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Hurricane Larry Forecast Discussion Number 17
2021-09-04 22:56:16| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sat Sep 04 2021 000 WTNT42 KNHC 042056 TCDAT2 Hurricane Larry Discussion Number 17 NWS National Hurricane Center Miami FL AL122021 500 PM AST Sat Sep 04 2021 Larry remains a formidable hurricane this afternoon. Both visible and infrared satellite bands show the hurricane has a well-defined and warm eye (greater than 10 C) surrounded by a cold ring of eyewall convection (-60 to -70 C). An AMSR2 microwave pass received at 1630 UTC showed the well-defined eye of Larry, though the 89 GHz channel hinted that the eyewall was weaker on its eastern side. Taking a look at the high-density atmospheric motion vectors (AMVs), available thanks to a GOES-16 1-minute updating domain over Larry, there is some restriction of Larry's outflow to the southwest. Flight-level wind data from the NASA-DC8 aircraft conducting a research mission around Larry also showed some light southwesterly flow between 10-20 kt just a few degrees to the south and west of the storm center. These data suggest that the upper-level wind environment is not as pristine as earlier suggested by SHIPS guidance, with the southwesterly flow likely restricting Larry's outflow in that quadrant. The SAB/TAFB subjective Dvorak intensity estimates are unchanged from this morning, and the latest UW-CIMSS ADT and SATCON estimates have also plateaued in the 105-110 kt range. Therefore, Larry was maintained as a 110 kt hurricane this advisory. Larry remains on a west-northwest track this afternoon, but a bit slower at 300/12 kt. There has been little change to the track philosophy over the next several days, as the guidance is in good agreement that Larry will maintain a continued west-northwest heading while gradually slowing down as it rounds the southern periphery of a large mid-level ridge. However, there has been a notable eastward shift in the track guidance in the short-term. The latest ECMWF run, which had previously been on the southwest side of the track guidance envelope, is now very similar or even a tad east of the latest GFS run. This shift has also resulted in an eastward adjustment in the consensus aids this afternoon. The latest NHC track forecast was adjusted a bit to the right early on, but not as far right as the HCCA and TVCA aids. After 72 hours, the track guidance actually converges very close to the previous track forecast, and few changes were needed after this time period. Based on this forecast, Larry will continue moving across the central Atlantic in the coming days, and be approaching Bermuda from the southeast in the day 4 to 5 forecast period. The existence of some light upper-level southwesterly flow ahead of Larry today was bit of a surprise, since the SHIPS guidance from the last few days suggested the shear-vector would be out of the east and weak. Indeed, the latest ECMWF-SHIPS guidance now shows moderate southwesterly vertical wind shear beginning earlier, and peaking between 20-25 knots in 36 to 48 hours. While the GFS-SHIPS shear remains much lower, given what I'm seeing from the latest upper-level flow in front of Larry, the ECMWF seems closer to correct. For this reason, the latest NHC intensity guidance now shows a bit of weakening after 24 hours, when the shear magnitude is expected to peak as the hurricane interacts with a large tropical upper-tropospheric trough (TUTT) located to its northwest. However, Larry has a large and vertically-deep circulation, and ultimately it will win the battle against the more vertically shallow TUTT, which is forecast to cut off and move away from the hurricane's expansive upper-level outflow. By 60 hours, this change in the synoptic pattern should once again reduce the vertical wind shear over Larry, and it will have a chance to achieve a secondary peak between the 60-84 hour time-frame. However, eyewall replacement cycles could also occur at any time over the next 2-5 days, providing additional intensity fluctuations that make this a challenging intensity forecast. The latest NHC intensity forecast is lower than the previous one, but still remains higher than the latest HCCA consensus aid. Regardless of the details, Larry is expected to remain a large major hurricane over the next 3-4 days. Key Messages: 1. Large swells generated by Larry are expected to first reach the Lesser Antilles on Sunday and will spread to portions of the Greater Antilles, the Bahamas, and Bermuda Monday and Tuesday. Significant swells will likely reach the east coast of the United States and Atlantic Canada around midweek. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming week. 2. Larry is forecast to approach Bermuda during the next several days, possibly as a major hurricane, bringing a risk of strong winds, heavy rainfall, and coastal flooding to the island by the middle of next week. While it is too soon to determine the magnitude of these hazards and potential impacts on Bermuda, interests there should monitor changes to the forecast during the next several days. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 17.4N 47.1W 110 KT 125 MPH 12H 05/0600Z 18.4N 48.6W 115 KT 130 MPH 24H 05/1800Z 19.8N 50.6W 120 KT 140 MPH 36H 06/0600Z 21.0N 52.3W 115 KT 130 MPH 48H 06/1800Z 22.2N 54.0W 110 KT 125 MPH 60H 07/0600Z 23.4N 55.6W 110 KT 125 MPH 72H 07/1800Z 24.7N 57.0W 115 KT 130 MPH 96H 08/1800Z 28.3N 59.9W 110 KT 125 MPH 120H 09/1800Z 33.4N 61.9W 95 KT 110 MPH $$ Forecaster Papin
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