ZCZC MIATCDEP5 ALL TTAA00 KNHC DDHHMM Tropical Storm Lidia Discussion Number 1 NWS National Hurricane Center Miami FL EP152023 300 AM MDT Tue Oct 03 2023 The system we have been monitoring for the last several days to the south of the coast of Mexico (Invest 98E) has become better organized early this morning. A small but growing burst of deep convection has formed near or just west of where the estimated center is, and some curved banding features are starting to take shape. In addition, we had two scatterometer passes between 04-05 UTC, showing the system has developed a well-defined closed circulation, with winds between 30-35 kt along the western flank of the circulation. Subjective intensity estimates were T1.5 from TAFB And T2.0 from SAB. However, based primarily on the earlier satellite wind-data, advisories are being initiated on Tropical Storm Lidia with an initial intensity of 35 kt for this advisory. Lidia's initial motion is somewhat uncertain since it only recently became well-defined, but it is estimated to be moving slowly to the west-northwest at 300/8 kt. This current motion is a result of the system moving along the southern extent of a mid-level ridge centered over Mexico and the western Gulf of Mexico. However, a mid- to upper-level trough located to its northwest is expected to erode this ridge somewhat, allowing the tropical cyclone to turn northwest or north-northwest in the next 24-48 hours. While the track guidance is in relatively good agreement on this initial poleward turn, the guidance spread increases quite dramatically after 48 hours, with uncertainty if this poleward motion continues, or if Lidia turns back westward as the trough lifts out and allows a mid-level ridge to build back in. For this initial track forecast, the NHC forecast favors the latest GFS and ECMWF global model solutions, which show a narrow ridge building back in, allowing for a slow westward turn by the end of the forecast period. This track forecast also lies between the simple consensus aid TVCE, and a blend of the GFS and ECMWF solutions (GFEX). For the intensity forecast, Lidia appears like it will have to deal with moderate easterly shear for the next several days, though the ECMWF-SHIPS has lower shear than the GFS-SHIPS. To complicate matters, the initial structure of Lidia appears rather small, potentially making the system prone to larger-than-usual changes in intensity, both up or down. Even though the shear may keep its intensity in relative check, other environmental factors are quite favorable, with sea-surface temperatures near 30 C, and ample mid-level moisture. In fact, with the lower shear, the ECMWF-SHIPS rapid intensification index (RII) shows a 31 percent chance of a 30 kt increase in intensity in 24 hours, and a 57 percent chance of 55 kt increase in 48 hours. However, the dynamical hurricane-regional model guidance is much more subdued, but they appear to be struggling to capture the current small structure of the system. Thus, the initial intensity forecast is a relative compromise, showing gradual intensification, with is higher than the regional-hurricane model guidance, but is lower than the ECMWF-based SHIPS and LGEM models. This initial forecast is somewhat higher than the consensus aids IVCN and HCCA early on, but they end up near the NHC intensity forecast by the end of the period. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 11.8N 107.0W 35 KT 40 MPH 12H 03/1800Z 12.7N 108.1W 40 KT 45 MPH 24H 04/0600Z 13.6N 108.9W 45 KT 50 MPH 36H 04/1800Z 14.4N 109.2W 50 KT 60 MPH 48H 05/0600Z 15.2N 109.5W 55 KT 65 MPH 60H 05/1800Z 15.5N 109.9W 60 KT 70 MPH 72H 06/0600Z 15.7N 110.6W 65 KT 75 MPH 96H 07/0600Z 15.7N 111.7W 65 KT 75 MPH 120H 08/0600Z 15.5N 113.5W 65 KT 75 MPH $$ Forecaster Papin NNNN