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#1013332 (Received by flhurricane at: 4:56 PM 22.Aug.2020) TCDAT4 Tropical Storm Marco Discussion Number 10 NWS National Hurricane Center Miami FL AL142020 400 PM CDT Sat Aug 22 2020 Cutting to the chase, there have been some big changes among the model guidance, and subsequently the NHC forecast, for Marco this afternoon. While at this point it's a little speculative, the data collected by this morning's NOAA G-IV flight in the environment around Marco and across the Gulf of Mexico may have played a key role in the significant eastward shift seen in nearly all the 12z models. This isn't to say that the uncertainty in the eventual track has diminished. In fact, various ensemble members from some of the global models still show a potential risk to the coast anywhere from Texas to Alabama, and it's entirely possible that the volatile shifts seen in the models could continue. That being said, the new NHC track forecast has been shifted significantly eastward and now shows the center of Marco reaching southeastern Louisiana in about 2 days, which is the scenario currently shown by the GFS, ECMWF, HCCA, Florida State Superensemble, and the TVCN multi-model consensus. After Marco reaches the coast, the western Atlantic ridge is expected to build westward and should cause the cyclone to move more slowly toward the west-northwest across southern portions of Louisiana. As far as the intensity is concerned, the last fix made by this morning's reconnaissance flight indicated that the pressure had leveled off, and no higher winds had been observed from what was measured earlier in the flight. The radar presentation from Cuban radar has also degraded a bit, so Marco's initial intensity is held at 55 kt. Marco is beginning to move into a zone of moderate southwesterly shear, but otherwise favorable conditions of warm ocean water and some upper-level divergence are expected to foster strengthening during the next day or so. With the exception of the HWRF, the other intensity models show Marco reaching hurricane strength, and the NHC foreast continues to show that possibility while Marco moves over the central Gulf. The shear is still expected to strengthen in 36-48 hours when the system is approaching the northern Gulf Coast, but with the shift in the forecast track, now there may not be enough time for Marco to weaken below hurricane intensity before it reaches land. The new NHC intensity forecast is near or just above the HCCA and Florida State Superensemble models and holds Marco as a hurricane until it reaches the coast. The forecast track changes now bring tropical storm force winds to the coast in 36-48 hours, which necessitates the issuance of Hurricane and Storm Surge Watches for a portion of the northern Gulf Coast. These watches will likely need to be upgraded to warnings later tonight. Key Messages: 1. Tropical storm conditions will continue over portions of extreme western Cuba through this evening. Heavy rainfall is also expected in the eastern portions of the Mexican states of Quintana Roo and Yucatan, and across far western Cuba, which could result in flash flooding. 2. Marco is expected to approach the central Gulf Coast as a hurricane on Monday. Hurricane conditions, life-threatening storm surge, and heavy rainfall are possible along portions of the Gulf Coast beginning on Monday, and Hurricane and Storm Surge watches have been issued. Interests in these areas should follow any advice given by local government officials. 3. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of next week. This could result in a prolonged period of hazardous weather for areas that could be affected by Marco. Interests there should monitor the progress of Marco and Laura and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 21.9N 85.7W 55 KT 65 MPH 12H 23/0600Z 23.3N 86.5W 65 KT 75 MPH 24H 23/1800Z 25.3N 87.3W 75 KT 85 MPH 36H 24/0600Z 27.5N 88.3W 75 KT 85 MPH 48H 24/1800Z 29.3N 89.6W 65 KT 75 MPH...ON THE COAST 60H 25/0600Z 30.4N 91.1W 50 KT 60 MPH...INLAND 72H 25/1800Z 31.1N 92.7W 40 KT 45 MPH...INLAND 96H 26/1800Z 32.3N 95.6W 25 KT 30 MPH...INLAND 120H 27/1800Z...DISSIPATED $$ Forecaster Berg