000 WTNT45 KNHC 160857 TCDAT5 Tropical Depression Fifteen Discussion Number 4 NWS National Hurricane Center Miami FL AL152023 500 AM AST Sat Sep 16 2023 Finding the exact center of the tropical depression this morning has been a challenge. While there is a large region of mid-level cyclonic rotation in the shortwave IR imagery, there isn't much evidence that the low-level circulation has tightened up any from the earlier ASCAT passes. This was confirmed by a 0501 UTC AMSR2 microwave pass which only had a subtle hint of banding with the ongoing convection in the depression's northern semicircle. Subjective Dvorak estimates were 25 kt and 35 kt from SAB and TAFB, respectively. Objective intensity estimates were also in the 30-35 kt range. Given the continued broad structure, the intensity is held at 30 kt for this advisory. Given the current structure of the depression, its initial motion has higher-than-normal uncertainty but is currently estimated at 330/12 kt. A general northwest to north-northwest motion is expected to continue for the next day or two, as the system is steered between a mid-level ridge centered to its northeast, and a deep-layer trough, centered to its west. The track guidance in the short-term has actually shifted a bit more poleward early on, and the NHC track was nudged in that direction over the first 48 hours or so. One interesting caveat to this track forecast is Margot, which has weakened the overall magnitude of the ridge steering TD15, and may explain the more poleward shift in the guidance the last few cycles. Thereafter, as Margot moves out of the picture, the mid-level ridge should briefly build back in, and the guidance this cycle shows a subtle bend more northwestward between 48-72 hours. However, another mid-latitude trough is expected to move off the eastern U.S. coastline in about 4-5 days, eroding this ridge enough to begin recurvature by the end of the forecast period. The NHC track forecast is in best agreement with the TVCN and HCCA consensus aids. The intensity forecast early on is largely dependent on how TD15's broad structure evolves, since environmentally vertical wind shear is low, sea-surface temperatures are anomalously warm, and the system is in a pocket of enhanced mid-level moisture. The uncertainty of structural evolution is highlighted by the regional hurricane models, which over the past few cycles have oscillated between the system developing a small inner core versus a broader structure that intensifies more slowly despite the favorable environment. The latest intensity forecast assumes it will take a day or so for the core to consolidate, only showing a slow rate of intensification for the next 24 hours, which is on the low side of the intensity guidance. However, assuming an inner core does form in the 36-48 hour time frame, a period of rapid intensification is possible, which is best highlighted by the ECMWF-SHIPS guidance which shows a 45 percent chance of a 65-kt increase in intensity in 72 hours, more than eight times the climatological mean. This possibility is shown in the NHC intensity forecast, which shows a peak intensity of 95 kt in 72 hours. While not explicitly shown, it still remains possible TD15 could become a major hurricane sometime between the 72-96 hour forecast period, though some increase in southwesterly shear and inner core fluctuations will likely result in gradual weakening by the end of the forecast. The NHC intensity forecast is somewhat higher than the consensus aids at peak intensity. FORECAST POSITIONS AND MAX WINDS INIT 16/0900Z 17.9N 45.3W 30 KT 35 MPH 12H 16/1800Z 19.6N 46.4W 35 KT 40 MPH 24H 17/0600Z 21.5N 48.3W 40 KT 45 MPH 36H 17/1800Z 23.1N 49.8W 50 KT 60 MPH 48H 18/0600Z 24.5N 51.3W 60 KT 70 MPH 60H 18/1800Z 25.9N 53.0W 75 KT 85 MPH 72H 19/0600Z 27.2N 55.1W 95 KT 110 MPH 96H 20/0600Z 31.0N 58.4W 90 KT 105 MPH 120H 21/0600Z 35.5N 56.5W 80 KT 90 MPH $$ Forecaster Papin