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Tropical Storm Larry Forecast Discussion Number 3
2021-09-01 10:59:00| National Hurricane Center (Atlantic)
Issued at 800 AM CVT Wed Sep 01 2021 000 WTNT42 KNHC 010858 TCDAT2 Tropical Storm Larry Discussion Number 3 NWS National Hurricane Center Miami FL AL122021 800 AM CVT Wed Sep 01 2021 Deep convection with cloud tops colder than -80C have increased over and to the west of the low-level center since the previous advisory. Subjective satellite intensity estimates at 0600 UTC from TAFB and SAB were T2.5/35 kt and T3.0/45 kt, while the most recent objective intensity estimates from UW-CIMSS are T2.8/41 kt from ADT and 37 kt from SATCON. An average of these intensity estimates support increasing the advisory intensity to 40 kt, making the cyclone Tropical Storm Larry. In addition, 0300 UTC and 0700 UTC observations from ship VRNF3, which recently passed through the center of Larry, reported a pressure of 1006.8 mb and winds near 25 kt. These data were the basis for the estimated central pressure of 1003 mb, a pressure value that also supports an intensity of 40 kt. Larry has turned more westward over the past several hours, and the new motion estimate is 280/17 kt. Larry is expected to move around the southern and southwestern periphery of the sprawling Bermuda-Azores ridge for the next 5 days, resulting in a general west motion for the next 36 hours or so, followed by a turn toward the west-northwest on Friday, and a northwestward motion over the weekend and continuing into early next week. There has been a pronounced westward shift in the track guidance for this cycle, with the greatest shift coming from the GFS model. Over the past 36 hours, the GFS has shifted its track westward by more than 500 nmi, and even the latest shift still keeps the GFS model the easternmost track forecast in the guidance suite. In contrast, the ECMWF and UKMET models, which lie along the westernmost portion of the guidance envelope, have been fairly stable. Owing to the westward shift in the overall guidance envelope, and considering the GFS solution as an outlier model, the new NHC forecast track has also been shifted westward, and lies between the NOAA-HCCA corrected-consensus model to the south, and the tightly packed TVCA simple-consensus model and FSSE corrected-consensus model to the north. Given the poor handling of the ridge to the north of Larry by the GFS, subsequent NHC forecast tracks may have to be shifted farther west. Given the improved inner-core wind field based on earlier ASCAT wind data and reports from ship VRNF3, along with warm sea-surface temperatures of 28 deg C and light easterly to southeasterly vertical shear of around 5 kt, steady strengthening is expected for the next 24 hours or so. By 36 hours when Larry is expected to be a hurricane and have a well-established and tighter inner-core wind field and possibly an eye, rapid intensification is forecast, with Larry becoming a major hurricane by 72 hour. This in large part due to the massive equatorward upper-level outflow pattern that all of the global and regional models are forecasting, which is the same type of outflow pattern that recently occurred with Hurricane Ida. The new official intensity forecast is above the previous advisory forecast by about 10 kt at all forecast times, and conservatively follows an average of the Decay-SHIPS, COAMPS-TC, FSSE, and ECMWF models. This intensity forecast is near the upper end of the guidance envelope and is above the other consensus intensity models. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 12.3N 24.8W 40 KT 45 MPH 12H 01/1800Z 12.5N 27.5W 50 KT 60 MPH 24H 02/0600Z 12.6N 30.6W 60 KT 70 MPH 36H 02/1800Z 13.1N 33.7W 70 KT 80 MPH 48H 03/0600Z 13.6N 36.8W 85 KT 100 MPH 60H 03/1800Z 14.4N 39.7W 95 KT 110 MPH 72H 04/0600Z 15.5N 42.0W 100 KT 115 MPH 96H 05/0600Z 18.2N 46.9W 105 KT 120 MPH 120H 06/0600Z 21.3N 50.6W 105 KT 120 MPH $$ Forecaster Stewart
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