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Hurricane Sam Forecast Discussion Number 14
2021-09-26 04:51:08| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sat Sep 25 2021 000 WTNT43 KNHC 260251 TCDAT3 Hurricane Sam Discussion Number 14 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Sat Sep 25 2021 It's been an interesting evening with regards to analyzing the various data from a NOAA Hurricane Hunter research flight into Hurricane Sam. Dropsonde data in both the southeast and northwest quadrants indicate that small eyewall mesovortices and possibly even tornado-scale vortices were present based on the wind profiles showing sharply opposite-direction winds from what would normally be expected in those regions of the hurricane. Some dropsonde surface winds have been as high as 162 kt, which is more representative of a gust, while SFMR surface winds have been as high as 133 kt. However, the strongest 700-mb flight-level winds have been 138-139 kt in the northeastern quadrant, which equates to about 125-kt tangential surface winds. Three dropsondes released in the eye indicate that the pressure had remained steady at 943-944 mb during the duration of the aircraft reconnoiter. That pressure equates to about 125 kt based on various pressure-wind relationships. Based on that estimate and the 700-mb flight-level to surface-wind conversion, the advisory intensity is 125 kt, which is representative of the mean tangential winds and no localized wind perturbations. The initial motion is toward the west-northwest, or 295/07 kt. Not to sound like a broken record, but no significant changes were made to the previous track forecast and reasoning. Sam is expected to move slowly west-northwestward and northwestward over the next few days around the southwestern periphery of a deep-layer subtropical ridge that is situated to the north and northeast of the small hurricane. On days 4 and 5, an upper-level trough/low is forecast to dig southward and amplify off the U.S. east coast and extend all the way to the Bahamas. This feature should act to lift Sam northward at a faster forward speed. The latest NHC model guidance based on 12Z and 18Z model runs has shifted noticeably to the east of the previous runs, and the new NHC track forecast has been nudged in that direction as well. However, since the NOAA G-IV jet aircraft has been out there sampling the environment around Sam, it's best to remain conservative and not shift the track any farther to the east until the new 00Z model runs with that new aircraft data come in for the next advisory package at 0600Z. The new NHC track forecast lies about halfway between the previous advisory track on the left and the tightly packed consensus track models on the right. The radar images from the reconnaissance aircraft indicated that the eyewall was thin in many locations due to dry-air intrusions, and the latest SHIPS intensity output indicates that Sam will remain within a fairly dry mid-level environment. Also, the depth of the warm water beneath the hurricane isn't overly deep, which could result in cold upwelling owing to Sam's slow forward motion of only 5-7 kt during the next couple of days. Eyewall replacement cycles are also likely now due to the hurricane's small size and strong intensity. Thus, fluctuations in intensity seem likely for the next couple of days even though the vertical wind shear is expected to remain quite low at only 5-10 kt. On days 3-5, however, the shear is forecast to increase to 15-20 kt from the southwest, which is expected to induce a slow weakening trend. However, it is likely that Sam will remain a major hurricane through 120 h, even on days 3-5 due to the cyclone moving over warmer and deeper water during that 3-day period. The new official intensity forecast is essentially the same as the previous advisory, and remains above the consensus model and is near the higher end of the intensity guidance. FORECAST POSITIONS AND MAX WINDS INIT 26/0300Z 13.5N 49.0W 125 KT 145 MPH 12H 26/1200Z 14.0N 49.9W 130 KT 150 MPH 24H 27/0000Z 14.7N 51.0W 125 KT 145 MPH 36H 27/1200Z 15.4N 52.1W 120 KT 140 MPH 48H 28/0000Z 16.3N 53.2W 120 KT 140 MPH 60H 28/1200Z 17.1N 54.3W 115 KT 130 MPH 72H 29/0000Z 18.0N 55.6W 115 KT 130 MPH 96H 30/0000Z 20.1N 58.5W 110 KT 125 MPH 120H 01/0000Z 23.7N 61.4W 110 KT 125 MPH $$ Forecaster Stewart
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