Eastern Pacific Basin Tropical Cyclones

Tropical Depression Eighteen-E Forecast Discussion

WTPZ43 KNHC 061453

Tropical Depression Eighteen-E Discussion Number   9
NWS National Hurricane Center Miami FL       EP182021
1000 AM CDT Sat Nov 06 2021

After struggling to produce much in the way of deep convection 
yesterday, a large nocturnal deep convective burst, with 
overshooting cloud tops as cold as -85 C, occurred near the 
estimated center of the depression, though this activity is 
beginning to wane. An 0815 UTC AMSR2 microwave pass also 
suggested modest organization under the cirrus, with some banding 
features present in the deep convection. While last night's 
scatterometer derived winds only topped out at 27 kt, the most 
recent subjective Dvorak intensity estimate from TAFB is at T2.0/30 
kt. The latest objective UW-CIMSS ADT estimate is even higher at 
T2.5/35 kt. These estimates support bringing the intensity back up 
to 30 kt for this advisory.

Using both scatterometer and microwave fixes, the depression has
maintained a south of due west heading over the past 12-24 hours,
with the latest motion estimated at 260/12 kt. This general heading
is expected to continue for the next several days with a gradual
bend poleward in the latter half of the forecast period as the
cyclone rounds the southern side of a expensive mid-level ridge
centered over Mexico. The latest NHC track forecast has been
adjusted just a bit faster, blending the tightly clustered
consensus aids TVCE and HCCA.

The depression's overnight convective burst was well anticipated by 
the both the GFS and ECMWF simulated IR brightness temperature and 
both models show more persistent activity continuing through the 
forecast period. Despite this factor, neither model shows much, if 
any, additional strengthening. This result is interesting, since 
both the GFS- and ECMWF-based SHIPS guidance indicate low 200-850 
hPa vertical wind shear, 27-28 C sea-surface temperatures, and a 
fairly moist mid-level environment. One factor possibly limiting the 
intensity forecast is the possibility of higher westerly mid-level 
shear undercutting the more favorable deep-layer shear. In addition, 
the tropical cyclone is forecast to remain at fairly low-latitude 
which can sometimes limit vortex spin-up in a lower Coriolis 
environment. Even the overzealous HWRF model has come back down to 
earth, showing only a peak intensity as a more modest tropical 
storm. The latest NHC intensity forecast does still show TD18-E 
eventually becoming a tropical storm in 36 h, but does not show any 
additional development. This forecast is a blend between the 
slightly higher intensity consensus aids (IVCN, HCCA) with the more 
pessimistic global model guidance (UKMET, GFS, ECMWF) which do not 
indicate any additional intensification over the forecast period.


INIT  06/1500Z  8.4N  96.6W   30 KT  35 MPH
 12H  07/0000Z  8.2N  98.3W   30 KT  35 MPH
 24H  07/1200Z  8.1N 100.7W   30 KT  35 MPH
 36H  08/0000Z  8.4N 103.2W   35 KT  40 MPH
 48H  08/1200Z  8.9N 105.5W   35 KT  40 MPH
 60H  09/0000Z  9.5N 108.0W   35 KT  40 MPH
 72H  09/1200Z 10.0N 110.4W   35 KT  40 MPH
 96H  10/1200Z 10.5N 115.5W   35 KT  40 MPH
120H  11/1200Z 10.7N 121.2W   35 KT  40 MPH

Forecaster Papin

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