000 WTNT45 KNHC 280315 TCDAT5 Hurricane Isaac Discussion Number 9 NWS National Hurricane Center Miami FL AL102024 300 AM GMT Sat Sep 28 2024 Isaac has maintained a well-defined eye on both visible and infrared imagery for the past 12 hours or so. The latest subjective Dvorak classifications are a consensus T-4.5/77 kt from TAFB and SAB. The latest objective intensity estimates from UW-CIMSS range from 72 to 92 kt. The initial intensity is adjusted upward to 80 kt based on a blend of the subjective and objective estimates. The hurricane is moving east-northeastward, or 075/14 kt. Isaac is currently moving in the zonal flow with the mid-latitude westerlies. A powerful deep-layer trough over Atlantic Canada will approach Isaac by Saturday night as a narrow mid-level ridge builds to the north of Isaac. These steering features will cause Isaac to bend a bit more toward the northeast while slowing down slightly. On Monday, after Isaac becomes post-tropical, a turn more toward the north-northeast is expected, as the deep-layer trough moves eastward and the ridge moves northward. No significant changes have been made to the official forecast track forecast through the first 60-72 h. After that time, a sizable westward adjustment is made to the official forecast track, but the new NHC forecast track is well east of the latest consensus models beyond 72 h. It is possible Isaac could strengthen a little more tonight, although the hurricane is likely near its peak intensity. The cyclone is crossing the 26 degree C isotherm now, but is forecast to remain over 24-26 degree C sea-surface temperatures for another 36 h or so. The cold upper-level temperatures that Isaac is forecast to traverse should help maintain enough instability such that only slow weakening is forecast through 36 h, and Isaac is expected to maintain hurricane strength through that time. Also, although moderate southwesterly shear is forecast to increase slightly, the shear direction is the same as the direction of Isaac's motion, so this should not be too unfavorable. Beyond 48 h, sea-surface temperatures decrease significantly and wind shear is forecast to become very strong. This should cause the cyclone to lose its deep convection by Monday and then become extratropical. Little to no change was made to the previous official intensity forecast, while lies near the middle of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 28/0300Z 38.2N 43.4W 80 KT 90 MPH 12H 28/1200Z 39.5N 40.9W 80 KT 90 MPH 24H 29/0000Z 41.0N 38.6W 75 KT 85 MPH 36H 29/1200Z 42.5N 37.0W 70 KT 80 MPH 48H 30/0000Z 43.8N 35.4W 60 KT 70 MPH 60H 30/1200Z 45.0N 33.7W 55 KT 65 MPH...POST-TROPICAL 72H 01/0000Z 46.9N 32.4W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 02/0000Z 50.9N 31.0W 40 KT 45 MPH...POST-TROP/EXTRATROP 120H 03/0000Z 53.8N 29.5W 35 KT 40 MPH...POST-TROP/EXTRATROP $$ Forecaster Hagen
Originally Posted at:
NATIONAL HURRICANE CENTER and CENTRAL PACIFIC HURRICANE CENTER
At The NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
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