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Hurricane Larry Forecast Discussion Number 9
2021-09-02 22:49:53| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 02 2021 000 WTNT42 KNHC 022049 TCDAT2 Hurricane Larry Discussion Number 9 NWS National Hurricane Center Miami FL AL122021 500 PM AST Thu Sep 02 2021 This afternoon, Larry's appearance has evolved from a small central dense overcast to a tightly coiled spiral banding pattern, with these bands wrapping into the estimated center. I finally received a useful SSMIS microwave overpass at 1857 UTC which provided evidence that Larry could be undergoing an eyewall replacement cycle (ERC). The imagery suggests a large outer eyewall is taking shape on the low-level 37-GHz channel, while an inner eyewall fragment is still seen on the mid-level 91-GHz channel. Despite these structural changes, the 1800 UTC subjective Dvorak estimates from TAFB and SAB were unchanged from this morning at CI 4.0/65 kt. In addition, the latest objective intensity estimates from SATCON and ADT have only varied slightly at 73 kt and 74 kt respectively. Taking a blend of the subjective and objective estimates still yields 70 kt for this advisory. Larry continues to move just north of due west, with the latest motion estimated at 280/16 kt. There is not much new to report with the track reasoning, with a large mid-level ridge currently north of Larry expected to steer the hurricane to the west or west-northwest over the next 2-3 days. Over time, the ridge axis is forecast to slowly shift further northeast of Larry's position, allowing the hurricane to gradually gain a more rightward component of motion by the end of the forecast period. In general, the track guidance remains tightly clustered for the first 2-3 days. There was a notable shift west in the latest 12z GFS deterministic track, while the 12z ECMWF shifted ever so slightly right. A quick look at the 12z ECMWF ensemble guidance reveals that the deterministic run is a bit to the northeast of the ensemble mean. The latest NHC track has been shifted slightly to the south and west of the previous track, following the reliable HFIP corrected consensus aid (HCCA) which also shifted a bit south and west this cycle. Once Larry completes the ongoing ERC, significant or even rapid intensification is anticipated. The short-term intensity forecast has been raised a bit more, since it appears more likely the present ERC will finish in the next 6-12 h. The latest NHC intensity forecast now makes Larry a major hurricane by tomorrow night, with further intensification up to Category 4 intensity anticipated by Sunday. Once again, there remains some discrepancy in the vertical wind shear forecast between the GFS- and ECMWF-based SHIPS guidance, with the ECMWF forecasting significantly more southwesterly shear beyond 72 hours. Both guidance tools also indicate a decrease in mid-level relative humidity which, if the shear materializes, could disrupt Larry's inner core structure. Thus, the NHC intensity forecast still shows gradual weakening from 72 to 120 hours. The latest intensity forecast is just a hair above HCCA in the short term, with the intensity forecast most closely following the latest COAMPS-TC (CTCI) guidance in the first three days. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 13.7N 35.8W 70 KT 80 MPH 12H 03/0600Z 14.0N 38.4W 80 KT 90 MPH 24H 03/1800Z 14.7N 41.3W 95 KT 110 MPH 36H 04/0600Z 15.7N 43.9W 105 KT 120 MPH 48H 04/1800Z 16.9N 46.3W 110 KT 125 MPH 60H 05/0600Z 18.3N 48.6W 120 KT 140 MPH 72H 05/1800Z 19.7N 50.6W 120 KT 140 MPH 96H 06/1800Z 22.2N 54.0W 115 KT 130 MPH 120H 07/1800Z 25.1N 57.0W 110 KT 125 MPH $$ Forecaster Papin
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