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Hurricane Larry Forecast Discussion Number 8
2021-09-02 16:51:18| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Sep 02 2021 000 WTNT42 KNHC 021451 TCDAT2 Hurricane Larry Discussion Number 8 NWS National Hurricane Center Miami FL AL122021 1100 AM AST Thu Sep 02 2021 Larry continues to gradually become better organized this morning, with a small but cold CDO near the estimated center, and well defined curved bands rotating completely around. Unfortunately, there have not been any recent microwave passes over the center in the last 6-9 hours. However, I did receive a helpful ASCAT-B pass at 1128 UTC which indicated that while the inner core remains quite small, the 34-kt wind radii have expanded dramatically in all quadrants. The most recent Dvorak subjective intensity estimates were both CI 4.0/65 kt from SAB and TAFB. However, the objective intensity estimates from SATCON and ADT are higher, at 70 kt and 77 kt respectively. Thus, the current intensity of Larry was nudged upward to 70 kt for this advisory. The hurricane's heading has changed little this morning, estimated at 280/15 kt. There also remains little change to the forecast track reasoning. A dominant mid-level ridge is located north of Larry and should maintain the hurricane on a west to west-northwest heading over the next 72 hours. The track guidance is in excellent agreement over this time period. After 72 hours, a bit more track guidance spread begins to take shape, which appears related to the evolution of the the steering ridge orientation. For example, the most recent GFS run begins a more pronounced rightward bend, as the ridge becomes positioned northeast of the tropical cyclone. In contrast, the ECMWF and a number of its left-leaning ensemble members maintain more ridging directly north of Larry, keeping the hurricane on a more leftward track with only a gradual bend to the west-northwest and northwest. These differences notwithstanding, the consensus aids has changed little from the previous forecast cycle, and the NHC forecast track is nearly identical to the previous track. This track lies very close to the HCCA consensus aid, which favors a track solution a bit closer to the ECMWF versus the GFS. Larry's environmental conditions appear very favorable for additional intensification over the next 60-72 hours. In fact, the primarily forecast challenge relates to how Larry's core structure evolves over the next several days. Right now, the inner-core and hurricane-force wind radii are very small relative to the expanding tropical-storm-force wind field around the storm. Thus, it appears likely that Larry will undergo some form of an eyewall replacement cycle (ERC) in the next 12 to 24 hours, which may slow down the short term intensification rate. However, once this cycle is complete, very light easterly vertical wind shear between 2-8 kt, abundant mid-level moisture, and sufficently warm sea-surface temperatures between 27-28 C should favor steady to rapid intensification. The latest NHC intensity forecast now calls for a peak intensity of 120 kt in roughly 72 hours, which is a faster and slightly higher peak than the prior forecast. Afterwards, the guidance is a bit conflicted. The GFS-based SHIPS continues to suggests low vertical wind shear through the end of the forecast period. However, the ECMWF-based SHIPS shows much higher southwesterly vertical wind shear beginning in 84 hours as Larry also moves into a drier environment. It is also possible additional ERCs may occur in the latter portion of this forecast, resulting in additional intensity fluctuations. For these reasons, the intensity forecast at the end of the period shows some modest weakening. This intensity forecast is a bit higher than the HCCA corrected consensus aid, but remains lower than some of the more aggressive guidance (COAMPS-TC, the experimental HAFS-B). FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 13.5N 34.2W 70 KT 80 MPH 12H 03/0000Z 13.8N 36.7W 75 KT 85 MPH 24H 03/1200Z 14.4N 39.7W 85 KT 100 MPH 36H 04/0000Z 15.3N 42.6W 95 KT 110 MPH 48H 04/1200Z 16.5N 45.2W 105 KT 120 MPH 60H 05/0000Z 17.9N 47.6W 115 KT 130 MPH 72H 05/1200Z 19.2N 49.7W 120 KT 140 MPH 96H 06/1200Z 21.8N 53.1W 115 KT 130 MPH 120H 07/1200Z 24.6N 56.0W 110 KT 125 MPH $$ Forecaster Papin
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