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Tropical Storm Marco Forecast Discussion Number 15
2020-08-24 04:56:43| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Sun Aug 23 2020 000 WTNT44 KNHC 240256 TCDAT4 Tropical Storm Marco Discussion Number 15 NWS National Hurricane Center Miami FL AL142020 1000 PM CDT Sun Aug 23 2020 Marco is feeling the effect of strong southwesterly shear. The center, as identified by an Air Force Reserve Hurricane Hunter aircraft, is displaced to the southwest edge of the convective canopy. That said, Marco has not quite fully decoupled yet and has moved well right of the previous forecast. Data from the plane indicates that Marco has weakened; the highest flight-level winds were only 58 kt with unflagged SFMR winds just above 50 kt. Marco's intensity has been lowered slightly to 60 kt, but this is probably generous. The intensity guidance remains maddeningly inconsistent. The most recent runs of the HWRF and HMON show that there is still a chance that Marco could reach the Gulf Coast as a hurricane, despite the shear. While this seems unlikely given the current structure of the cyclone, it also seems slightly premature to rule it out entirely. After all, the shear lessened enough to allow Marco to briefly strengthen after it was strongly sheared for a time last night. The rest of the guidance shows Marco weakening further as it approaches the coast, and this seems like a more likely solution. The NHC forecast just holds Marco at 60 kt for the next 24 h, but its certainly possible it could already be weaker than that. Rapid weakening is likely by 36 h if it hasn't happened sooner and Marco is forecast to become a remnant low by early Wednesday. Marco's track is tied entirely to its intensity and structure. Once Marco becomes fully decoupled it will slow and turn westward. It is not out of the question that this could happen before Marco reaches the coast and the system never makes landfall. However, as long as the strong southwesterly upper-level flow contributes to the steering, Marco will continue to move more northward to north-northwestward. Due to the recent northward movement of Marco's center, the NHC track forecast has been adjusted in that direction, but it is blended to the previous forecast by about 36 h and onward. Confidence in this forecast unfortunately remains low since the model spread remains usually high. It is worth noting that Marco is a small tropical cyclone. The large area of Tropical Storm and Hurricane watches and warnings along the northern Gulf Coast is a reflection of the unusually high uncertainty in the forecast, and it is unlikely that all of those regions will experience tropical-storm-force winds or life-threatening storm surge associated with Marco. However, impacts will likely occur in some portions of the watch/warning area beginning on Monday, and heavy rain is likely across most of the region during the next couple of days. Changes to the watches and warnings are likely on Monday and users should consult products from their local weather forecast office for more information about potential hazards in their area. Key Messages: 1. Strong winds, life-threatening storm surge, and heavy rainfall are expected from Marco along portions of the Gulf Coast beginning on Monday. Interests in these areas should follow any advice given by local government officials. 2. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of the 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 24/0300Z 26.8N 87.6W 60 KT 70 MPH 12H 24/1200Z 28.0N 88.7W 60 KT 70 MPH 24H 25/0000Z 29.0N 90.2W 60 KT 70 MPH 36H 25/1200Z 29.6N 92.0W 40 KT 45 MPH...INLAND 48H 26/0000Z 29.8N 93.6W 30 KT 35 MPH...INLAND 60H 26/1200Z 29.7N 94.9W 25 KT 30 MPH...POST-TROP/INLAND 72H 27/0000Z...DISSIPATED $$ Forecaster Zelinsky
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