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Tropical Depression Thirteen Discussion Number 6
NWS National Hurricane Center Miami FL AL132020
500 AM AST Fri Aug 21 2020
Tropical Depression Thirteen remains very disorganized this morning.
The associated convection is elongated from northwest to southeast,
and the low-level center is located near the northwestern end of the
convective area. Additionally, satellite imagery and model analyses
indicate that the mid-level center is located several hundred miles
to the southeast of the low-level center. Earlier aircraft and
scatterometer data suggested the possibility that the system was an
open wave. However, the currently available data is ambiguous on
whether the system still has a closed circulation, so it will be
maintained as a tropical depression. The initial intensity remains
30 kt.
The initial motion is west-northwestward or 285/18 kt. There is
little change to the track forecast philosophy through about 96 h.
A strong subtropical ridge over the western Atlantic should remain
north of the depression during the next few days, steering the
cyclone at a fast pace to the west-northwest. After that, the ridge
weakens some over the eastern Gulf of Mexico, and Tropical
Depression Thirteen may interact with Tropical Depression Fourteen,
which is also forecast to be in the Gulf by that time, with the
result of these things being a turn toward the northwest or
north-northwest. The track guidance has shifted southward since the
last advisory, with the UKMET shifting far enough to the south that
it takes the system over the Caribbean south of Cuba. The new
forecast track is also shifted a bit southward from the previous
track. However, it lies to the north of the GFS, the UKMET, and the
various consensus models. It also lies north of the ECMWF model
from 24-72 h.
The intensity forecast remains low confidence. The separation
between the low- and mid-level centers, as well as some westerly
shear and dry air entrainment, suggests that significant
strengthening is unlikely during the next 24 h or so. The dynamical
models suggests the centers will become more vertically aligned
around 36-48 h and that the shear should diminish. However, the
system could be close to Hispaniola during this time, and be near
Cuba thereafter, especially if it moves south of the forecast track.
The upper-level winds over the Gulf of Mexico should be generally
favorable for development if the cyclone doesn't get too close to
Tropical Depression Fourteen. The possibilities range from the
system degenerating to an open wave as seen in the GFS and ECMWF to
a major hurricane as seen in the HWRF. Given the uncertainty, the
intensity forecast is unchanged from the previous advisory, and it
lies a little below the intensity consensus.
Key Messages:
1. Tropical storm conditions are possible across portions of the
northern Leeward Islands, the Virgin Islands, Puerto Rico, the
southeastern Bahamas, and the Turks and Caicos islands later today
through Saturday night, and Tropical Storm Watches have been
issued for some of these islands. Heavy rainfall is likely across
this area beginning today and could cause mudslides and flash
and urban flooding through Sunday.
2. The details of the long-range track and intensity forecasts are
more uncertain than usual since the system could move over portions
of the Greater Antilles this weekend. However, this system could
bring some storm surge, rainfall and wind impacts to portions of
Hispaniola, Cuba, the Bahamas, and Florida this weekend and early
next week. Interests there should monitor this system's progress
and updates to the forecast over the next few days.
FORECAST POSITIONS AND MAX WINDS
INIT 21/0900Z 17.8N 58.5W 30 KT 35 MPH
12H 21/1800Z 18.1N 60.7W 30 KT 35 MPH
24H 22/0600Z 18.8N 64.0W 35 KT 40 MPH
36H 22/1800Z 19.7N 67.5W 40 KT 45 MPH
48H 23/0600Z 20.6N 71.1W 45 KT 50 MPH
60H 23/1800Z 21.8N 74.9W 50 KT 60 MPH
72H 24/0600Z 23.2N 78.5W 55 KT 65 MPH
96H 25/0600Z 26.5N 84.0W 65 KT 75 MPH
120H 26/0600Z 30.0N 87.0W 65 KT 75 MPH
$$
Forecaster Beven
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