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Tropical Storm Marco Forecast Discussion Number 7
2020-08-22 04:54:37| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Fri Aug 21 2020 000 WTNT44 KNHC 220254 TCDAT4 Tropical Storm Marco Discussion Number 7 NWS National Hurricane Center Miami FL AL142020 1100 PM EDT Fri Aug 21 2020 An Air Force Reserve Hurricane Hunter aircraft has investigated the system over the northwest Caribbean during the past few hours. The plane reported a number of unflagged SFMR winds between 35 and 40 kt and max flight level winds of 41 kt. A blend of these data supports an intensity of 35 kt, and therefore, this system has been designated as Tropical Storm Marco. Deep convection has increased near and to the east of Marco's center during the past few hours. Although there still isn't much evidence of inner-core banding, the data from the plane does indicate that the center of Marco has become better defined since the afternoon and that the minimum pressure has dropped. Unfortunately the intensity forecast has not become any clearer and confidence in that aspect of the forecast is quite low. Marco is embedded within an environment that could support a fast rate of strengthening. However, recent microwave data does not indicate that the system has developed an inner-core, and only gradual strengthening is likely until it does. The intensity guidance spread is quite high, with the GFS and ECMWF global models both showing little further strengthening, while the HMON regional model rapidly makes Marco a hurricane before it reaches the northeast tip of the Yucatan peninsula. That possibility can not be ruled out, but a majority of the intensity guidance favors the weaker solution of the global models. Even with the HMON outlier included, the NHC intensity forecast is above the model consensus. Once Marco moves over the central Gulf of Mexico, a rapid increase in wind shear associated with an upper-level trough should limit the potential for further strengthening, and weakening is still anticipated before Marco nears the northern Gulf Coast, as shown in the previous official forecast. Confidence in the track forecast is also lower than normal, as the models spread remains quite high. Only small adjustments were made to the NHC forecast which heavily favors the GFS and ECMWF solutions on the left side of the track guidance. It is worth noting that the NHC track forecast is near middle of the GFS and ECMWF ensembles. Marco is currently forecast to move northwestward toward a weakness in the subtropical ridge over the central Gulf of Mexico for the next day or two, before the ridge builds and turns the tropical cyclone farther west. Near the end of the period, Marco's track and intensity could be also influenced by Tropical Storm Laura which is also forecast to be over the Gulf of Mexico, however the details of that interaction are highly uncertain at this time. Given the high uncertainty in the forecast, larger than normal changes could be required to future advisories. Key Messages: 1. Marco is forecast to strengthen over the northwestern Caribbean Sea through Saturday as it approaches the northeast coast of the Yucatan Peninsula. A Hurricane Watch and a Tropical Storm Warning are in effect for portions of that region. 2. The system is expected to move into the south-central Gulf of Mexico as a tropical storm on Sunday. Although some strengthening is anticipated on Sunday, weakening is forecast as the system approaches the northwestern Gulf coast on Tuesday. It is still too soon to know exactly the location and magnitude of impacts the system will produce along the central or northwestern Gulf Coast, and interests in that area should continue monitoring the progress of this system over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 18.7N 84.9W 35 KT 40 MPH 12H 22/1200Z 19.7N 85.7W 45 KT 50 MPH 24H 23/0000Z 21.1N 86.7W 50 KT 60 MPH 36H 23/1200Z 22.7N 88.0W 55 KT 65 MPH 48H 24/0000Z 24.4N 89.5W 60 KT 70 MPH 60H 24/1200Z 25.9N 91.0W 60 KT 70 MPH 72H 25/0000Z 27.5N 92.8W 60 KT 70 MPH 96H 26/0000Z 29.0N 95.4W 45 KT 50 MPH...INLAND 120H 27/0000Z 30.0N 97.4W 30 KT 35 MPH...INLAND $$ Forecaster Zelinsky
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