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Hurricane Larry Forecast Discussion Number 13
2021-09-03 22:54:48| National Hurricane Center (Atlantic)
Issued at 500 PM AST Fri Sep 03 2021 000 WTNT42 KNHC 032054 TCDAT2 Hurricane Larry Discussion Number 13 NWS National Hurricane Center Miami FL AL122021 500 PM AST Fri Sep 03 2021 Over the course of the day, Larry's appearance has become more impressive. The eye has been gradually becoming better defined on visible satellite imagery as the center cloud top temperature has been warming on the infrared channel. Several microwave passes have been received since the last advisory. The most recent pass at 1900 UTC suggested the eye and surrounding eyewall convection is becoming better defined, though still weaker on the east side. 1800 UTC subjective Dvorak classifications were still both CI 4.5/77 kt from TAFB and SAB. However, the objective satellite estimates have been rising quickly, with the most recent UW-CIMSS SATCON estimate up to 84 kt. Thus, the NHC advisory intensity has been raised to 85 kt for this advisory, making Larry a category 2 hurricane. Larry's motion has remained fairly steady to the west-northwest throughout the day, at 285/14 kt. The latest track reasoning remains unchanged, with Larry being steered to the west-northwest around the southern periphery of an expansive mid-level ridge centered north of the hurricane. This ridge axis will gradually become oriented to the northeast of Larry with time, allowing the tropical cyclone to gradually gain more latitude. Because the ridge is quite large, even as its axis shifts eastward, a portion of the ridge will still remain to the north of the storm. The end result is that Larry should only slowly gain a more poleward component of motion as the system also gradually slows down. While the track guidance spread remains small for the first 48-60 hours of the forecast, a bit more longitudinal spread becomes apparent thereafter, which appears related to how much mid-level ridging remains poleward of the cyclone. The latest 12z ECMWF run has a distinctly stronger ridge, oriented more poleward in comparison to the 12z GFS run. Thus, it is not surprising to see that the ECMWF track is a bit further west and slower than the GFS. The latest NHC track forecast this cycle was also adjusted a bit slower, following the TVCN and HCCA consensus aids. However, it is interesting to note the latest 12z ECMWF ensembles are even slower than this track forecast, and future slower adjustments may be needed if these track solutions verify. It remains too soon to determine what impacts Larry may pose to the Island of Bermuda, but interests there should monitor updates in the forecast in the subsequent days. With the recent improvement in Larry's structure on satellite imagery, the hurricane may be starting another rapid intensification (RI) cycle, as suggested by the high RI probabilities given in the DTOPS guidance. However, the earlier microwave data also suggested that there were a lot of concentric bands outside of the primary eyewall, and it is not out of the question that another eyewall replacement cycle could begin in the next 24-36 hours. Instead of attempting to forecast these often stochastic processes, the latest NHC intensity forecast will instead show a healthy rate of deepening over the next several days, now taking Larry to 120 kt in 48 hours. Afterwards, there still remain questions as to how much the vertical wind shear will increase over Larry, with the ECMWF-SHIPS indicating higher shear that could affect Larry's inner-core structure, while the GFS-SHIPS showing much lower shear. Warmer sea-surface temperatures also await the hurricane in the 60-96 hour period. Due to these competing factors, the intensity forecast during this period will only show very gradual weakening. The latest NHC intensity forecast remains on the high end of the guidance, but still remains lower than the higher HAFS-B and COAMPS-TC models. It should be noted that the majority of the guidance also shows Larry becoming a very large hurricane, with a significant expansion of the 34-, 50-, and 64-kt wind radii also forecast. Significant ocean swells generated by the increasingly large wind field of Larry are expected to reach the Lesser Antilles on Sunday, increasing the risk of life-threatening rip currents and high surf conditions on those islands early next week. Large swells are likely to spread to areas surrounding the western Atlantic later in the week as well. FORECAST POSITIONS AND MAX WINDS INIT 03/2100Z 15.0N 42.0W 85 KT 100 MPH 12H 04/0600Z 15.8N 44.1W 95 KT 110 MPH 24H 04/1800Z 16.9N 46.5W 105 KT 120 MPH 36H 05/0600Z 18.1N 48.7W 110 KT 125 MPH 48H 05/1800Z 19.4N 50.8W 120 KT 140 MPH 60H 06/0600Z 20.7N 52.8W 120 KT 140 MPH 72H 06/1800Z 21.9N 54.5W 115 KT 130 MPH 96H 07/1800Z 24.7N 57.6W 115 KT 130 MPH 120H 08/1800Z 28.7N 60.5W 105 KT 120 MPH $$ Forecaster Papin
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