Tropical Storm Sam Forms East of Lesser Antilles, Now Rapidly Intensifying

Tropical Storm Sam has formed in the Atlantic Ocean, approximately 2,220 kilometers east of Trinidad and Tobago, as of 11:00 PM Thursday 23rd September 2021.

Sam formed from Tropical Wave 49 in the far Eastern Atlantic and developed into Tropical Depression Eighteen at 5:00 PM Wednesday 22nd September 2021.

Tropical Storm Sam became the 18th named tropical cyclone in the Atlantic, the second-most by 23rd September in the Atlantic Ocean. Only the 2020 Atlantic Hurricane Season had more storms at this point – 23 named storms by 23rd September.

Sam is forecast to become a major hurricane by Saturday 25th September 2021, joining the other three major hurricanes that have formed in the Atlantic this year, Grace, Ida, and Larry.

Sam is forecast to slowly near the northeastern Lesser Antilles by the middle to end of the upcoming week. However, it is still too soon to tell if this tropical cyclone could impact the Lesser Antilles. Interests in the Leeward Islands should monitor this system closely.

What we know

Tropical Storm Sam's Forecast Track as of 11:00 PM Thursday 23rd September 2021. (National Hurricane Center)
Tropical Storm Sam’s Forecast Track as of 11:00 PM Thursday 23rd September 2021. (National Hurricane Center)

Through Thursday, the satellite structure of now Tropical Storm Sam has markedly improved. Data indicates that the tropical storm has developed a small inner core, a central dense overcast, and overshooting convective cloud tops – all indicative of a rapidly strengthening and organizing tropical cyclone.

Presently, Tropical Storm Sam is located in an area with warm sea surface temperatures between 28°C and 29°C, low wind shear, and favorable upper-level winds.

Based on the latest information from the National Hurricane Center (NHC), the center of Tropical Storm Sam was located near 11.2°N, 40.9°W. Sam is moving toward the west near 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.

Tropical Storm Sam in the Main Development Region of the Atlantic Ocean. (Weathernerds)
Tropical Storm Sam in the Main Development Region of the Atlantic Ocean. (Weathernerds)

Microwave data also has shown Sam’s inner core has become well defined with a nearly closed eyewall feature. Sam’s intensity has risen to sustained winds of 60 knots or 110 KM/H based on satellite data. Since Sam’s intensity has increased quickly, by 30 knots in the last 24 hours, the tropical storm has met the definition for rapid intensification.

Sam continues to intensify rapidly, according to the NHC. Rapid intensification is forecast to continue through early Saturday. Sam is likely to become a hurricane very soon and 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 75 kilometers from the center with an estimated minimum central pressure of 997 millibars.

Where is Tropical Storm Sam Going?

The takeaway: Everyone in the Lesser Antilles should monitor this system, but those in the Leewards should pay particularly close attention to the eventual forecast track. This system is approximately 6-7 days away from its closest approach to the Leewards. This system poses no direct threat to the Windward Islands, including Trinidad and Tobago at this time.

Tropical Storm Sam's forecast tracks from top dynamical and consensus hurricane models as of 8:00 PM Thursday 23rd September 2021. (NCAR)
Tropical Storm Sam’s forecast tracks from top dynamical and consensus hurricane models as of 8:00 PM Thursday 23rd September 2021. (NCAR)

Tropical Storm Sam is riding along an anomalously strong and large high-pressure ridge to the north, which is why the system will continue to mostly westward at relatively low latitude for the next 2-3 days (by Sunday, latest). This ridge is forecast to build northwest, ahead of Sam, which will contribute to the unusually slow forward speed of the cyclone.

As Sam strengthens, it should gain latitude, taking the Southern Windward Islands out of the eventual hurricane’s crosshairs.

By next week (Monday and Tuesday), a weakness in the ridge is forecast to develop due to a trough system moves off the eastern United States. This weakness should allow the then Hurricane Sam to gain more latitude at a faster forward motion as the ridge shifts eastward. This steering pattern should take the hurricane northwestward, potentially missing the Lesser Antilles.

European (ECENS) versus the American (GEFS) latest ensemble model runs showing the stark differences in the track of Tropical Depression Eighteen approximately five days out (by Thursday 30th September 2021). A weaker system (seen on the ECENS) will remain further south but this is unlikely due to Sam's rapid intensification. The GEFS models reflect the current and forecast trajectory more accurately. (Weathernerds)
European (ECENS) versus the American (GEFS) latest ensemble model runs showing the stark differences in the track of Tropical Depression Eighteen approximately five days out (by Thursday 30th September 2021). A weaker system (seen on the ECENS) will remain further south, but this is unlikely due to Sam’s rapid intensification. The GEFS models reflect the current and forecast trajectory more accurately. (Weathernerds)

Looking at the two top global models at a longer range, the American GFS takes the tropical cyclone well northeast of the Leewards. In contrast, the European EMCWF takes the system uncomfortably close to the northernmost Leewards. Nearly all long-range models show the eventual hurricane moving either across or northeast of the Leewards by next Wednesday into Thursday.

Effectively, with the slow movement, this means it will be a few more days before models, and the National Hurricane Center, have a better idea of the eventual path of this system as it nears or potentially moves close to the Leewards.

How Strong Can Tropical Storm Sam Get?

The takeaway: By the end of the upcoming weekend, a slow-moving major hurricane is forecast east of the Lesser Antilles and remain a major hurricane through much of next week.

Tropical Storm Sam's forecast intensity guidance from top dynamical, statistical, and consensus hurricane models as of 8:00 PM Thursday 23rd September 2021. (NCAR)
Tropical Storm Sams forecast intensity guidance from top dynamical, statistical, and consensus hurricane models as of 8:00 PM Thursday 23rd September 2021. (NCAR)

Environmental conditions, Sam’s structure, and size are ideal for continued rapid intensification. Based on the current forecast, the cyclone is forecast to move over warm sea surface temperatures between 28°C and 29°C in an environment with low wind shear, less than 10 knots, and relatively high moisture.

Rapid strengthening is forecast over the next 36 hours, with Sam becoming a hurricane on Friday morning and a major hurricane by Friday night into Saturday morning. Intensity models then level off from Saturday, keeping Sam a major Category 3 hurricane into next week. These fluctuations are likely due to normal internal dynamics within the storm and the cyclone’s response to the surrounding environment in relation to its small size.

Tropical Storm Sam's probabilities for hurricane-force winds as of 11:00 PM Thursday 23rd September 2021. (National Hurricane Center)
Tropical Storm Sam’s probabilities for hurricane-force winds as of 11:00 PM Thursday 23rd September 2021. (National Hurricane Center)

What impacts can we expect to see from Tropical Storm Sam?

This system is 6-7 days away from the closest approach to the Lesser Antilles. Based on the latest track, feeder band activity is likely for the Leeward Islands. Swells are forecast to affect the entire island chain from Monday through the upcoming week. Sam will also influence winds across the island chain, allowing for hot temperatures, anomalous southerly winds depending on its proximity to the Lesser Antilles, and localized afternoon showers/thunderstorms in western areas.

Tropical Storm Sam's probabilities for tropical-storm-force winds as of 11:00 AM Thursday 23rd September 2021. (National Hurricane Center)
Tropical Storm Sam’s probabilities for tropical-storm-force winds as of 11:00 AM Thursday 23rd September 2021. (National Hurricane Center)

This system is still several days away, so it is too soon to tell where, if any, may see tropical-storm-force or hurricane-force winds and rainbands directly associated with the eventual Tropical Storm/Hurricane Sam. As the track stands presently, the Leeward Islands have the highest chances of experiencing outer bands from the tropical cyclone, with a present radius of 350 kilometers from the center of circulation.

Regardless of its path, based on the forecast strength, long-period swells are likely to begin affecting coastlines of the Lesser Antilles from Monday 27th September 2021, which can produce life-threatening surf and rip currents.

As this system moves to the northwest, potentially taking the cyclone northeast of the Leewards, a trough extending from the cyclone’s core to the southwest will slacken the pressure gradient, causing light winds. Light winds, high moisture, and a somewhat favorable atmospheric setup will allow for high temperatures triggering localized, slow-moving afternoon showers and thunderstorms across western and hilly areas. In addition, funnel clouds and waterspouts will also be possible.

Specifically for Trinidad and Tobago, in addition to the localized activity outlined above and the long-period swells, winds from the south will also be possible. This wind profile will result in showers and thunderstorms moving from the south to the north, possibly intensifying over the Gulf of Paria and the Northern Range and producing hazardous seas as well as street/flash flooding along the East-West Corridor.

But this model shows…

Individual model runs are just one possible outcome from a myriad of outcomes. Weather does not always follow what is modeled, and even what may be forecast. Beware of individual model runs being posted on social media.

Always check the National Hurricane Center for the latest information for tropical cyclones and your local meteorological offices for country-specific advisories.

What should I do?

This system is not forecast to directly affect the Southern Windward Islands, including Trinidad and Tobago. Don’t panic. There are no immediate or direct tropical storm threats heading to T&T, so feel free to ignore the “storm coming” chain messages that pass through WhatsApp every hurricane season.

If you are a risk-averse person, now is a good time to check your inclement weather, flood, or hurricane season plan, ensuring your preparedness supplies are not expired, stocked, and in a safe location.

The Office of Disaster Preparedness and Management has compiled a comprehensive guide for preparing for the Wet and Hurricane Season.

Tropical Cyclone Climatology

2021 has already produced 18 tropical cyclones in the Atlantic Basin, with the following system being named Teresa for the 2021 Atlantic Hurricane Season.

As we head through the second half of September and into October, we’ll continue to monitor the entire Atlantic closely as tropical cyclones could form from tropical waves, non-tropical low-pressure systems in the North Atlantic, and from the Central America Gyre in the Western Caribbean Sea or the southwestern Gulf of Mexico.

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