Tropical Depression NINE-E (Text)

Tropical Depression Nine-E Discussion Number   5
NWS National Hurricane Center Miami FL       EP092021
1100 AM HST Sat Jul 31 2021
Visible and scatterometer satellite data continue to show depression 
getting stretched further from west-to-east, with the surface 
circulation now elliptical-shaped about 500 nmi long and 250 nmi 
wide. A 1757Z ASCAT-B pass revealed an ill-defined low-level 
circulation center and one 26-kt wind vector 60-70 nmi west of the 
center. Unlike a few hours ago, deep convection has begun to wane 
with cloud tops now warmer than -70 deg C. The intensity has been 
held at 25 kt based on the ASCAT wind data and the overall poor 
appearance in satellite imagery.
The initial motion estimate is an uncertain 270/04 kt. The 
aforementioned ASCAT data suggest that either the center has 
reformed farther to the southwest near a small burst of deep 
convection, or that that feature is just a smaller swirl rotating 
around the larger gyre envelope. Thus, the estimated center location 
is an average between the previous center position and the small 
center noted in the ASCAT data. Otherwise, there is no significant 
change to the previous forecast tack or reasoning. After 72 hours, 
the models remain is major disagreement on how much, if any, binary 
interaction occurs between the depression and Tropical Storm Hilda, 
located about 650 nmi to the east. The new 12Z GFS model remains 
the most extreme and takes the depression northward on days 4 and 
5, with the ECMWF again the weakest with little interaction with 
Hilda. The remainder of the NHC track guidance lies somewhere in 
between these two extremes. As in the previous advisory, the best 
call for now is to punt by remaining close to the previous forecast 
track, with the new official NHC track forecast still lying inside 
the southern edge of the guidance envelope between the ECMWF 
solution to the south and the consensus models farther north.
The depression's future intensity, and even its existence as a 
tropical cyclone, depends heavily on the track over the next 120 
hours. A more westward motion as per the ECMWF would keep the 
cyclone over warmer water and in a more favorable upper-level 
pattern, whereas a sharp northward motion like the GFS is predicting 
would take the cyclone over cold SSTs below 25 deg C and into a 
strong wind shear environment. Another negative factor continues to 
be the west-to-east stretching of the depression's low-level wind 
field caused by Tropical Storm Hilda's larger and stronger 
circulation. As more of the southerly/southwesterly 
cross-equatorial low-level inflow gets drawn away from the elongated 
cyclone and into Hilda, most of the depression's inflow will be 
cooler and drier air trade wind flow coming into the northern 
semicircle, which would induce weakening. The previous intensity 
forecast is being maintained for this advisory, which continues to 
show little strengthening for the next 48 hours, followed by only 
modest intensification thereafter. However, this remains a 
low-confidence forecast.
INIT  31/2100Z 11.6N 128.4W   25 KT  30 MPH
 12H  01/0600Z 11.5N 129.1W   25 KT  30 MPH
 24H  01/1800Z 11.6N 130.4W   30 KT  35 MPH
 36H  02/0600Z 11.8N 131.9W   30 KT  35 MPH
 48H  02/1800Z 12.2N 133.5W   35 KT  40 MPH
 60H  03/0600Z 12.6N 134.7W   35 KT  40 MPH
 72H  03/1800Z 13.0N 135.8W   40 KT  45 MPH
 96H  04/1800Z 14.0N 138.1W   45 KT  50 MPH
120H  05/1800Z 15.4N 140.8W   45 KT  50 MPH
Forecaster Stewart

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Page last modified: Friday, 31-Dec-2021 12:09:56 UTC