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Tropical Storm Larry Forecast Discussion Number 6
2021-09-02 04:40:42| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Sep 01 2021 000 WTNT42 KNHC 020240 TCDAT2 Tropical Storm Larry Discussion Number 6 NWS National Hurricane Center Miami FL AL122021 1100 PM AST Wed Sep 01 2021 Larry's strengthening phase appears to have taken a pause this evening. While an earlier 2055 UTC GMI microwave overpass indicated that the low-level structure was fairly impressive with a tight low-level eye-like feature, the deep convection waned briefly, but has since returned in a band around the eastern portion of the circulation. The various satellite-based intensity estimates have an unusually large spread from about 45 to 75 kt this evening. At the higher end of the estimates are UW/CIMSS ADT T-numbers of T4.5 and at the lower end were scatterometer wind data showing peak wind retrievals of about 45 kt. But given the small-inner core seen in the microwave data, there is likely some undersampling occuring with the ASCAT instrument. Subjective Dvorak data T-numbers from both TAFB and SAB were T3.5 (55 kt), and the latest SATCON estimate is 57 kt. The NHC initial intensity estimate leans toward the subjective Dvorak numbers and SATCON, and therefore remains at 60 kt, but there is larger-than-normal uncertainty regarding the initial intensity at this time. Larry is moving westward or 275/18 kt. The track forecast philosophy has not changed this evening. Larry is expected to move around the south and southwestern portion of a strong mid-level ridge that is anchored over the east-central Atlantic. This should steer the tropical cyclone steadily westward to west-northwestward during the next few days, with a turn toward the northwest by days 4 and 5 as Larry nears the southwestern portion of the ridge. The latest model envelope is largely unchanged through 120 hours, and the updated official forecast is very similar to the previous advisory. Although the track guidance is fairly tightly clustered there are some differences in how fast Larry will move across the tropical Atlantic, with the UKMET depicting the fastest forward speed. The NHC track forecast is slightly slower than the consensus aids to be in better agreement with the bulk of the guidance and the GFS ensemble mean. Conditions are expected to remain favorable for steady to rapid strengthening. Despite the recent pause in intensification, Larry is likely to resume strengthening later tonight, and the short-term portion of the intensity forecast is unchanged from before. Larry is forecast to become a hurricane by Thursday morning, and a major hurricane in 36-48 hours. That portion of the intensity forecast is in good agreement with the SHIPS guidance and is supported by the global model guidance which significantly deepen Larry over the next couple of days. After that time, there are some mixed signals on the amount of shear over the system with the GFS-based SHIPS guidance predicting moderate shear and some dry air, while the ECMWF-based SHIPS model diagnoses more favorable conditions. For now, the NHC intensity forecast shows a peak slightly higher than before by 72 hours, and then indicates little overall change in strength thereafter as some fluctuations in intensity could occur due to eyewall replacement cycles. The latter portion of the forecast is in best agreement with the HFIP corrected consensus aid. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 12.8N 30.9W 60 KT 70 MPH 12H 02/1200Z 13.0N 33.3W 70 KT 80 MPH 24H 03/0000Z 13.6N 36.5W 85 KT 100 MPH 36H 03/1200Z 14.2N 39.5W 95 KT 110 MPH 48H 04/0000Z 15.1N 42.2W 100 KT 115 MPH 60H 04/1200Z 16.2N 44.8W 105 KT 120 MPH 72H 05/0000Z 17.4N 47.3W 110 KT 125 MPH 96H 06/0000Z 20.1N 51.4W 110 KT 125 MPH 120H 07/0000Z 22.9N 55.0W 110 KT 125 MPH $$ Forecaster Brown
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