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Hurricane Sam Forecast Discussion Number 17
2021-09-26 22:35:23| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sun Sep 26 2021 000 WTNT43 KNHC 262035 TCDAT3 Hurricane Sam Discussion Number 17 NWS National Hurricane Center Miami FL AL182021 500 PM AST Sun Sep 26 2021 Sam has found a way to strengthen some more today. The ring of intense convection surrounding the 15-n-mi-diameter eye has expanded in size over the past several hours, while cloud tops colder than -70C still completely surround the center. An earlier GMI microwave overpass revealed very strong eyewall convection tightly wrapped around the small eye, a feature oftentimes seen with intense hurricanes. The latest Dvorak intensity estimates from TAFB and SAB remain T6.5/127 kt. However, the UW-CIMSS ADT estimate has recently increased to T6.6/130 kt. Based on the expansion of the ring of convection and a blend of these Dvorak estimates, the initial intensity is raised to 130 kt for this advisory. Sam remains compact, with winds of tropical storm force extending only 70 n mi from the center. A NOAA Hurricane Hunter aircraft is en route to investigate Sam and will provide more detailed intensity data. It would not take much further expansion of the convection and cooling of the cloud tops over the inner-most core of Sam for it to become a rare Category 5 hurricane on the Saffir-Simpson hurricane wind scale. Although plausible given the ideal environmental conditions over the next couple of days, this strengthening is not explicitly forecast to occur. The only factors within the next 2-3 days that could cause Sam to weaken would be an eyewall replacement cycle or the upwelling of cooler waters due to the hurricane's relatively slow forward motion. By 72 h, vertical wind shear is forecast to increase, which should cause gradual weakening. However, given the current strength of Sam, the new forecast calls for it to remain a major hurricane throughout the 5-day forecast period. The latest NHC intensity forecast was increased by 5 kt through 60 h due to the change in the initial intensity and remains on the high end of the guidance through that time. At 72 h and beyond, the intensity forecast is near the HFIP corrected consensus approach (HCCA) model. Sam has been wobbling a little north-northwestward and has slowed down over the past few hours, but the 12-h-average motion is about 305/06 kt. A subtropical ridge centered to the northeast of the hurricane should steer it northwestward for the next 3-4 days. By late Thursday, Sam is expected to reach the western periphery of the ridge and turn north-northwestward and begin to increase its forward speed as the cyclone starts to get caught up in the southerly flow between the ridge and an approaching mid-latitude trough. The various track model guidance is in better agreement today compared to yesterday, especially in the day 3-5 time frame. The latest NHC track forecast is little changed from the previous one, and lies near the various consensus track model solutions. FORECAST POSITIONS AND MAX WINDS INIT 26/2100Z 14.2N 50.5W 130 KT 150 MPH 12H 27/0600Z 14.7N 51.3W 130 KT 150 MPH 24H 27/1800Z 15.5N 52.5W 125 KT 145 MPH 36H 28/0600Z 16.3N 53.6W 125 KT 145 MPH 48H 28/1800Z 17.2N 54.7W 120 KT 140 MPH 60H 29/0600Z 18.1N 55.8W 120 KT 140 MPH 72H 29/1800Z 19.2N 57.4W 115 KT 130 MPH 96H 30/1800Z 22.1N 60.5W 110 KT 125 MPH 120H 01/1800Z 26.6N 62.9W 110 KT 125 MPH $$ Forecaster Latto
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