FLORIDA
STORMS
Hurricane Sam
LOCATED
1365 MI ESE OF THE NORTHERN LEEWARD ISLANDS
WINDS
75 MPH
PRESSURE
993 MB
MOVING
W AT 14 MPH
From the National Hurricane Center at 1100 AM AST Fri Sep 24 2021
Sam taking a momentary pause in strengthening.
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Alerts
hazards
summary
DISCUSSION

There are no coastal watches or warnings in effect.

There are no coastal watches or warnings in effect.

None.

None.

At 1100 AM AST (1500 UTC), the center of Hurricane Sam was located near latitude 11.8 North, longitude 43.7 West. Sam is moving just north of due west near 14 mph (22 km/h) and this motion is expected to continue through tonight. A decrease in forward speed and a motion toward the west northwest are expected over the weekend. Maximum sustained winds remain near 75 mph (120 km/h) with higher gusts. Rapid intensification is forecast to resume over the next several days and Sam is likely to become a major hurricane on Saturday. Hurricane force winds extend outward up to 15 miles (30 km) from the center and tropical storm force winds extend outward up to 60 miles (95 km). The estimated minimum central pressure is 993 mb (29.33 inches).

At 1100 AM AST (1500 UTC), the center of Hurricane Sam was located near latitude 11.8 North, longitude 43.7 West. Sam is moving just north of due west near 14 mph (22 km/h) and this motion is expected to continue through tonight. A decrease in forward speed and a motion toward the west northwest are expected over the weekend. Maximum sustained winds remain near 75 mph (120 km/h) with higher gusts. Rapid intensification is forecast to resume over the next several days and Sam is likely to become a major hurricane on Saturday. Hurricane force winds extend outward up to 15 miles (30 km) from the center and tropical storm force winds extend outward up to 60 miles (95 km). The estimated minimum central pressure is 993 mb (29.33 inches).

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.

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.

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