Eastern Pacific Basin Tropical Cyclones

Tropical Depression Eighteen-E Forecast Discussion


000
WTPZ43 KNHC 050246
TCDEP3

Tropical Depression Eighteen-E Discussion Number   3
NWS National Hurricane Center Miami FL       EP182021
1000 PM CDT Thu Nov 04 2021

TD18-E is a poorly organized system this evening. Most of the 
deeper convection that was near the center earlier today has 
collapsed, with only a few skeletal outer bands remaining south of 
the low-level circulation. Last light visible satellite imagery 
also indicated that the low-level center was not vertically 
aligned, located to the west ahead of the mid-level center. While I 
have yet to receive new scatterometer data which should be 
arriving soon, the current satellite structure argues against 
any development. Indeed, the latest TAFB and SAB Dvorak satellite 
estimates have held steady or decreased from earlier today. The 
current intensity will remain at 30 kt for this advisory, and even 
this might be a bit generous.

After a motion to the west-northwest earlier, the depression appears 
to have resumed a due westward motion, estimated at 270/6 kt. A 
narrow mid-tropospheric ridge north of the cyclone has been steering 
the system slowly westward, and this ridge is expected to strengthen 
and merge with a larger mid-level ridge currently centered over the 
Baja California peninsula. The end result of this pattern 
reconfiguration on the depression should be a faster motion to the 
west, or even west-southwest due to the orientation of the ridge 
complex. By the end of the forecast period, the ridge is expected to 
shift a bit more to the east, which will provide an opportunity for 
the depression to gain some latitude by the end of the forecast 
period. The model track guidance continues to be in good agreement, 
though with a bit of along-track spread. The guidance overall has 
made another shift to the south this cycle, and the latest NHC track 
was also once again nudged further south, staying on the northern 
side of the guidance envelope but not too far from the TVCE and HCCA 
consensus solutions.

The intensity forecast is somewhat challenging. While the latest 
SHIPS guidance indicates the depression will remain in low 200-850 
hPa vertical wind shear, have ample mid-level moisture, and be over 
27-28 C sea-surface temperatures, the majority of the intensity 
guidance shows only modest strengthening over the next 2-3 days. 
Model-derived soundings from the GFS and HWRF suggest that higher 
mid-level shear could be undercutting this favorable upper-level 
flow pattern over the time period. This undercutting shear may help 
explain why the low and mid-level centers of the depression are 
currently not aligned. In addition, surface-based backwards 
trajectories suggest that the tropical cyclone is ingesting air 
originating over the equator, which currently is dominated by an 
environment of stable stratocumulus clouds. This may help explain 
the anemic convective appearance of the system currently. An 
additional forecast concern is the low-latitude track of the 
cyclone, with the forecast showing the system losing additional 
latitude over the next several days. All these factors make me 
skeptical that there will be significant intensification over the 
next 2-3 days and the intensity forecast has been lowered somewhat 
from the previous one over this time period. The latest NHC 
intensity forecast is a bit under the latest IVCN and NOAA-HCCA 
consensus, but is higher than the current LGEM and ECMWF guidance, 
which essentially show no intensification through the forecast 
period. 


FORECAST POSITIONS AND MAX WINDS

INIT  05/0300Z  9.7N  91.2W   30 KT  35 MPH
 12H  05/1200Z  9.6N  92.0W   30 KT  35 MPH
 24H  06/0000Z  9.2N  93.5W   30 KT  35 MPH
 36H  06/1200Z  9.1N  95.4W   35 KT  40 MPH
 48H  07/0000Z  8.9N  97.4W   35 KT  40 MPH
 60H  07/1200Z  8.9N  99.5W   35 KT  40 MPH
 72H  08/0000Z  9.3N 102.0W   40 KT  45 MPH
 96H  09/0000Z 10.0N 106.7W   50 KT  60 MPH
120H  10/0000Z 10.6N 111.5W   55 KT  65 MPH

$$
Forecaster Papin



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