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Tropical Storm Fred Discussion Number   6
NWS National Hurricane Center Miami FL       AL062021
1100 PM AST Tue Aug 10 2021
 
Data from satellites, the San Juan NOAA Doppler weather radar, and
an Air Force Reserve Unit reconnaissance aircraft indicate that the
low-level circulation has become better defined since the previous
advisory and,thus, the disturbance has been upgraded to Tropical
Storm Fred. Although the surface and low-level circulations are
vertically aligned, radar data indicate that the mid-level
circulation is still tilted about 40 n mi to the south. However,
that mid-level feature has been steadily gaining ground from its
earlier 100-nmi southeastward displacement. Radar Doppler velocity
data, aircraft 850-mb flight-level wind data of 43 kt and SFMR
surface winds of 33 kt, plus earlier surface observations of
sustained 33-kt winds, support an initial intensity of 35 kt.
 
The initial motion estimate is westward or 270/15 kt. However, this
should be a short-term motion overnight due to interaction with the
mountainous terrain of Puerto Rico to the north. By early Wednesday
morning, Fred is expected to resume a west-northwestward motion and
pass through the Mona Passage, reaching the eastern Dominican
Republic shortly after sunrise. A strong subtropical ridge to the
north of the cyclone is forecast by the global models to basically
remain static across the central and western Atlantic Ocean for the
next five days.  Given this pattern, Fred is expected to maintain a
west-northwestward motion through 96 hours, remaining just offshore
the north coast of Haiti and Cuba during that time.  After emerging
over the southeastern Gulf of Mexico on day 4, Fred is forecast to
gradually turn more north-northwestward as the cyclone rounds the
southwestern periphery of the aforementioned ridge.  The latest NHC
track forecast models strongly support this track scenario, although
there was a slight northward shift in the guidance envelope on this
cycle. The new official track forecast was shifted a little north of
the previous one through 72 hours, and then lies down the middle
of the tightly packed track consensus models.
 
Conditions remain favorable for some strengthening before Fred
reaches Hispaniola in about 12 h, but the intensity likely won't be
much stronger than 40-45 kt at landfall. Interaction with the
mountainous terrain of Hispaniola on Wednesday should result in some
weakening, but tropical-storm-force winds could occur along the the
north coast of the island if the center shifts a little more to
right of the forecast track. After emerging back over water by late
Wednesday, only slow strengthening is forecast in the 36-60-h time
period due to westerly vertical wind shear of about 20 kt expected
to affect the cyclone. On days 4 and 5, however, Fred is expected to
be moving over warmer sea-surface temperatures and into a much
weaker wind shear regime, which would favor more robust
strengthening. As a result, some of the regional and statistical-
dynamical intensity models bring Fred near or to hurricane strength
during that time. However, there is uncertainty Fred's structure
after interaction with Hispaniola and possibly Cuba, and continued
disagreement between the global models on how the upper-level wind
flow pattern near the cyclone will evolve, which ranges from
favorable anticyclonic outflow to strong southwesterly shear. Given
these factors, the NHC intensity forecast was only nudged slightly
higher than the previous advisory, and lies between the consensus
models HCCA and IVCN, and the SHIPS and LGEM models.
 
KEY MESSAGES:
 
1. Tropical storm conditions are expected in portions of the
U.S. Virgin Islands and Puerto Rico tonight, and are expected to
begin in the Dominican Republic Wednesday morning.  Tropical storm
conditions are possible in northern Haiti and the southeastern
Bahamas by late Wednesday.
 
2. Heavy rainfall could lead to flash, urban, and small stream
flooding, along with possible rapid river rises and the potential
for mudslides across the U.S. Virgin Islands, Puerto Rico, and the
Dominican Republic. The greatest threat for flooding impacts will be
across eastern and southern Puerto Rico.
 
3. There is a risk of wind and rainfall impacts elsewhere in
portions of Hispaniola, the Bahamas, and Cuba during the next
several days, although the forecast is more uncertain than usual
since Fred is forecast to interact with Hispaniola on Wednesday.
Interests in these areas should monitor Fred's progress and updates
to the forecast.
 
4. There is a risk of wind and rainfall impacts in Florida beginning
Friday in the Keys and spreading northward through portions of the
Peninsula and the Panhandle this weekend. However, it is too soon to
determine the exact timing, location, and magnitude of any potential
impacts. Interests throughout Florida should monitor Fred's progress
and check updates to the forecast.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  11/0300Z 17.4N  66.8W   35 KT  40 MPH
 12H  11/1200Z 18.6N  68.9W   40 KT  45 MPH...INLAND
 24H  12/0000Z 19.9N  71.4W   35 KT  40 MPH...INLAND
 36H  12/1200Z 20.9N  73.7W   35 KT  40 MPH...OVER WATER
 48H  13/0000Z 21.8N  76.0W   35 KT  40 MPH
 60H  13/1200Z 22.8N  78.0W   35 KT  40 MPH
 72H  14/0000Z 23.8N  79.9W   40 KT  45 MPH
 96H  15/0000Z 26.5N  82.8W   45 KT  50 MPH
120H  16/0000Z 29.3N  84.6W   55 KT  65 MPH
 
$$
Forecaster Stewart
 
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