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Tropical Depression Eighteen Discussion Number   1
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Wed Sep 22 2021

The tropical wave that NHC has been monitoring over the last several 
days has gradually become better organized. The satellite structure 
in particular is quite impressive for a tropical depression, with 
both the 1800 UTC subjective Dvorak estimates from TAFB and SAB 
at T2.5/35 kt. This bigger question, however, was if the system 
possessed a well-defined closed earth-relative circulation. An 
ASCAT-C pass from earlier this morning hinted that the circulation 
was becoming better defined, with the development of westerly 
low-level winds to the south of the convective shield. This westerly 
low-level flow is also confirmed by atmospheric motion vectors 
(AMVs) available from the GOES-16 meso domain over the system. While 
the low-level circulation may still be somewhat broad, it now 
appears to be well-defined enough to mark the formation of a 
tropical cyclone. The initial intensity is set at 30-kt, in 
agreement with peak wind retrievals of 28-30 kt by the earlier 
scatterometer data.  

The initial motion is estimated to be at 270/13 kt, though this is 
somewhat uncertain given that the center has only recently formed. 
An expansive mid-level ridge is located to the north and west of 
the cyclone, which should maintain its heading toward the west, 
though with a gradual gain in latitude as the system approaches 
the western extent of the ridge by day 5. The track guidance is in 
excellent agreement on this track evolution for the first three 
days, with just a bit more spread in the guidance thereafter. The 
ECMWF and its ensemble mean towards the end of the forecast is on 
the left side of the guidance envelope, while the GFS and HWRF 
models are currently on the right side. For the first NHC track 
forecast, I have elected to stay close to the track consensus aids, 
taking a blend of the HCCA and TCVN aids which are near the 
middle of the guidance envelope.

The environment ahead of the tropical depression appears quite 
favorable for intensification. Vertical wind shear is forecast by 
both the ECWMF- and GFS-based SHIPS guidance to stay at or under 10 
kt for the next 3-5 days as the storm traverses warm 28-29 C sea 
surface temperatures. Most of the guidance responds to this 
environment by indicating strengthening, and the NHC intensity 
guidance follows suit, showing a steady increase in intensity 
throughout the forecast period. While it might take a bit of time 
for the formative low-level circulation to become vertically aligned 
with the mid-level center, once that occurs, it is possible a period 
of rapid intensification could occur during the five day forecast. 
The forecast intensity by 120 hours (100 kt) is on the higher end of 
the guidance envelope, but not as high as the latest HWRF or HAFS-B 
forecasts. 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  22/2100Z 10.1N  33.9W   30 KT  35 MPH
 12H  23/0600Z 10.1N  36.0W   35 KT  40 MPH
 24H  23/1800Z 10.5N  38.6W   40 KT  45 MPH
 36H  24/0600Z 11.0N  41.1W   50 KT  60 MPH
 48H  24/1800Z 11.7N  43.4W   60 KT  70 MPH
 60H  25/0600Z 12.4N  45.3W   70 KT  80 MPH
 72H  25/1800Z 13.0N  46.8W   80 KT  90 MPH
 96H  26/1800Z 14.2N  49.5W   90 KT 105 MPH
120H  27/1800Z 16.0N  52.5W  100 KT 115 MPH
 
$$
Forecaster Papin
 
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