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| #982631 (Received by flhurricane at: 10:59 AM 17.Sep.2019) TCDAT4 Hurricane Humberto Discussion Number 20 NWS National Hurricane Center Miami FL AL092019 1100 AM EDT Tue Sep 17 2019 An Air Force Reserve reconnaissance aircraft that investigated Humberto earlier this morning found 700-mb flight-level winds of 96 kt and 82-kt surface winds on a dropsonde in the southwestern quadrant, and 81-kt SFMR surface winds in the northwestern quadrant. More importantly, the wind data indicate that Humberto's wind field has expanded significantly and is becoming asymmetric with the largest wind radii now located in the southern semicircle. The aircraft also measured a central pressure of 961 mb, down 18 mb since this time yesterday. The central pressure typically corresponds to an intensity of around 100 kt, but the expansion of the wind field has resulted in little increase in the eyewall wind field for now. Based on these data, the initial intensity has been increased to 85 kt. Humberto has maintained an east-northeastward motion of 075/07 kt. The hurricane is forecast to remain embedded within the mid-latitude westerlies though the forecast period. A deep-layer trough is forecast to dig southward to the west of Humberto in the 24-48 hour period, causing the hurricane to gradually gain latitude and pass just to the northwest of Bermuda in around 36 hours. The models are in excellent agreement on this developing track scenario and are tightly clustered around the previous advisory track. However, the recent expansion of the wind field and the expected additional increase in the wind on the southeast side of Humberto's circulation will result in the island being in near-hurricane-force conditions. By 72 hours and beyond, Humberto will accelerate northeastward over the cold waters of the north Atlantic and interact with a strong frontal system, resulting in the transition into a large extratropical low. The official track forecast is similar to the previous advisory track through 48 hours, and then is a little faster in the 72-120 hour period, close to a blend of the various consensus models. Due to the expansion of Humberto's wind field and large 35-40-n mi wide eye, only modest strengthening is expected during the next 36 hours or so. Although not explicitly shown, Humberto could still become a major hurricane between 24-36 hours when the cyclone will be located over the warmest water of about 29C. Thereafter, gradual wakening is forecast due to cooler waters and strong southwesterly vertical wind shear in excess of 40 kt. However, the typical rate of weakening due to such hostile shear conditions will be tempered owing to the increasing baroclinic forcing in the right-rear entrance region of a polar jet maximum. The new intensity forecast is nearly identical to the previous advisory, and follows a blend of the consensus models HCCA and FSSE. Key Messages: 1. Tropical-storm-force winds are expected to reach Bermuda by Wednesday afternoon, with hurricane conditions possible Wednesday night and Thursday morning. Residents there should follow advice given by local officials. 2. Large swells will increase along the coast of Bermuda by Wednesday. Dangerous breaking waves, especially along south-facing beaches, could cause coastal flooding Wednesday night and Thursday. 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 17/1500Z 30.7N 73.5W 85 KT 100 MPH 12H 18/0000Z 31.1N 72.0W 90 KT 105 MPH 24H 18/1200Z 31.8N 69.4W 95 KT 110 MPH 36H 19/0000Z 33.1N 66.1W 95 KT 110 MPH 48H 19/1200Z 35.3N 62.6W 90 KT 105 MPH 72H 20/1200Z 39.6N 58.4W 70 KT 80 MPH 96H 21/1200Z 43.0N 52.2W 60 KT 70 MPH...POST-TROP/EXTRATROP 120H 22/1200Z 44.5N 40.0W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart |
