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Tropical Depression Eighteen-E Discussion Number   6
NWS National Hurricane Center Miami FL       EP182021
400 PM CDT Fri Nov 05 2021
 
For the last 6-9 hours, TD18-E has not produced enough organized 
deep convection to be classifiable as a tropical cyclone. Indeed, 
the current satellite structure primarily consists of a low-level 
cloud swirl, with only a few disorganized puffs of colder cloud tops 
that quickly decay after they form.  In addition, an earlier ASCAT-B 
pass valid at 1608 UTC suggested that the circulation was also 
becoming increasingly fragile, with less than 10 kt winds on the 
southwest side of the vortex and a peak wind retrieval of only 25 
kt. Subjective satellite Dvorak estimates have been decreasing, with 
the latest T numbers down to T1.5 from TAFB and T1.0 from SAB. 
Assuming some undersampling could still be occuring from the earlier 
ASCAT wind data, the intensity was held at 30 kt this advisory, 
though this is likely generous.
 
The motion continues to be south of due west, with the latest 
estimate now at 260/9 kt. There has not been much change to the 
latest track reasoning. An expansive mid-level ridge is expected to 
build eastward over Mexico over the next several days, helping to 
maintain the tropical cyclone on a westward heading throughout the 
forecast period. However, the cyclone is vertically shallow, 
and the influence of the low-level gap wind flow emanating from the 
Gulf of Tehuantepec could be causing the short-term south of due 
west bend in the track. Yet again, the track guidance has come in 
further south and a bit faster compared to the previous cycle, and 
the latest NHC track forecast has been shifted in that direction, 
though still not quite as far south and west as the HCCA and TVCE 
consensus aids.  
 
Without any organized convection occuring near the center of the 
depression currently, its future status as a tropical cyclone could 
be in jeopardy. Apparently stable air has infiltrated the 
circulation and is limiting convective output despite sufficently 
warm sea-surface temperatures and vertical wind shear that does not 
seem too inhibiting.  The GFS, ECMWF, and HWRF model simulated IR 
brightness temperature forecasts suggest that better organized 
convection should return near the low-level center during tonight's 
diurnal maximum. Even so, the global model guidance continues to 
trend downward in the intensity forecast, delaying intensification 
further out into the future. Indeed the most recent GFS run 
maintains status quo intensity over the next 48 hours, joining the 
ECMWF, SHIPS and LGEM guidance. The only guidance that shows 
significant intensification over the next several days is the HWRF 
and this model has had a persistent high bias. The latest intensity 
forecast has been lowered a bit more beyond 36 h. As alluded above, 
if organized deep convection does not return soon, it's possible 
this depression could degenerate into a remnant low as early as 
tomorrow.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  05/2100Z  9.1N  93.5W   30 KT  35 MPH
 12H  06/0600Z  8.9N  94.7W   30 KT  35 MPH
 24H  06/1800Z  8.6N  96.7W   30 KT  35 MPH
 36H  07/0600Z  8.4N  99.0W   30 KT  35 MPH
 48H  07/1800Z  8.7N 101.3W   30 KT  35 MPH
 60H  08/0600Z  9.2N 103.6W   35 KT  40 MPH
 72H  08/1800Z  9.6N 106.3W   40 KT  45 MPH
 96H  09/1800Z 10.6N 111.1W   45 KT  50 MPH
120H  10/1800Z 11.0N 116.0W   50 KT  60 MPH
 
$$
Forecaster Papin
 
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