000 WTNT43 KNHC 040842 TCDAT3 Hurricane Sam Discussion Number 47 NWS National Hurricane Center Miami FL AL182021 500 AM AST Mon Oct 04 2021 A 0255Z GPM microwave pass indicated that Sam had continued to go through an eyewall replacement cycle, consisting of a 20-nmi-wide inner eye and an 85-nmi-wide outer eye. The inner eye was rapidly eroding on the northwest side compared to microwave data from just a few hours earlier. Additionally, late-arriving ASCAT surface wind data from 0000-0100Z indicated that the inner-core wind field had contracted or weakened, while the outer wind field (34-kt radii) had continued to expand in all quadrants. Satellite intensity estimates have decreased to around 80 kt. However, the advisory intensity has been maintained at 90 kt based on the 10-kt increase in forward speed, which should offset any weakening of the tangential winds. Gradual weakening is expected now that Sam has passed north of the warm Gulf Stream and is moving over sub-25-deg-C sea-surface temperatures (SST), with much colder SSTs lying ahead of the hurricane. Vertical wind shear is forecast to increase to more than 30 kt by 18 h, which should cause further weakening. By 24 h and beyond, interaction with a strong upper-level trough/low should cause Sam to transition into a powerful extratropical cyclone by early Tuesday to the east of Newfoundland. This baroclinic interaction is expected to cause extratropical-Sam to maintain hurricane-force winds until early Wednesday. Slow weakening is forecast thereafter as the very large cyclone slowly spins down as an occluded low pressure system. No significant changes were made to the previous intensity forecast. Sam has continued to accelerate and is now moving at 045/26 kt. A northeastward motion along with continued acceleration is forecast through today as Sam moves into stronger southwesterly flow ahead of an approaching mid-latitude trough. The large cyclone is expected to remain the primary surface low center as it merges with the trough in a couple days. A gradual turn toward the north and northwest is expected on days 4 and 5 when extratropical-Sam moves around another high-latitude trough. The new NHC forecast track is essentially just an update of the previous advisory track, and lies close to the tightly packed consensus track models. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, the eastern United States and Atlantic Canada for the next couple of days. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 04/0900Z 40.8N 48.7W 90 KT 105 MPH 12H 04/1800Z 43.7N 45.0W 80 KT 90 MPH 24H 05/0600Z 47.8N 41.5W 75 KT 85 MPH...POST-TROP/EXTRATROP 36H 05/1800Z 50.1N 39.3W 70 KT 80 MPH...POST-TROP/EXTRATROP 48H 06/0600Z 51.0N 35.8W 60 KT 70 MPH...POST-TROP/EXTRATROP 60H 06/1800Z 52.8N 31.2W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 07/0600Z 56.0N 28.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 08/0600Z 60.6N 29.3W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 09/0600Z 61.5N 30.0W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart