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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.
FORECAST POSITIONS AND MAX WINDS
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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