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Hurricane Sam Forecast Discussion Number 8
2021-09-24 16:55:11| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Fri Sep 24 2021 000 WTNT43 KNHC 241455 TCDAT3 Hurricane Sam Discussion Number 8 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Fri Sep 24 2021 After rapidly intensifying from a tropical depression to a hurricane over the past 24 hours, Sam is taking a momentary pause from intensification. The structure on visible satellite imagery remains quite healthy, with tightly coiled outer bands and a small but very cold central dense overcast. However, microwave passes at 0957 UTC and 1211 UTC on the 89-91 GHz channels show an erosion of the inner-core structure on the west side, which could be related to the dry-air slots mentioned in the previous advisory entraining into the core, temporarily disrupting the structure underneath the cirrus. The latest subjective Dvorak intensity estimates from TAFB and SAB were T4.0/4.0-65 kt and T4.0/4.5-75 kt while the latest UW-CIMSS ADT and SATCON objective estimates were both at 59 kt. The current intensity is being maintained at 65 kt for this advisory, taking a blend of these subjective and objective estimates. Sam remains a very small tropical cyclone, with the latest scatterometer wind data indicating that tropical-storm and hurricane-force winds have have not expanded much from last night. Sam continues to move just north of due west, but is beginning to gradually slow down at 280/12 kt. Over the next 24-36 hours, Sam is expected to maintain a general west to west-northwest heading but will slow down gradually as the strongest mid-level ridging becomes oriented more to the northwest of the hurricane. This ridge repositioning is thanks in part to a strong deep-layer trough well northeast of Sam that is digging in south of the Azores. Towards the end of the forecast, this trough will cutoff and gradually decay, allow the mid-level ridging to build-in back east as another deep-layer trough becomes established just off the eastern United States coastline. In general, the track guidance is in fairly good agreement on this general solution. However, differences remain in the details between how strong the ridge will be immediately to the north of Sam at the end of the 5-day forecast. The deterministic GFS and Canadian models show weaker ridging that erodes sooner, allowing a bit more poleward motion, while the ECMWF has stronger ridging that places its track on the equatorward side of the guidance envelope. The consensus track aid TCVN blends the aforementioned model guidance and has changed little this cycle, and the latest NHC track forecast is largely an update of the previous advisory. While the shear remains very low over Sam as diagnosed by the GFS- and ECMWF-based SHIPS guidance, the 700-500 mb layer mean relative humidity is a bit on the low side, and could possibly explain why Sam was susceptible to the dry-air intrusion that disrupted the core structure this morning. However, the vertical wind shear is expected to remain very low for the next 48-60 hours, and the hurricane should be able to easily mix out the dry air over the inner-core. The wind field also remains very compact, so once the inner-core closes back off, rapid intensification is expected to resume. The latest NHC intensity forecast still shows Sam becoming a major hurricane by tomorrow and currently has a peak intensity as a category 4 hurricane in 48-72 hours. Thereafter, a bit more vertical wind shear, and the likelihood of eyewall replacement cycles are likely to lead to fluctuations in the intensity that are difficult to predict ahead of time. The current intensity forecast remains close to the HCCA and IVCN consensus guidance. Regardless of the details, Sam is expected to be a significant hurricane through the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 11.8N 43.7W 65 KT 75 MPH 12H 25/0000Z 12.0N 45.4W 75 KT 85 MPH 24H 25/1200Z 12.3N 47.1W 90 KT 105 MPH 36H 26/0000Z 12.7N 48.4W 105 KT 120 MPH 48H 26/1200Z 13.1N 49.7W 115 KT 130 MPH 60H 27/0000Z 13.7N 50.9W 115 KT 130 MPH 72H 27/1200Z 14.6N 52.1W 115 KT 130 MPH 96H 28/1200Z 16.4N 54.8W 110 KT 125 MPH 120H 29/1200Z 18.2N 57.7W 110 KT 125 MPH $$ Forecaster Papin
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