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| #1108842 (Received by flhurricane at: 4:45 AM 25.Sep.2022) TCDAT4 Tropical Storm Ian Discussion Number 9 NWS National Hurricane Center Miami FL AL092022 500 AM EDT Sun Sep 25 2022 Bands of deep convection have developed primarily over the northern portion of Ian's circulation overnight, however data from an Air Force Reserve reconnaissance aircraft show that the low- and mid-level centers of the tropical storm have not yet become in better alignment. The Air Force plane essentially performed a low-level invest-like mission at about 2500 ft and found an east-southeast to west-northwest elongated area of light and variable winds that is located to the southeast of the mid-level center seen in infrared satellite imagery. Given the time spent searching for the low-level center, the aircraft did not fully sample the northern portion of the circulation where the strongest winds are likely located. Therefore, the initial intensity is held at 45 kt, which is in line with the latest subjective and objective satellite estimates. It should be noted that the advisory position is a compromise between the low- and mid-level centers as it is quite likely that a new low-level center will form closer to the convection and the mid-level center very soon. Due to the current lack of center definition, the initial motion estimate is a somewhat uncertain 285/10 kt. The track forecast philosophy remains unchanged, with Ian forecast to move around the western periphery of a subtropical ridge located over the western Atlantic. Ian is forecast to turn northwestward later today, passing near or southwest of the Cayman Islands on Monday, and approach western Cuba on Monday night or early Tuesday. While the models agree on the overall scenario, there are still significant differences regarding the exact track of the storm, especially after 72 hours. Even with the addition of the NOAA G-IV synoptic surveillance dropsonde data and additional upper-air balloon releases across much of the United States, the spread in the guidance has not narrowed from before. The UKMET and ECMWF models continue to hold firm along the eastern side of the guidance and show a track into west-central Florida, while the GFS and HWRF remain one the western side, taking the Ian into the central or western Florida panhandle. The updated NHC track continues to split these differences and remains closest to the TVCA multi-model consensus, and the latest GFS ensemble mean. The new track is very similar to the previous advisory. With the cross-track spreading remaining between 200-220 n mi at days 4 and 5, it cannot be overstated that significant uncertainty remains in Ian's long-range prediction. Another NOAA G-IV synoptic surveillance mission is already underway collecting data around the storm which will hopefully reduce some of the model spread. Ian remains within an environment that appears quite conducive for strengthening. Once the circulation become more vertically coherent, low vertical wind shear conditions and high ocean heat content are expected to allow for rapid intensification while Ian moves over the northwestern Caribbean Sea. The Deterministic to Probabilistic Statistical Rapid Intensification Index (DTOPS) once again calls for a 90 percent chance of rapid strengthening during the following 48- and 72-hour forecast periods. The NHC intensity forecast calls for rapid intensification to begin later today, and forecasts Ian to be a major hurricane when it nears western Cuba in about 48 hours. The latest official intensity forecast shows a similar peak intensity around 72 h over the southeastern Gulf of Mexico as the previous advisory. After that time, a significant increase in southwesterly shear is predicted by the global models, and weakening is forecast to occur while Ian approaches the Florida coast. Despite the reduction in intensity, Ian is likely to have an expanding wind field and will be slowing down by that time, which will have the potential to produce significant wind and storm surge impacts. Users are urged to not focus on specific forecast intensities in the 4- and 5-day forecasts and instead focus on the potential hazards Ian may produce across portions of Florida. Key Messages: 1. Ian is expected to produce heavy rainfall and instances of flash flooding and possible mudslides in areas of higher terrain, particularly over Jamaica and Cuba. Flash and urban flooding is possible with rainfall across the Florida Keys and Florida peninsula through mid week. Additional flooding on rivers across northern Florida and parts of the Southeast cannot be ruled out. 2. Hurricane or tropical storm conditions are expected on Grand Cayman beginning early Monday. 3. Ian is forecast to be a major hurricane when it passes near or over western Cuba, and there is increasing confidence in a life-threatening storm surge and hurricane-force winds in portions of western Cuba beginning late Monday. Hurricane and tropical storm watches are now in effect for much of western Cuba. 4. Ian is expected to remain a major hurricane when it moves generally northward across the eastern Gulf of Mexico during the middle of the week, but uncertainty in the long-term track and intensity forecast is higher than usual. Regardless of Ian’s exact track and intensity, there is a risk of dangerous storm surge, hurricane-force winds, and heavy rainfall along the west coast of Florida and the Florida Panhandle by the middle of the week, and residents in Florida should ensure they have their hurricane plan in place, follow any advice given by local officials, and closely monitor updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 14.9N 78.8W 45 KT 50 MPH 12H 25/1800Z 15.7N 80.0W 55 KT 65 MPH 24H 26/0600Z 17.3N 81.7W 70 KT 80 MPH 36H 26/1800Z 19.1N 83.1W 90 KT 105 MPH 48H 27/0600Z 21.0N 84.1W 105 KT 120 MPH 60H 27/1800Z 23.0N 84.6W 115 KT 130 MPH 72H 28/0600Z 24.8N 84.8W 120 KT 140 MPH 96H 29/0600Z 27.5N 84.6W 105 KT 120 MPH 120H 30/0600Z 29.8N 83.9W 80 KT 90 MPH $$ Forecaster Brown |
