FLORIDA
STORMS
Tropical Storm Sam
LOCATED
1560 MI ESE OF THE NORTHERN LEEWARD ISLANDS
WINDS
70 MPH
PRESSURE
997 MB
MOVING
W AT 15 MPH
From the National Hurricane Center at 1100 PM AST Thu Sep 23 2021
Small Sam rapidly intensifying.
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Alerts
hazards
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DISCUSSION

There are no coastal watches or warnings in effect.

There are no coastal watches or warnings in effect.

None.

None.

At 1100 PM AST (0300 UTC), the center of Tropical Storm Sam was located near latitude 11.2 North, longitude 40.9 West. Sam is moving toward the west near 15 mph (24 km/h), and this motion with a decrease in forward speed is expected through Saturday. A motion toward the west northwest at an even slower forward speed is expected late Saturday into Sunday. Sam is rapidly intensifying, and maximum sustained winds have increased to near 70 mph (110 km/h) with higher gusts. Rapid intensification is forecast to continue through early Saturday. Sam is likely to become a hurricane very soon and then could be a major hurricane by Friday night or early Saturday. Sam is a small tropical storm. Tropical storm force winds extend outward up to 45 miles (75 km) from the center. The estimated minimum central pressure is 997 mb (29.44 inches).

At 1100 PM AST (0300 UTC), the center of Tropical Storm Sam was located near latitude 11.2 North, longitude 40.9 West. Sam is moving toward the west near 15 mph (24 km/h), and this motion with a decrease in forward speed is expected through Saturday. A motion toward the west northwest at an even slower forward speed is expected late Saturday into Sunday. Sam is rapidly intensifying, and maximum sustained winds have increased to near 70 mph (110 km/h) with higher gusts. Rapid intensification is forecast to continue through early Saturday. Sam is likely to become a hurricane very soon and then could be a major hurricane by Friday night or early Saturday. Sam is a small tropical storm. Tropical storm force winds extend outward up to 45 miles (75 km) from the center. The estimated minimum central pressure is 997 mb (29.44 inches).

Microwave data from around the time of the previous advisory revealed that Sam has developed a well-defined inner core, including a nearly closed eyewall feature in the 91-GHz channel. Dvorak estimates from TAFB and SAB have increased to T3.5 and T4.0, respectively, and the initial intensity is therefore estimated to be 60 kt. Sam's 24-hour intensity change qualifies as rapid intensification, since the cyclone was only a 30-kt tropical depression at this time yesterday. Sam is small, however. Recent scatterometer data showed that tropical-storm-force winds only extend a maximum of 40 n mi from the center, and the radius of maximum winds is estimated to be about 15 n mi. This small size is likely to have implications on Sam's future intensity.

Sam is moving a little slower toward the west (280/13 kt). A mid-tropospheric ridge is expected to build across the Caribbean Sea and extend northeastward over the central Atlantic during the next few days. This blocking high is expected to cause Sam to slow down to 6 to 7 kt by late Saturday and Sunday while it maintains a westward to west-northwestward heading. Global models suggest the ridge may weaken and shift eastward a bit by days 4 and 5, allowing Sam to turn to the northwest and speed up a little by the end of the forecast period. The ECMWF, HWRF, and HCCA models lie along the southern edge of the main pack of models, while the ECENS ensemble mean is even farther south. The GFS and the HMON are on the northern side. Partially due to an adjustment of the initial position, the new NHC track forecast has been shifted southward a bit from the previous forecast, and it lies just south of the TVCA consensus aid--but not as far to the left as the previously mentioned southern models.

Environmental conditions and Sam's structure and size are ideal for continued rapid intensification (RI). The SHIPS RI index for a 30-kt increase in 24 hours is nearly 50 percent. The DTOPS methodology, which uses inputs from the statistical-dynamical and deterministic models, shows RI indices for various forecast periods exceeding 90 percent. Therefore, rapid strengthening is being forecast at least for the next 36 hours, with Sam becoming a hurricane very soon and then a major hurricane by Friday night or Saturday morning. One caveat to this forecast is that although Sam's small size more readily allows for RI to occur, it can also make the cyclone more prone to weakening if, for example, vertical shear increases. By 48 hours, the intensity models show the strengthening trend leveling off, and the NHC intensity forecast follows suit. Fluctuations in intensity from days 3 through 5 are likely, both due to normal internal dynamics within the storm and the cyclone's response to the surrounding environment in relation to its small size.

Sam continues to rapidly intensify. Satellite images show a small, but well-developed inner core and pronounced curved bands that wrap most of the way around the center. There are some dry slots, however, between the core and bands. The latest satellite intensity estimates range from 55 to 77 kt, and based on that data and the continued improvement in the cyclone's structure, the initial wind speed is increased to 65 kt. This makes Sam a hurricane, the seventh one of the 2021 Atlantic hurricane season. It should be noted that Sam has a compact wind field, with hurricane-force and tropical-storm-force winds estimated to only extend outward up to 15 and 50 n mi from the center, respectively.

Sam is moving westward at about 13 kt, and this general motion should continue for another 12 to 24 hours as it moves in the flow on the south side of a mid-level ridge. After that time, a decrease in forward speed and a turn to the west-northwest are expected, and the official forecast has Sam moving at a slow pace of only 6-8 kt during the 48-96-hour time period. By the middle of next week, the ridge is forecast to slide eastward as a trough moves over the western Atlantic. In response, the hurricane will likely turn northwestward as it approaches the northern Leeward Islands. The models have changed little this cycle with the GFS still on the northern side of the guidance envelope and the ECMWF on the southern side. The NHC track forecast is essentially an update of the previous one and lies between the GFS and ECMWF models, and near the consensus aids.

The large scale environmental conditions all appear favorable for continued rapid intensification during the next day or so as the hurricane is expected to remain over warm 29 deg C waters and in very low wind shear conditions. All of the SHIPS rapid intensification indices are well above the climatological means, and the NHC intensity forecast calls for Sam to become a major hurricane by early Saturday. Beyond a couple of days, the environment is likely to become a little less ideal, and most of the models show Sam leveling off in strength, and so does the official forecast. This intensity prediction lies near a blend of the FSSE, HCCA, and IVCN consensus models. Regardless of the details, Sam is expected to be a significant hurricane during the next several days.

Microwave data from around the time of the previous advisory revealed that Sam has developed a well-defined inner core, including a nearly closed eyewall feature in the 91-GHz channel. Dvorak estimates from TAFB and SAB have increased to T3.5 and T4.0, respectively, and the initial intensity is therefore estimated to be 60 kt. Sam's 24-hour intensity change qualifies as rapid intensification, since the cyclone was only a 30-kt tropical depression at this time yesterday. Sam is small, however. Recent scatterometer data showed that tropical-storm-force winds only extend a maximum of 40 n mi from the center, and the radius of maximum winds is estimated to be about 15 n mi. This small size is likely to have implications on Sam's future intensity.

Sam is moving a little slower toward the west (280/13 kt). A mid-tropospheric ridge is expected to build across the Caribbean Sea and extend northeastward over the central Atlantic during the next few days. This blocking high is expected to cause Sam to slow down to 6 to 7 kt by late Saturday and Sunday while it maintains a westward to west-northwestward heading. Global models suggest the ridge may weaken and shift eastward a bit by days 4 and 5, allowing Sam to turn to the northwest and speed up a little by the end of the forecast period. The ECMWF, HWRF, and HCCA models lie along the southern edge of the main pack of models, while the ECENS ensemble mean is even farther south. The GFS and the HMON are on the northern side. Partially due to an adjustment of the initial position, the new NHC track forecast has been shifted southward a bit from the previous forecast, and it lies just south of the TVCA consensus aid--but not as far to the left as the previously mentioned southern models.

Environmental conditions and Sam's structure and size are ideal for continued rapid intensification (RI). The SHIPS RI index for a 30-kt increase in 24 hours is nearly 50 percent. The DTOPS methodology, which uses inputs from the statistical-dynamical and deterministic models, shows RI indices for various forecast periods exceeding 90 percent. Therefore, rapid strengthening is being forecast at least for the next 36 hours, with Sam becoming a hurricane very soon and then a major hurricane by Friday night or Saturday morning. One caveat to this forecast is that although Sam's small size more readily allows for RI to occur, it can also make the cyclone more prone to weakening if, for example, vertical shear increases. By 48 hours, the intensity models show the strengthening trend leveling off, and the NHC intensity forecast follows suit. Fluctuations in intensity from days 3 through 5 are likely, both due to normal internal dynamics within the storm and the cyclone's response to the surrounding environment in relation to its small size.

Sam continues to rapidly intensify. Satellite images show a small, but well-developed inner core and pronounced curved bands that wrap most of the way around the center. There are some dry slots, however, between the core and bands. The latest satellite intensity estimates range from 55 to 77 kt, and based on that data and the continued improvement in the cyclone's structure, the initial wind speed is increased to 65 kt. This makes Sam a hurricane, the seventh one of the 2021 Atlantic hurricane season. It should be noted that Sam has a compact wind field, with hurricane-force and tropical-storm-force winds estimated to only extend outward up to 15 and 50 n mi from the center, respectively.

Sam is moving westward at about 13 kt, and this general motion should continue for another 12 to 24 hours as it moves in the flow on the south side of a mid-level ridge. After that time, a decrease in forward speed and a turn to the west-northwest are expected, and the official forecast has Sam moving at a slow pace of only 6-8 kt during the 48-96-hour time period. By the middle of next week, the ridge is forecast to slide eastward as a trough moves over the western Atlantic. In response, the hurricane will likely turn northwestward as it approaches the northern Leeward Islands. The models have changed little this cycle with the GFS still on the northern side of the guidance envelope and the ECMWF on the southern side. The NHC track forecast is essentially an update of the previous one and lies between the GFS and ECMWF models, and near the consensus aids.

The large scale environmental conditions all appear favorable for continued rapid intensification during the next day or so as the hurricane is expected to remain over warm 29 deg C waters and in very low wind shear conditions. All of the SHIPS rapid intensification indices are well above the climatological means, and the NHC intensity forecast calls for Sam to become a major hurricane by early Saturday. Beyond a couple of days, the environment is likely to become a little less ideal, and most of the models show Sam leveling off in strength, and so does the official forecast. This intensity prediction lies near a blend of the FSSE, HCCA, and IVCN consensus models. Regardless of the details, Sam is expected to be a significant hurricane during the next several days.

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