FLORIDA
STORMS
Tropical Storm Sam
LOCATED
1635 MI ESE OF THE NORTHERN LEEWARD ISLANDS
WINDS
60 MPH
PRESSURE
1000 MB
MOVING
W AT 16 MPH
From the National Hurricane Center at 500 PM AST Thu Sep 23 2021
Sam continues to strengthen.
TAP LINKS BELOW TO FOCUS
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 500 PM AST (2100 UTC), the center of Tropical Storm Sam was located near latitude 11.1 North, longitude 39.7 West. Sam is moving toward the west near 16 mph (26 km/h) and a general west to west northwest motion is expected to continue over the next several days, with a gradual slowdown in forward motion. Maximum sustained winds have increased to near 60 mph (95 km/h) with higher gusts. Rapid intensification is forecast during the next several days. Sam is forecast to become a hurricane tomorrow morning and could become a major hurricane by Saturday. Tropical storm force winds extend outward up to 60 miles (95 km) from the center. The estimated minimum central pressure is 1000 mb (29.53 inches).

At 500 PM AST (2100 UTC), the center of Tropical Storm Sam was located near latitude 11.1 North, longitude 39.7 West. Sam is moving toward the west near 16 mph (26 km/h) and a general west to west northwest motion is expected to continue over the next several days, with a gradual slowdown in forward motion. Maximum sustained winds have increased to near 60 mph (95 km/h) with higher gusts. Rapid intensification is forecast during the next several days. Sam is forecast to become a hurricane tomorrow morning and could become a major hurricane by Saturday. Tropical storm force winds extend outward up to 60 miles (95 km) from the center. The estimated minimum central pressure is 1000 mb (29.53 inches).

Sam could be in the beginning stages of developing a small inner-core this afternoon as a central dense overcast is beginning to take shape. On geostationary visible satellite imagery, the storm has occasionally exhibited a clear area, though this feature has been tilted a bit southeast of the estimated low-level center position. In addition, overshooting convective cloud tops have been recirculating cyclonically around this feature, suggesting convective symmetrization that may help shield the low-level vortex from dry-air intrusion as seen earlier on the northwest side of Sam. Unfortunately, I have not received a high resolution microwave pass over Sam in 9-12 hours, so there remains some uncertainty on the structure underneath the cirrus canopy. The latest subjective Dvorak estimates from SAB and TAFB were 55-kt and 45-kt respectively. Taking a blend of these intensity estimates yields 50-kt for this advisory.

Sam's estimated motion is nearly identical to this morning, moving north of due west at 280/14 kt. A large and anomalously strong mid-level ridge is located poleward of Sam and should maintain the current west to west-northwest heading over the next 2-3 days. However, this ridge-axis is forecast to build in to the northwest ahead of Sam, which is likely to gradually slow down this forward motion over the next few days. In fact, Sam is forecast to move unusually slow for a tropical cyclone in the deep tropics east of the Lesser Antilles. Afterwards, a weakness in the ridge to the northwest is forecast to gradually develop as a broad deep-layer trough parks itself just offshore of the eastern United States by early next week. This could potentially allow Sam to gain a bit more latitude at a somewhat faster forward motion as the ridge axis shifts back east. The track guidance this cycle has slowed down a bit compared to this morning and is also a bit further south in the early portion of the forecast. The NHC official track will follow suit this advisory, though is not quite as slow as the GFS model. It is interesting to note that both the GFS and ECMWF ensemble guidance this afternoon shows a significant spread in track solutions by day 5 in the cross-track direction, so this track forecast is a bit more uncertain than usual.

With the apparent improvement in inner-core structure inferred on satellite today, Sam appears poised to rapidly intensify in the short-term. In fact, the latest GFS-SHIPS guidance is now explicitly forecasting rapid intensification over the next 24-48 hours. This seems feasible given that the guidance is forecasting 5 kt or less vertical wind shear over the next 60 hours as the cyclone traverses over warm 28-29 C sea-surface temperatures. Thus, the intensity guidance in the short-term has been raised once again, now showing a 50-kt increase in intensity over the next 48 hours taking Sam to major hurricane intensity by Saturday afternoon. Remarkably, this rapid intensification is still below the latest HCCA and LGEM guidance over this time span. After this period, Sam is likely to start undergoing inner core fluctuations, related to eyewall replacement cycles that are challenging to forecast far in advance. A modest increase in southwesterly vertical wind shear could also occur in the day 3-5 period. Thus, the NHC intensity forecast still tops off Sam as 110-kt hurricane by the end of the forecast period, which is on the high end of the intensity guidance envelope.

Sam could be in the beginning stages of developing a small inner-core this afternoon as a central dense overcast is beginning to take shape. On geostationary visible satellite imagery, the storm has occasionally exhibited a clear area, though this feature has been tilted a bit southeast of the estimated low-level center position. In addition, overshooting convective cloud tops have been recirculating cyclonically around this feature, suggesting convective symmetrization that may help shield the low-level vortex from dry-air intrusion as seen earlier on the northwest side of Sam. Unfortunately, I have not received a high resolution microwave pass over Sam in 9-12 hours, so there remains some uncertainty on the structure underneath the cirrus canopy. The latest subjective Dvorak estimates from SAB and TAFB were 55-kt and 45-kt respectively. Taking a blend of these intensity estimates yields 50-kt for this advisory.

Sam's estimated motion is nearly identical to this morning, moving north of due west at 280/14 kt. A large and anomalously strong mid-level ridge is located poleward of Sam and should maintain the current west to west-northwest heading over the next 2-3 days. However, this ridge-axis is forecast to build in to the northwest ahead of Sam, which is likely to gradually slow down this forward motion over the next few days. In fact, Sam is forecast to move unusually slow for a tropical cyclone in the deep tropics east of the Lesser Antilles. Afterwards, a weakness in the ridge to the northwest is forecast to gradually develop as a broad deep-layer trough parks itself just offshore of the eastern United States by early next week. This could potentially allow Sam to gain a bit more latitude at a somewhat faster forward motion as the ridge axis shifts back east. The track guidance this cycle has slowed down a bit compared to this morning and is also a bit further south in the early portion of the forecast. The NHC official track will follow suit this advisory, though is not quite as slow as the GFS model. It is interesting to note that both the GFS and ECMWF ensemble guidance this afternoon shows a significant spread in track solutions by day 5 in the cross-track direction, so this track forecast is a bit more uncertain than usual.

With the apparent improvement in inner-core structure inferred on satellite today, Sam appears poised to rapidly intensify in the short-term. In fact, the latest GFS-SHIPS guidance is now explicitly forecasting rapid intensification over the next 24-48 hours. This seems feasible given that the guidance is forecasting 5 kt or less vertical wind shear over the next 60 hours as the cyclone traverses over warm 28-29 C sea-surface temperatures. Thus, the intensity guidance in the short-term has been raised once again, now showing a 50-kt increase in intensity over the next 48 hours taking Sam to major hurricane intensity by Saturday afternoon. Remarkably, this rapid intensification is still below the latest HCCA and LGEM guidance over this time span. After this period, Sam is likely to start undergoing inner core fluctuations, related to eyewall replacement cycles that are challenging to forecast far in advance. A modest increase in southwesterly vertical wind shear could also occur in the day 3-5 period. Thus, the NHC intensity forecast still tops off Sam as 110-kt hurricane by the end of the forecast period, which is on the high end of the intensity guidance envelope.

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