Hurricane Sam Strengthens Major Category 4 Hurricane (Again) East of The Lesser Antilles

What you need to know:
– Sam is forecast to continue moving northeast of the Lesser Antilles, safely northeast of the Leeward Islands, posing no direct threat to T&T and the Lesser Antilles.
– The National Hurricane Center is forecasting Sam to remain at Category 4 strength, with fluctuations in intensity through the next 24 hours.
Swells from Hurricane Sam are forecast to continue affecting the northern and eastern coastlines of the Lesser Antilles through the week.
– Due to near calm winds induced by Sam, very hot temperatures are forecast across the region with localized, slow-moving, possibly severe, afternoon showers and thunderstorms across western and hilly areas.

The Latest From The National Hurricane Center

Hurricane Sam's Forecast Track as of 5:00 AM Tuesday 28th September 2021. (National Hurricane Center)
Hurricane Sam’s Forecast Track as of 5:00 AM Tuesday 28th September 2021. (National Hurricane Center)

At 5:00 AM AST, the center of Hurricane Sam was located near latitude 17.2°N, 53.9°W. Sam is moving toward the northwest near 15 km/h, and this motion is expected to continue for the next few days, with an increase in forward speed beginning on Thursday. A turn toward the north is expected by Friday. On the forecast track, Sam will pass well to the northeast of the northern Leeward Islands on Wednesday and Thursday.

Data from an Air Force Reserve Hurricane Hunter aircraft indicate that maximum sustained winds have increased to near 215 KM/H with higher gusts. Sam is a category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale. Some fluctuations in intensity are expected during the next couple of days, but Sam is forecast to remain a major hurricane through late this week.

Hurricane-force winds extend outward up to 65 kilometers from the center and tropical-storm-force winds extend outward up to 205 kilometers. The estimated minimum central pressure based on dropsonde data from the aircraft is 953 millibars.

Hurricane Sam's latest information as of 5:00 AM Tuesday 28th September 2021. (National Hurricane Center)
Hurricane Sam’s latest information as of 5:00 AM Tuesday 28th September 2021. (National Hurricane Center)

Hurricane Sam’s Watches & Warnings

Hurricane Sam's Tropical Storm Watches and Warnings as of 5:00 AM Tuesday 28th September 2021. (National Hurricane Center)
Hurricane Sam’s Tropical Storm Watches and Warnings as of 5:00 AM Tuesday 28th September 2021. (National Hurricane Center)

There are no coastal watches or warnings in effect at this time.

However, swells generated by Sam will impact the Lesser Antilles for the next several days. Swells are expected to reach Bermuda and the Bahamas in a couple of days, and then spread to the United States east coast late this week. These swells could cause life-threatening surf and rip current conditions.

Swells have been affecting Trinidad and Tobago’s northern and eastern coastlines, causing choppy seas in nearshore areas.

Hurricane Sam's probabilities for tropical-storm-force winds as of 5:00 AM Tuesday 28th September 2021. (National Hurricane Center)
Hurricane Sam’s probabilities for tropical-storm-force winds as of 5:00 AM Tuesday 28th September 2021. (National Hurricane Center)

This system is of no direct threat to Trinidad and Tobago.

Hurricane Sam’s Forecast Discussion

Hurricane Sam (Weathernerds.org)

The Air Force Reserve Hurricane Hunters that investigated Sam early this morning found evidence that the hurricane has strengthened. Peak 700-mb flight-level winds of 126 kt were reported in the northeastern quadrant, which after the standard adjustment would support an intensity of 110-115 kt. Satellite data also indicate that Sam’s structure has improved overnight. Sam’s inner core appears to have consolidated into a single, primary eyewall in recent GMI and AMSR2 microwave imagery, and the latest GOES-16 infrared imagery shows a colder ring of convective cloud tops developing around the eye of Sam. This has brought the latest UW-CIMSS ADT and SATCON estimates up to around 115 kt. Based on the flight-level wind data and ongoing satellite trends, the initial intensity is raised to 115 kt for this advisory. The minimum central pressure of 953 mb is based on a center dropsonde of 954 mb with 12-kt surface winds.

In the short-term, internal dynamic processes that are difficult to predict could cause some fluctuations in Sam’s intensity. If Sam’s eyewall is able to contract today, some additional strengthening could occur, and this possibility is reflected in the latest NHC intensity forecast. Thereafter, the warm sea-surface temperatures and weak to moderate vertical wind shear along Sam’s forecast track suggest it should remain a major hurricane for the next several days. Thus, the NHC forecast only shows gradual weakening through 72-96 h, in agreement with the consensus aids IVCN and HCCA. By days 4-5, increasing southerly wind shear along with decreasing SSTs should increase Sam’s rate of weakening as it recurves deeper into the mid-latitudes.

Aircraft and microwave data indicate that Sam is slightly tilted in the vertical, with the low-level center displaced just a bit to the south of the mid-level eye. Based on recent aircraft fixes, Sam’s initial motion is northwestward, or 310/8 kt. This motion is expected to continue for the next few days as Sam moves around the southwestern periphery of the subtropical ridge. A mid- to upper-level trough is forecast to dig southward over the western Atlantic later this week, which should steer Sam toward the north by Friday. Then, Sam is expected to accelerate north-northeastward within the deep-layer southerly flow ahead of the trough this weekend. The official NHC track forecast is very similar to the previous one, with just a slight adjustment to the left based on the latest track guidance consensus aids. The along-track spread in the guidance noticeably increases as Sam recurves over the western Atlantic, with the ECMWF much slower than the GFS. Once again, the NHC forecast trends closer to the faster GFS solution at longer ranges, given its better overall performance this season.

Key Messages:

  1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend.

Tropical Cyclone Climatology

2021 has already produced 19 tropical cyclones in the Atlantic Basin, with the next system being named Victor for the 2021 Atlantic Hurricane Season. For this time of year, most systems form in the Caribbean, the Gulf of Mexico, and off of the Eastern United States.

As we head through the last few days 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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