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| #1013459 (Received by flhurricane at: 10:59 AM 23.Aug.2020) TCDAT4 Tropical Storm Marco Discussion Number 13 NWS National Hurricane Center Miami FL AL142020 1000 AM CDT Sun Aug 23 2020 Deep convection with cloud tops of -75 to -80 degrees C has persisted over the center of Marco for the past several hours. An Air Force Reserve Hurricane Hunter aircraft currently investigating the cyclone measured winds equivalent to 62 kt both with the SFMR and in a dropsonde in the northeastern eyewall. However, the aircraft reported peak 850-mb flight level winds of only 64 kt. Based on a blend of these data it appears that Marco is on the cusp of becoming a hurricane, but is not quite there yet. Therefore, the initial intensity remains 60 kt. Based on the SHIPS guidance, Marco has about a 12-24 hour window to intensify in an environment characterized by moderate southwesterly shear, very warm waters, and plenty of atmospheric moisture. After that time, the vertical wind shear is expected to increase and this should begin to dominate the cyclone's environment. The latest NHC intensity forecast is unchanged from the previous one, forecasting Marco to become a hurricane later today, and maintaining hurricane intensity up until landfall in agreement with the latest LGEM intensity guidance. While it is possible that Marco will weaken just prior to landfall due to the increasing shear, there is little difference in the impacts between a 60 and 65 kt system. Marco is moving north-northwestward or 340/12 kt. Although the overall guidance has not changed much since the previous advisory, there remains considerable spread in this guidance by the time Marco reaches the northern Gulf coast. This spread could be attributed to the varying ways the models handle the system as it encounters the more hostile environment near the coast. Since the track consensus aids have changed little through 36 h, the official forecast is essentially the same as the previous one through that time. The model guidance has shifted a little northward beyond 36 h, so the official forecast was nudged to the north during that time as well. Key Messages: 1. Hurricane conditions, life-threatening storm surge, and heavy rainfall are expected from Marco along portions of the Gulf Coast beginning on Monday, and Hurricane and Storm Surge Warnings have been issued. 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 23/1500Z 24.7N 87.3W 60 KT 70 MPH 12H 24/0000Z 26.4N 88.1W 65 KT 75 MPH 24H 24/1200Z 28.2N 89.1W 65 KT 75 MPH 36H 25/0000Z 29.5N 90.4W 55 KT 65 MPH...INLAND 48H 25/1200Z 30.5N 92.1W 35 KT 40 MPH...INLAND 60H 26/0000Z 31.2N 93.8W 30 KT 35 MPH...INLAND 72H 26/1200Z 31.8N 95.4W 25 KT 30 MPH...INLAND 96H 27/1200Z...DISSIPATED $$ Forecaster Latto |
