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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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