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Tropical Storm Marco Forecast Discussion Number 11
2020-08-23 04:45:54| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Sat Aug 22 2020 000 WTNT44 KNHC 230245 TCDAT4 Tropical Storm Marco Discussion Number 11 NWS National Hurricane Center Miami FL AL142020 1000 PM CDT Sat Aug 22 2020 Marco has taken on distinctly sheared appearance. Reports from an Air Force Reserve Hurricane Hunter plane, microwave imagery, and radar imagery from Cuba all indicate that deep convection is limited to the east side of the tropical storm and that it no longer has a nearly closed eyewall. The degradation of Marco's structure appears to be primarily due to strong upper-level southwesterly flow. Despite the shear, the plane still measured SFMR winds near 55 kt and the intensity is held at that value. Marco is a small tropical storm and will be susceptible to rapid changes in structure and intensity until it reaches the northern Gulf Coast. Such systems are often not very resilient in a high-shear environment, however even a brief relaxation of the shear could result in quick strengthening. It would not be surprising if Marco's intensity evolves in step-wise fashion consisting of periods of arrested development followed by fast strengthening if/when the shear relaxes. While the statistical models still show Marco becoming a hurricane within 24 h, the run-to-run consistency of the dynamical guidance remains poor. The latest HWRF, HMON and GFS forecasts show Marco weakening as it approaches the northern Gulf Coast, and this remains a distinct possibility if the shear remains consistently high. The NHC intensity forecast has not been changed substantially, in large part due to the low confidence of the forecast, and is consequently above all of the guidance at 36 and 48 h when Marco is forecast to be near the northern Gulf Coast. Additional adjustments to the forecast are likely on Sunday. In sharp contrast to earlier today, no large changes were made to the track forecast, though that should not be interpreted as an increase in forecast confidence. Marco is forecast to move north-northwestward and approach the northern Gulf Coast on Monday. As it moves inland and weakens, a turn toward the west at a slower forward speed is anticipated. This turn could occur before or after Marco moves inland, and will be tied in part to exactly when Marco begins to weaken since a stronger, deeper storm should continue to feel the affects of the upper-level southwesterly flow and move farther north while a weaker system will be steered westward by a low- to mid-level ridge extending over the southeastern US. The NHC forecast is nearly on top of the multi-model consensus, but the spread in the guidance is still higher than normal. Key Messages: 1. Tropical storm conditions will continue over portions of extreme western Cuba for a few more hours. Heavy rainfall is also expected overnight 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 be at or near hurricane strength when it approaches 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 may also 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 23/0300Z 22.8N 86.3W 55 KT 65 MPH 12H 23/1200Z 24.3N 87.1W 65 KT 75 MPH 24H 24/0000Z 26.3N 87.9W 70 KT 80 MPH 36H 24/1200Z 28.3N 88.9W 70 KT 80 MPH 48H 25/0000Z 29.6N 90.4W 55 KT 65 MPH...INLAND 60H 25/1200Z 30.5N 92.0W 35 KT 40 MPH...INLAND 72H 26/0000Z 30.9N 93.5W 30 KT 35 MPH...INLAND 96H 27/0000Z...DISSIPATED $$ Forecaster Zelinsky
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