islands this weekend, 
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 
conditions are possible across portions of the southern Windward 
Islands.  Hurricane Watches are currently in effect for Barbados 
and St. Vincent and the Grenadines, and additional watches for 
other islands could be required later today.  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  23/1500Z  9.6N  48.3W   55 KT  65 MPH
12H  24/0000Z  9.8N  50.0W   60 KT  70 MPH
24H  24/1200Z 10.1N  52.7W   65 KT  75 MPH
36H  25/0000Z 10.5N  55.7W   70 KT  80 MPH
48H  25/1200Z 11.2N  59.0W   70 KT  80 MPH
60H  26/0000Z 11.9N  62.3W   70 KT  80 MPH
72H  26/1200Z 12.6N  65.4W   65 KT  75 MPH
96H  27/1200Z 14.0N  71.4W   50 KT  60 MPH
120H  28/1200Z 15.5N  77.5W   40 KT  45 MPH

$$
Forecaster Berg