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| #927131 (Received by flhurricane at: 5:00 PM 25.May.2018) TCDAT1 Subtropical Storm Alberto Discussion Number 2 NWS National Hurricane Center Miami FL AL012018 400 PM CDT Fri May 25 2018 The inner-core low-level wind field of Alberto has changed little since the previous advisory based on recent data from an Air Force Reserve reconnaissance aircraft, along with land and ship observations. However, the convective structure of the cyclone has degraded over the past several hours due a pronounced intrusion of dry mid-/upper-level air and the cloud pattern continues to exhibit the structure of a subtropical cyclone. The lowest pressure measured thus far by the aircraft has been 1006 mb. The initial position is a little north of the recon position of a pronounced swirl due to the broad overall nature of the low-level circulation. The initial motion estimate is 090/02 kt, but this is considered to be a short-term motion. A gradual turn toward the north should begin later tonight due to a strong ridge located to the east across the Greater Antilles. A steadier northward motion is forecast to occur by Saturday evening and continue into Sunday as a sharp mid/upper-level trough digging southward into the central Gulf of Mexico combines with southerly flow around the western portion of a large subtropical ridge to produce deep-layer southerly flow across Alberto. By 48 hours and continuing through 72 hours, the developing mid/upper-level low over the central Gulf should cause the cyclone to turn northwestward and accelerate until it nears the Gulf Coast by Monday night. After that, steering currents are forecast to collapse as a broad weakness develops in the subtropical ridge axis located along the Gulf coast. Slow but steady recurvature into the westerlies across the Deep South is expected to begin by 96-120 h. The new NHC forecast track is similar to the previous advisory, and closely follows the consensus models TVCN and HCCA, and the Florida State Superensemble (FSSE) model. The broad nature of the inner-core wind field, along with strong westerly wind shear in excess of 20 kt is forecast to continue for the next 24 hours. The latest model runs actually decrease the shear sooner than previously forecast, but the ragged nature of the wind field should prevent any significant intensification until after 48 hours. As a result, only slow but steady strengthening is expected for the next 3 days, and Alberto could peak around 60 kt around 60 hours when the storm will be in a low wind shear regime and over SSTs greater than 28 deg C. However, proximity to dry mid- level air around landfall could hinder any additional strengthening, and the NHC intensity forecast remains similar to the previous advisory, closely following a blend of the HCCA and ICON consensus models and the FSSE model. The new NHC forecast necessitates the issuance of tropical storm and storm surge watches for portions of the U.S. Gulf Coast at this time. Note that if the intensity forecast increases with later advisories, a hurricane watch could be needed for a portion of the Gulf Coast. KEY MESSAGES: 1. Regardless of its exact track and intensity, Alberto is expected to produce heavy rainfall and flash flooding over the northeaster Yucatan Peninsula of Mexico, western Cuba, southern Florida and the Florida Keys. Rainfall and flooding potential will increase across the central U.S. Gulf Coast region and the southeastern United States later this weekend and early next week when Alberto is expected to slow down after it moves inland. 2. Tropical-storm-force winds and hazardous storm surge are possible along portions of the central and eastern U.S. Gulf Coast beginning on Sunday, including areas well east of the track of Alberto`s center, and tropical storm and storm surge watches have been issued for portions of these areas. Residents in the watch areas are encouraged not to focus on the details of the forecast track of Alberto and should follow any guidance given by their local government officials. 3. Dangerous surf and rip current conditions are affecting portions of the Yucatan Peninsula and western Cuba and will likely spread along the eastern and central U.S. Gulf Coast later this weekend. FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 19.4N 86.3W 35 KT 40 MPH 12H 26/0600Z 20.7N 86.0W 40 KT 45 MPH 24H 26/1800Z 22.5N 85.7W 45 KT 50 MPH 36H 27/0600Z 24.8N 85.6W 50 KT 60 MPH 48H 27/1800Z 26.8N 86.2W 55 KT 65 MPH...TROPICAL STORM 72H 28/1800Z 29.0N 87.8W 55 KT 65 MPH...TROPICAL STORM 96H 29/1800Z 31.7N 88.6W 30 KT 35 MPH...INLAND 120H 30/1800Z 35.2N 87.6W 20 KT 25 MPH...INLAND $$ Forecaster Stewart |
