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| #1063829 (Received by flhurricane at: 11:12 PM 29.Sep.2021) TCDAT3 Hurricane Sam Discussion Number 30 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Wed Sep 29 2021 Sam is intensifying tonight after completing an eyewall replacement cycle. A recent NOAA Hurricane Hunter aircraft showed peak 700 mb flight-level winds of 138 kt, multiple SFMR surface winds of 120 kt or greater, plus falling central pressure to about 940 mb. This deepening trend is also seen on recent satellite imagery, with significant warming within the eye noted. These data support raising the initial intensity to 125 kt for this advisory. It is of note that the NOAA mission has been quite helpful for both the size and intensity analysis, with dropsondes and SFMR data showing that the hurricane has grown quite a bit since earlier today, mostly on the eastern side. Additionally, reflectivity data from the core confirmed the end of the eyewall replacement cycle. The hurricane could strengthen a little more overnight while it moves over a warm ocean eddy in fairly light shear. Afterward, gradually cooling SSTs, falling ocean heat content, and the inevitable future eyewall replacement cycles should cause a weakening trend. However, this will likely not be as steady as shown below, and significant deviations (upward or downward) from the forecast can be expected. Faster weakening is expected by the weekend due to cool SSTs and increasing shear. The new forecast is close to the intensity consensus, and is a little higher than the previous NHC prediction early on, primarily due to the initial wind speed. Sam has sped up tonight, now estimated at 315/10 kt. It sounds like a broken record, but there are no changes to the forecast during the first few days as model guidance is in excellent agreement on a gradually rightward-curving track, passing a couple of hundred n mi or more east of Bermuda. Afterward, guidance is actually in worse agreement over the weekend, with widely divergent solutions from an upper-level trough picking up Sam and turning it north toward Newfoundland or the trough leaving Sam behind, causing a slow eastward motion. The trend in the guidance is for a slower motion at long range, so I`ve elected to keep the day 4 and 5 points almost the same as the previous advisory, just a hair faster than the latest consensus aids. Key Messages: 1. Swells generated by Sam will impact the Lesser Antilles during the next several days. Swells are expected to reach Bermuda and the Bahamas in a day or so, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. A Tropical Storm Watch could be required for Bermuda on Thursday, and interests there should monitor the progress of Sam. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 20.6N 58.4W 125 KT 145 MPH 12H 30/1200Z 21.9N 59.6W 130 KT 150 MPH 24H 01/0000Z 24.1N 60.9W 125 KT 145 MPH 36H 01/1200Z 26.8N 61.8W 120 KT 140 MPH 48H 02/0000Z 29.6N 61.8W 115 KT 130 MPH 60H 02/1200Z 32.2N 60.7W 110 KT 125 MPH 72H 03/0000Z 34.3N 59.2W 100 KT 115 MPH 96H 04/0000Z 37.5N 54.5W 85 KT 100 MPH 120H 05/0000Z 42.0N 48.5W 70 KT 80 MPH $$ Forecaster Blake |
