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Tropical Depression Nine-E Discussion Number   4
NWS National Hurricane Center Miami FL       EP092021
800 AM PDT Sat Jul 31 2021
 
The depression has become increasingly elongated west-to-east since 
the previous advisory. Although deep convection has increased 
somewhat near the center, with cloud tops now colder than -70C, the 
convection isn't organized very well. The initial intensity is being
maintained at 25 kt until new ASCAT data arrives this afternoon.
 
The initial motion estimate is 270/04 kt. The latest NHC track 
guidance is in decent agreement on the depression moving in a slow 
westward direction for the next 24 hours or so, followed by a 
west-northwestward motion as the ridge to north of the cyclone 
weakens slightly. By around 72 hours, some binary interaction 
between the depression and rapidly strengthening Tropical Storm 
Hilda, located about 650 nmi to the east, is expected. However, the 
degree of interaction varies widely among the models. The GFS is the 
most extreme and takes the depression northward on days 4 and 5, 
with the ECMWF being the weakest, showing little if any interaction 
and keeps the cyclone moving generally westward through 120 h; the 
other global and regional models lie somewhere in between these two 
extremes. For now, the new official NHC track forecast was only 
nudged slightly northward, and lies inside the southern edge of the 
guidance envelope near the TVCE consensus model.
 
The depression's future intensity and existence depends heavily on 
the track over the next 5 days. A more westward motion would keep 
the cyclone over warmer water and in a better upper-level flow 
regime. In contrast, a sharp northward motion, as per the GFS 
solution, would place the system over colder water and within strong 
vertical wind shear. Another negative factor is the effect of 
intensifying Hilda to the east, which has started to draw in the 
southerly/southwesterly cross-equatorial inflow away from TD-9E. 
This deflection of the low-level flow eastward away from the 
depression could result in the cyclone's cyclone getting stretched 
out/elongated east-to-west even further, which would induce 
weakening or even dissipation of the cyclone. For now, the previous 
intensity forecast is being maintained for this advisory until the 
track model guidance comes into better agreement, and the 
interactive effects of Tropical Storm Hilda become clearer.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  31/1500Z 12.1N 127.6W   25 KT  30 MPH
 12H  01/0000Z 12.1N 128.2W   25 KT  30 MPH
 24H  01/1200Z 12.1N 129.2W   30 KT  35 MPH
 36H  02/0000Z 12.3N 130.7W   30 KT  35 MPH
 48H  02/1200Z 12.5N 132.3W   35 KT  40 MPH
 60H  03/0000Z 12.8N 133.7W   35 KT  40 MPH
 72H  03/1200Z 13.1N 135.0W   40 KT  45 MPH
 96H  04/1200Z 13.8N 137.2W   45 KT  50 MPH
120H  05/1200Z 15.5N 139.9W   45 KT  50 MPH
 
$$
Forecaster Stewart
 
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