AXNT20 KNHC 290001

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
0005 UTC Wed Sep 29 2021

Tropical Weather Discussion for North America, Central America Gulf of Mexico, Caribbean Sea, northern sections of South America, and Atlantic Ocean to the African coast from the Equator to 31N. The following information is based on satellite imagery, weather observations, radar and meteorological analysis.

Based on 1800 UTC surface analysis and satellite imagery through 2345 UTC.


Sam, a category 4 hurricane, was centered near 17.9N 55.0W at 28/2100 UTC or 460 nm E of the Northern Leeward Islands, and moving NW at 8 kt. Estimated minimum central pressure is 947 mb. Maximum sustained wind speed is 115 kt with gusts to 140 kt. Seas are peaking at 35 to 38 ft near the center, and from 28 to 35 ft up to 70 nm farther out. Latest satellite imagery shows a faintly visible eye feature. The imagery shows numerous moderate to scattered strong convection within 120 nm of the eye in the NE quadrant and within 90 nm of the eye in the SE quadrant. Scattered moderate convection is elsewhere within 240 nm in the SE quadrant, 180 nm in the NE quadrant and 150 nm in the NW quadrant. An outer band consists of scattered moderate convection within 30 nm of a line from 13N56W to 13N54W and to 15N52W. The current northwest motion will continue through the next couple of days, with an increase in forward speed. A turn toward the north is forecast by Fri. On the forecast track, Sam will pass well to the east or northeast of the northern Leeward Islands through Wed. 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. Swells generated by Sam will impact the Lesser Antilles for the next several days. Swells are expected to reach Bermuda and the Bahamas within 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. Please read the latest High Seas Forecast issued by NHC at http://www.nhc.noaa.gov/text/MIAHSFAT2.shtml and the latest NHC Public Advisory and Forecast/Advisory at https://nhc.noaa.gov/cyclones/

A large area of numerous moderate to strong convection from 05N to 10N between 20W-27W is associated with a low pressure system that is located over the far eastern tropical Atlantic several hundred nautical miles south of the Cabo Verde Islands. This system is gradually becoming better defined. The associated aforementioned convection is showing signs of organization, and a tropical depression is expected to form during the next day or so while the disturbance moves west-northwestward at 8 to 13 kt over the eastern tropical Atlantic. This system has a high chance of becoming a tropical cyclone during the next 48 hours.

Scattered moderate to isolated strong convection from 03N to 10N between 27W-36W in association with a broad area of low pressure located several nautical miles southwest of the Cabo Verde Islands remains disorganized. Environmental conditions are conducive for gradual development, and a tropical depression could still form during the next couple of days if the disturbance does not get absorbed into the stronger low pressure system located to its east. The broad low is expected to move slowly west-northwestward to westward over the central tropical Atlantic during the next few days. This system has a medium chance of becoming a tropical cyclone during the next 48 hours.

Please refer to the latest NHC Tropical Weather Outlook at www.hurricanes.gov for more details on the two tropical disturbances.


A far eastern Atlantic tropical wave is along 21W. Please see the Special Features section above for details.

Another far eastern Atlantic tropical wave is along 38W from 03N to 17N, moving westward near 15 kt. Scattered moderate convection is within 120 nm east of the wave from 09N to 11N.

A Caribbean tropical wave is along 80W south of to across the central section of Panama. It is moving westward near 10 kt. Scattered moderate to isolated strong convection is noted across and near the southern part of the wave south of 12N and west of 75W. This activity is being further enhanced by he eastern segment of the eastern Pacific monsoon trough that extends into the southwestern Caribbean.


A monsoon trough enters the Atlantic near the Guinea Bissau-Guinea border near 11N15W southwestward to the aforementioned 1010 mb Special Features 1010 low near 07N22W and continues to 09N28N to a 1010 mb low near 08N32W and to 06N40W. The ITCZ extends from 06N40W to 06N76W and to 08N54W. Aside from convection attributed to the Special Features systems and the tropical wave along 38W, numerous moderate to strong convection is just inland and along the coast of Africa from 07N to 11N. Scattered moderate convection is within 60 nm north of the ITCZ between 50W-54W. Latest satellite scatterometer and altimeter data reveal moderate to fresh with locally strong monsoon winds and seas of 7-9 ft near and south of the monsoon trough, from 01N to 07N between 18W and 30W.


Numerous moderate to strong convection is along and inland the coastal areas from the far western Florida panhandle to extreme northeastern Texas. This activity is due to an approaching vigorous short-wave trough that is presently over eastern Texas and moving through a very moist and unstable air mass. Scattered showers and isolated thunderstorms are over the western part of the NE Gulf, while isolated showers and thunderstorms are over the central and southeastern Gulf. Latest ASCAT data indicates mainly light to gentle winds across the basin under a weak pressure pattern. Seas are relatively low, 4 ft or less throughout.

For the forecast, the present conditions will continue through late Wed. Moderate to fresh winds will follow a weak cold front that will move into the eastern Gulf Fri into Sat before stalling and dissipating through Sun.


Modest convergent trade winds are causing isolated thunderstorms near the Cayman Islands and over the southeast part of the basin. Refer to the Tropical Waves section for additional convection in the basin. Moderate to locally fresh trades and seas in the 4-5 ft range are over the south-central section of the sea. Light to gentle trades along with seas of 2-3 ft prevail for the rest of the Caribbean Basin.

For the forecast, major Hurricane Sam centered near 17.9N 55.0W, or 460 nm E of the Northern Leeward Islands 947 mb at 2100 UTC and moving NW at 8 kt with maximum sustained wind speed of 115 kt with gusts to 140 kt will move pass well to the east or northeast of the northern Leeward Islands through Wed. Meanwhile, moderate trades will continue over much of the Caribbean today. Gentle trades will then prevail over the Caribbean through Fri. Swells generated by Sam are forecast to persist near the Leeward Islands through the middle of the week.


Please read the Special Features section above for details on Hurricane Sam and on two tropical disturbances in the eastern part of the Atlantic Basin.

The Cumbre Vieja volcano on the island of La Palma in the Canary Islands has been erupting since 19/1410 UTC. Medium concentration of volcanic ash is expected up 40 nm east-southeast of the volcano. Marine and aviation interests should monitor this ongoing situation by reading Volcanic Ash Advisory issued by Meteo-France at http://vaac.meteo.fr/volcanoes/la-palma/. The next advisory will be issued no later than 29/0300 UTC.

A frontal trough extends from near Bermuda southwestward to 26N70W and to near 28N74W. Another trough to its east extends from the remnant 1010 mb low of Peter north of the area near 34N59W south-southwest to 28N60W and to 26N64W. Scattered moderate convection moving quickly northeast is between the troughs from 25N to 30N. Scattered showers and isolated thunderstorms are elsewhere north of 25N between 59W-70W.

In the eastern Atlantic, a surface trough extends from near 30N30W to 28N36W and to 24N39W. Scattered showers and isolated thunderstorms are within 90 nm either side of the the trough from 27N to 29N. Another trough extends from 22N29W to 16N27W. There is no significant convection with this trough as it is surrounded by a very stable environment.

North of 22N, light to gentle winds and seas at 3 to 4 ft are present between 62W and the Georgia-Florida coast, while gentle winds with locally moderate winds and seas of 5-7 ft are between 30W-62W. Farther east, moderate to fresh with locally strong northeast trades and seas of 8-11 ft in northerly swell are present near the Canary Islands, N of 20N between the NW African coast and 30W. Outside the influence of Hurricane Sam, gentle to moderate trades and seas of 5-7 ft are from 10N to 22N between the African coast and the Lesser Antilles. Besides stronger monsoon winds and higher seas mentioned in the Monsoon Trough/ITCZ section, mainly gentle monsoon winds and seas in the range of 4-7 ft prevail across the remainder of the basin.

For the forecast, light to gentle breezes are expected to prevail with slight seas through Thu W of 65W. Meanwhile, major Hurricane Sam as near 17.9N 55.0W 947 mb at 2100 UTC moving NW at 8 kt. Maximum sustained winds are 115 kt with gusts to 140 kt. Sam will move to 18.6N 55.9W Wed morning, 19.7N 57.4W Wed afternoon, 21.0N 59.1W Thu morning, and 22.8N 60.6W Thu afternoon. Sam will stay east of the region and change little in intensity as it moves north of the area by early Sat, staying east of Bermuda. In addition to hurricane conditions northeast of the Leeward Islands through Wed, expect rough seas in the form of swell from Sam east of 70W Thu through Fri. Farther west, a weak pressure pattern will maintain light to gentle breezes and slight seas through Thu. A cold front will move from the Carolinas to the central Bahamas Fri and Sat, then stall and dissipate through late Sun.

$$ Aguirre