Subtropical Storm Alberto Forecast Discussion # 1

Subtropical Storm Alberto Forecast Discussion

StartFragmentSubtropical Storm Alberto Discussion Number 1 NWS National Hurricane Center Miami FL AL012018 1000 AM CDT Fri May 25 2018 The broad low pressure system that the NHC has been tracking for the past several days over the Yucatan Peninsula has finally moved offshore over the waters of the northwestern Caribbean Sea. Although the system possesses multiple low-level circulations, the overall larger circulation has improved since yester

day. Given that the system has been interacting with a sharp upper-level trough, the strongly sheared low has been designated a subtropical storm. The initial intensity is based on buoy and ship observations of 30-35 kt. Ship 3ETA7 located just northeast of the center at 1100Z reported 45-kt winds at 50 meters elevation. Those winds equate to 35-40 kt at 10 meters elevation. An Air Force Reserve Hurricane Hunter Aircraft is scheduled to investigate Alberto later this afternoon and provide more information on the storm's structure and intensity. The initial motion estimate is an uncertain 020/05 kt. The broad inner-core wind field and multiple swirls makes the short-term motion forecast a little tricky. However, a large subtropical ridge to the east should generally induce a slow north to north- northeastward motion for the next 24 hours or so. After that, the ridge across the western Atlantic and Florida, along with a mid/upper-level low forecast to develop over the central Gulf of Mexico, should result in a faster northward motion at 36-48 hours, followed by a gradual turn toward the northwest around the northern fringe of the aforementioned mid/upper-level low. By 96 hours, the cyclone is forecast to slow down significantly as it nears the north-central Gulf Coast due to a large weakness in the subtropical ridge forecast to develop over the Deep South. The official forecast track closely follows the consensus models TVCN and HCCA. Given the broad inner-core wind field and belligerent westerly wind shear forecast to persist for the next 48 hours or so, only gradual intensification is expected. By 72 hours, however, when the cyclone is forecast to move slowly over above-normal SSTs of 28-29C and into an upper-level col and weak wind shear, some additional strengthening could occur. For now, the intensity forecast will remain conservative due to possible intrusion of dry mid-level air before landfall, and closely follows the HCCA consensus model. KEY MESSAGES: 1. Regardless of its exact track and intensity, Alberto is expected to produce heavy rainfall and flash flooding over the northeastern Yucatan Peninsula of Mexico, western Cuba, southern Florida and the Florida Keys. Rainfall and flooding potential will increase across the central 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. Alberto could bring tropical storm conditions and storm surge to portions of the central and eastern Gulf Coast later this weekend and early next week, although it is too soon to specify the exact location and magnitude of these impacts. Residents in these areas should monitor the progress of Alberto, as tropical storm and storm surge watches may be required later today or tonight. 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 25/1500Z 19.7N 86.8W 35 KT 40 MPH 12H 26/0000Z 20.5N 86.6W 35 KT 40 MPH 24H 26/1200Z 22.0N 86.2W 40 KT 45 MPH 36H 27/0000Z 24.1N 85.8W 45 KT 50 MPH 48H 27/1200Z 26.7N 86.1W 50 KT 60 MPH 72H 28/1200Z 29.3N 87.9W 55 KT 65 MPH 96H 29/1200Z 31.0N 89.1W 30 KT 35 MPH...INLAND 120H 30/1200Z 33.6N 88.7W 20 KT 25 MPH...INLAND $$ Forecaster Stewart EndFragment