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Tropical Depression Nine-E Forecast Discussion Number 4
2021-07-31 16:47:12| National Hurricane Center (East Pacific)
Issued at 800 AM PDT Sat Jul 31 2021 798 WTPZ44 KNHC 311447 TCDEP4 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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