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Tropical Depression Nine-E Forecast Discussion Number 5
2021-07-31 22:39:21| National Hurricane Center (East Pacific)
Issued at 1100 AM HST Sat Jul 31 2021 000 WTPZ44 KNHC 312039 TCDEP4 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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