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Chill falls over the Atlantic as the official season heads towards close
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| #982682 (Received by flhurricane at: 4:47 PM 17.Sep.2019) TCDAT4 Hurricane Humberto Discussion Number 21 NWS National Hurricane Center Miami FL AL092019 500 PM EDT Tue Sep 17 2019 Humberto's satellite appearance has improved somewhat since the previous advisory, with the eye clearing out and becoming more distinct. This has resulted in satellite subjective intensity estimates increasing to T5.5/102 kt from TAFB and SAB, with objective ADT estimates from UW-CIMSS increasing to T5.9/112 kt. However, during the past 36 h or so, the satellite estimates have been running higher than the actual surface winds by about 10-15 percent. A 1425Z ASCAT-C overpass indicated surface winds of 72 kt in the southeastern eyewall, which is significant given that the scatterometer is well undersampling the actual peak winds due to the 25-km footprint of the instrument. The intensity has been increased to 90 kt based on the clearing and warming of the eye and allowing for some overestimation by the satellite agencies. The scatterometer wind data also showed that Humberto's wind field has expanded more since the earlier recon wind data, thus some additional adjustments were made to all of the wind radii in this advisory. Humberto continues to move east-northeastward or 075/10 kt. The large hurricane remains on track, and the latest NHC model guidance remains tightly packed around the previous advisory track. Thus, no significant changes were made to the previous forecast track through 48 hours. By 72 hours and beyond, however, the models are now in better agreement on Humberto accelerating and moving faster toward the northeast through 96 hours as an extratropical cyclone, followed by a turn toward the east-northeast on day 5. The official forecast track is based on a blend of the consensus models TVCN, HCCA, and FSSE, and shows the center of Humberto passing just to the northwest and north of Bermuda between 24-36 hours or late Wednesday night. The latest SHIPS intensity guidance shows near 30 kt of deep-layer vertical shear allegedly affecting Humberto, which obviously is a significant overestimate based on the presence of a well-defined eye and smooth CDO feature in visible satellite imagery. Given the likely overestimation of the shear values, Humberto is expected to strengthen to major hurricane status in the next 24 h. Thereafter, Humberto is forecast to steadily weaken due to cold upwelling as the hurricane moves over cooler waters, and into an environment consisting of very strong shear exceeding 40-50 kt and more stable, drier air. However, the usual rate of weakening due to the strong shear conditions is expected to be tempered by the increasing baroclinic forcing in the right-rear entrance region of a strong jetstream wind maximum. The NHC intensity closely follows a blend of the intensity consensus models IVCN, HCCA, and FSSE. Key Messages: 1. Hurricane conditions are expected in Bermuda Wednesday night and Thursday morning, with tropical-storm-force winds expected by Wednesday afternoon. Residents there should follow advice given by local officials. 2. Storm surge and dangerous breaking waves could cause coastal flooding Wednesday night and Thursday along the southern coast of Bermuda. 3. Swells will continue to affect the northwestern Bahamas and the southeastern coast of the United States from east-central Florida to North Carolina during the next couple of days, creating life- threatening surf and rip current conditions. FORECAST POSITIONS AND MAX WINDS INIT 17/2100Z 31.0N 72.3W 90 KT 105 MPH 12H 18/0600Z 31.5N 70.5W 95 KT 110 MPH 24H 18/1800Z 32.6N 67.6W 100 KT 115 MPH 36H 19/0600Z 34.6N 63.9W 95 KT 110 MPH 48H 19/1800Z 37.4N 61.0W 85 KT 100 MPH 72H 20/1800Z 41.5N 57.2W 65 KT 75 MPH 96H 21/1800Z 43.5N 48.5W 55 KT 65 MPH...POST-TROP/EXTRATROP 120H 22/1800Z 46.0N 34.0W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart |
