[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT41 KNHC 120252
TCDAT1

Tropical Depression Fred Discussion Number  10
NWS National Hurricane Center Miami FL       AL062021
1100 PM EDT Wed Aug 11 2021

The center of Fred is estimated to be located over extreme eastern
Haiti. This position is based on continuity with the previous
forecast and a small low-level cyclonic circulation noted in
cloud-drift wind vectors that were based on the last few GOES-16
high-resolution visible satellite images. A NOAA reconnaissance
aircraft performed a quasi-synoptic surveillance mission this
evening and released dropsondes while circumnavigating Hispaniola.
Some of the dropsondes measured surface winds of 24-27 kt about 150
nmi north and northwest (in the Windward Passage) of the center, and
these data support maintaining an intensity of 30 kt for this
advisory. The main convective cloud mass has been displaced to the
southeast of the low-level center due to the west-northwesterly
vertical wind shear of about 20 kt.

The initial motion estimate is 300/13 kt. The global and regional
models remain in surprisingly good agreement on Fred maintaining a
steady west-northwestward motion through 48-60 hours as a strong
ridge remains entrenched over the western Atlantic during that time
period. Thereafter, however, the NHC model guidance diverges
significantly, with the now much weaker HWRF model taking Fred more
westward, while the GFS and ECMWF models move the cyclone
northwestward over the eastern Gulf of Mexico near or just offshore
the Florida west coast; the UKMET model lies between these two track
extremes. The weaker HWRF track has been discounted given the NHC
forecast that Fred will remain vertically coherent and restrengthen
after moving back over water. Thus, the new NHC track forecast is
essentially just an update of the previous advisory track, and lies
close to the NOAA-HCCA corrected-consensus model. NOTE: the
aforementioned NOAA WP-3D reconnaissance aircraft has been releasing
dropsondes from approximately the 450-mb level and has also been
collecting tail-Doppler radar data. These data should have been
assimilated into the 0000 UTC model runs, which hopefully will
provide better track and intensity model forecasts for the next
advisory package.

Fred is forecast remain within a moderate to strong westerly to
southwesterly vertical wind shear regime throughout the forecast
period by almost all of the global and regional models. As a result,
rapid strengthening is not expected after Fred moves back over
water. There is one caveat, however, and that is the ECMWF model
forecasting the shear to weaken to 5-10 kt during the 36-72 hour
period, which could allow for a little more strengthening than what
the official forecast is currently indicating. The new NHC intensity
prediction is identical to the previous one owing to the
expectation that some degree of westerly shear will be affecting
Fred throughout the entire forecast period. The official forecast
follows a blend of the IVCN and HCCA consensus intensity models.

KEY MESSAGES:

1. Through early Thursday morning, heavy rainfall could lead to
flash, urban, and small stream flooding, along with possible rapid
river rises and potential mudslides in the Dominican Republic.

2. Tropical storm conditions are possible in northern Haiti, the
Turks and Caicos, and the southeastern Bahamas tonight, and in
portions of Cuba Thursday.

3. Beginning Friday, heavy rainfall could lead to areal, urban, and
small stream flooding, and possible rapid river rises across
southern Florida. Heavy rainfall associated with Fred will impact
the remainder of Florida and parts of the Southeast this weekend
and into next week.

4. There is a risk of tropical storm conditions beginning
early Saturday in the Florida Keys and spreading northward along
portions of the Florida west coast and the Florida Panhandle through
Monday.

FORECAST POSITIONS AND MAX WINDS

INIT  12/0300Z 19.5N  72.2W   30 KT  35 MPH...INLAND HISPANIOLA
 12H  12/1200Z 20.4N  73.9W   30 KT  35 MPH...OVER WATER
 24H  13/0000Z 21.6N  76.1W   30 KT  35 MPH
 36H  13/1200Z 22.5N  78.1W   35 KT  40 MPH
 48H  14/0000Z 23.5N  79.8W   35 KT  40 MPH
 60H  14/1200Z 24.7N  81.2W   40 KT  45 MPH
 72H  15/0000Z 26.2N  82.4W   45 KT  50 MPH
 96H  16/0000Z 29.2N  84.2W   50 KT  60 MPH
120H  17/0000Z 31.7N  84.7W   35 KT  40 MPH...INLAND

$$
Forecaster Stewart
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