[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT42 KNHC 231445
TCDAT2

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
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