Official Discussion issued by the National Hurricane Center

Seventeen (AL172020) DATA RELEASED: 9/7/2020 3:00:00 AM UTC

Copy of official data

Tropical Depression Seventeen Discussion Number   1
NWS National Hurricane Center Miami FL       AL172020
1100 PM AST Sun Sep 06 2020

The tropical low that the National Hurricane Center has been 
tracking the past few days west of the Cabo Verde Islands has 
developed enough convective banding for the system to be classified 
as a tropical depression. A 06/2345Z ASCAT-B scatterometer pass 
indicated surface wind speeds of 30-31 kt north of the center, which 
are supported by a T2.0/30 kt Dvorak satellite intensity estimate 
from TAFB at 07/0000 UTC.

The initial motion estimate is an uncertain 270/04kt. The center has 
been jumping around somewhat due to small, brief convective bursts 
that then dissipate, leaving small swirls rotating around the mean 
circulation center. However, the latest ASCAT data showed a much 
better defined overall circulation, especially in the inner core 
wind field region, so the forward motion should stabilize fairly 
soon. The depression is expected to remain caught in weak steering 
flow for the next 5 days, so only a slow westward motion is expected 
tonight and Monday, followed by a west-northwestward motion by later 
Monday and Tuesday that will continue through the remainder of the 
120-h forecast period. Possible track forecast complications could 
develop by days 3-5 if binary interaction occurs between the 
depression and the large low (Invest 93L) currently located 
southeast of the Cabo Verde Islands, resulting in TD-17 not gaining 
as much latitude as currently indicated. The NHC forecast track lies 
down the middle of the simple- and corrected-consensus model 
guidance envelope.

The depression is only forecast to slowly strengthen for the next 4 
days or so due to expected intrusions of dry mid-level air. By day 
5, increasing southerly vertical wind shear on the east side of a 
large upper-level low that is forecast to cut off north of Puerto 
Rico is expected to induce gradual weakening. However, both the 
timing of the development of the low and its west-southwestward 
motion will determine if sufficient shear will prevent further 
strengthening after 96 h; a slower formation of the low and/or a 
slower retrograding motion would result in at least less weakening. 
The NHC intensity forecast closely follows an average of the IVCN 
and NOAA-HCCA intensity consensus models.


INIT  07/0300Z 17.0N  41.5W   30 KT  35 MPH
12H  07/1200Z 17.0N  42.1W   30 KT  35 MPH
24H  08/0000Z 17.2N  42.7W   35 KT  40 MPH
36H  08/1200Z 17.5N  43.4W   40 KT  45 MPH
48H  09/0000Z 18.0N  44.6W   45 KT  50 MPH
60H  09/1200Z 18.6N  46.3W   50 KT  60 MPH
72H  10/0000Z 19.2N  48.2W   55 KT  65 MPH
96H  11/0000Z 20.4N  51.9W   50 KT  60 MPH
120H  12/0000Z 21.2N  55.5W   45 KT  50 MPH

Forecaster Stewart