Tropical Depression Thirteen Forecast Discussion

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814 
WTNT43 KNHC 210849
TCDAT3

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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