FLORIDA
STORMS
Tropical Storm Sam
LOCATED
1745 MI ESE OF THE NORTHERN LEEWARD ISLANDS
WINDS
50 MPH
PRESSURE
1003 MB
MOVING
W AT 16 MPH
From the National Hurricane Center at 1100 AM AST Thu Sep 23 2021
Depression strengthens into tropical storm Sam, the 18th named storm of the season.
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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 AM AST (1500 UTC), the center of Tropical Storm Sam was located near latitude 10.9 North, longitude 38.1 West. Sam is moving toward the west near 16 mph (26 km/h) and this general motion is expected to continue over the next several days, but with a gradual slowdown in forward motion. Recent sallite wind data indicates maximum sustained winds have increased to near 50 mph (85 km/h) with higher gusts. Additional strengthening is forecast over the next several days, and Sam is now forecast to become a hurricane tomorrow and be near major hurricane intensity by the end of the weekend. Tropical storm force winds extend outward up to 45 miles (75 km) from the center. The estimated minimum central pressure is 1003 mb (29.62 inches).

At 1100 AM AST (1500 UTC), the center of Tropical Storm Sam was located near latitude 10.9 North, longitude 38.1 West. Sam is moving toward the west near 16 mph (26 km/h) and this general motion is expected to continue over the next several days, but with a gradual slowdown in forward motion. Recent sallite wind data indicates maximum sustained winds have increased to near 50 mph (85 km/h) with higher gusts. Additional strengthening is forecast over the next several days, and Sam is now forecast to become a hurricane tomorrow and be near major hurricane intensity by the end of the weekend. Tropical storm force winds extend outward up to 45 miles (75 km) from the center. The estimated minimum central pressure is 1003 mb (29.62 inches).

Overnight and this morning, the satellite structure of the tropical cyclone has been improving, with prominent curved bands, and obvious low-level cloud motions that indicate the circulation is becoming established within the convection. In particular, an SSMIS microwave pass at 0653 UTC indicated a substantial improvement in the convective structure, with a well-defined curved band wrapping three-quarters of the way around the center in both the 91- and 37-GHz channels. ASCAT-B wind retrievals at 1234 UTC also indicated a tight, well-defined circulation had formed, with peak winds of 44-kt on the north side of the vortex. Subjective Dvorak satellite intensity estimates are now T3.5/55-kt from SAB and T2.5/35-kt from TAFB. In addition, the latest objective intensity estimates from UW-CIMSS ADT and SATCON were at 35-kt and 43-kt respectively. Given the recent scatterometer data, the intensity has been set to 45-kt for this advisory. Thus, Tropical Depression 18 has been upgraded to Tropical Storm Sam. It is noteworthy that this is the 2nd earliest formation of the 18th named storm in the Atlantic basin, moving ahead of the 2005 hurricane season, and only trailing last year.

Sam continues to move to the west-northwest with an estimated motion at 280/14 kt. A prominent mid-level ridge remains entrenched to the north of the cyclone, and this feature should continue to steer Sam to the west-northwest over the next 2-3 days. Over this period, the ridge orientation actually shifts more westward ahead of the cyclone as a deep-layer trough digs in well northeast of Sam. This gradual change in the synoptic pattern should also result in a slowdown in Sam's forward motion over the next 72 hours. While the track guidance remains in good agreement over this period, larger spread begins to take shape beyond the day 3 forecast. Similar to yesterday, the ECMWF model and its ensemble mean flanks the southwest side of the guidance envelop, while the GFS is on the northeast side by day 5. These differences appear to be partially related to the mid-level ridge intensity on the western end, which could begin to be eroded by a mid-latitude trough off the eastern United States in 120-h. In addition, differences in both the size and intensity of Sam may also play a role in its ultimate track evolution. For this advisory, the latest NHC track forecast remains close to the consensus aids TCVN and TCVE, which represents a slight shift westward and a bit faster motion compared to the previous forecast. Because Sam is forecast to slow down, it remains too early to determine what impacts, direct or indirect, could be felt by the Lesser Antilles by this cyclone.

The previously mentioned SSMIS microwave imagery also indicated that the low-level center has become better aligned with the mid-level vortex, perhaps a bit earlier than anticipated given the scatterometer data from last night. This improvement in structure, combined with favorable low vertical wind shear under 10 kt and warm 28-29 C sea-surface temperatures, argues for significant intensification. In fact, the GFS-SHIPS rapid intensification index now give Sam a 39 percent chance of a 65-kt increase over the next 72 hours, which is more than 7 times the climatological value. Thus, the NHC intensity forecast has been raised quite a bit from the prior one, especially in the short-term. The latest forecast now makes Sam a hurricane in 36 hours, and a major hurricane in 72 hours. This intensity forecast is just a shade under the latest HCCA consensus aid. Thereafter, a more gradual intensification rate is forecast. By this period it is possible that Sam could undergo hard to predict inner-core changes such as eyewall replacement cycles. In addition, the ECMWF-SHIPS suggests that the shear may also increase a little in days 4-5 which also argues for a bit slower rate of intensification for this period.

Overnight and this morning, the satellite structure of the tropical cyclone has been improving, with prominent curved bands, and obvious low-level cloud motions that indicate the circulation is becoming established within the convection. In particular, an SSMIS microwave pass at 0653 UTC indicated a substantial improvement in the convective structure, with a well-defined curved band wrapping three-quarters of the way around the center in both the 91- and 37-GHz channels. ASCAT-B wind retrievals at 1234 UTC also indicated a tight, well-defined circulation had formed, with peak winds of 44-kt on the north side of the vortex. Subjective Dvorak satellite intensity estimates are now T3.5/55-kt from SAB and T2.5/35-kt from TAFB. In addition, the latest objective intensity estimates from UW-CIMSS ADT and SATCON were at 35-kt and 43-kt respectively. Given the recent scatterometer data, the intensity has been set to 45-kt for this advisory. Thus, Tropical Depression 18 has been upgraded to Tropical Storm Sam. It is noteworthy that this is the 2nd earliest formation of the 18th named storm in the Atlantic basin, moving ahead of the 2005 hurricane season, and only trailing last year.

Sam continues to move to the west-northwest with an estimated motion at 280/14 kt. A prominent mid-level ridge remains entrenched to the north of the cyclone, and this feature should continue to steer Sam to the west-northwest over the next 2-3 days. Over this period, the ridge orientation actually shifts more westward ahead of the cyclone as a deep-layer trough digs in well northeast of Sam. This gradual change in the synoptic pattern should also result in a slowdown in Sam's forward motion over the next 72 hours. While the track guidance remains in good agreement over this period, larger spread begins to take shape beyond the day 3 forecast. Similar to yesterday, the ECMWF model and its ensemble mean flanks the southwest side of the guidance envelop, while the GFS is on the northeast side by day 5. These differences appear to be partially related to the mid-level ridge intensity on the western end, which could begin to be eroded by a mid-latitude trough off the eastern United States in 120-h. In addition, differences in both the size and intensity of Sam may also play a role in its ultimate track evolution. For this advisory, the latest NHC track forecast remains close to the consensus aids TCVN and TCVE, which represents a slight shift westward and a bit faster motion compared to the previous forecast. Because Sam is forecast to slow down, it remains too early to determine what impacts, direct or indirect, could be felt by the Lesser Antilles by this cyclone.

The previously mentioned SSMIS microwave imagery also indicated that the low-level center has become better aligned with the mid-level vortex, perhaps a bit earlier than anticipated given the scatterometer data from last night. This improvement in structure, combined with favorable low vertical wind shear under 10 kt and warm 28-29 C sea-surface temperatures, argues for significant intensification. In fact, the GFS-SHIPS rapid intensification index now give Sam a 39 percent chance of a 65-kt increase over the next 72 hours, which is more than 7 times the climatological value. Thus, the NHC intensity forecast has been raised quite a bit from the prior one, especially in the short-term. The latest forecast now makes Sam a hurricane in 36 hours, and a major hurricane in 72 hours. This intensity forecast is just a shade under the latest HCCA consensus aid. Thereafter, a more gradual intensification rate is forecast. By this period it is possible that Sam could undergo hard to predict inner-core changes such as eyewall replacement cycles. In addition, the ECMWF-SHIPS suggests that the shear may also increase a little in days 4-5 which also argues for a bit slower rate of intensification for this period.

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