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Atlantic Tropical Weather Discussion
NWS National Hurricane Center Miami FL
2:05PM EDT (1805 UTC)
Friday September 24 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 1200 UTC surface analysis and satellite imagery through 1745 UTC.

Active and Developing Storms
Hurricane Sam is centered near 11.8N 43.7W at 24/1500 UTC or about 1280 nm ESE of the Northern Leeward Islands moving W at 12 kt. Estimated minimum central pressure is 993 mb. Maximum sustained wind speed is 65 kt with gusts to 80 kt. Satellite imagery shows quite a healthy system with well pronounced tightly coiled outer rain bands. Scattered moderate to strong convection within the central dense overcast feature are within 45 nm of center of Sam. Scattered moderate to strong convection are with the outer rain bands within 30 nm of a line from 11N42W to 13N43W and to 12N45W. Similar convection in another outer rain band is within 30 nm of a line from 12N46W to 10N44W. Peak seas are currently near 21 ft, but are forecast to reach near 34 ft Sun. A decrease in forward speed and a motion toward the west- northwest are expected over the weekend. Rapid intensification is forecast to continue, and Sam is likely to become a major hurricane on Sat. Please read the latest HIGH SEAS FORECAST issued by the National Hurricane Center at website- https://www.nhc.noaa.gov/text/MIAHSFAT2.shtml and the latest Sam NHC Forecast/Advisory and Public Advisory at www.hurricanes.gov for more details.

Surface observations and recent satellite wind data indicate that a surface low located a couple hundred miles north-northeast of Bermuda is gradually becoming better defined as it interacts with an upper-level trough. This system is producing winds of 40 to 45 mph north of the low center and additional development into a subtropical or tropical storm appears likely. Advisories could be initiated as soon as this afternoon as the system moves slowly toward the north-northwest over the next day or so. This system has a high chance of tropical cyclone formation during the next 48 hours.

Tropical Waves
A far eastern Atlantic tropical wave has its axis along 26W from 04N to 19N. It is moving westward at 10 kt. Scattered moderate to isolated strong convection is within 180 nm east of the wave from 09N to 12N. Per latest GOES-16 SAL imagery, the northern part of the wave remains embedded in dry Saharan air, inhibiting the development of shower and thunderstorm activity.

A central Atlantic tropical wave has its axis near 58W from 07N to 19N. It is moving westward at 10-15 kt. Scattered showers and isolated thunderstorms are seen from 11N to 13N between 56W- 59W. This is where the TPW animation imagery indicates a maximum in moisture content.

Monsoon Trough And Intertropical Convergence Zone - ITCZ
(The ITCZ is also known by sailors as the doldrums)
The monsoon trough axis passes through coast of Senegal near 14N17W to 06N28W. A segment of the ITCZ extends from 06N28W to 08N36W and another segment continues from 07N46W to 06N55W. Aside from convection associated to the tropical waves and Sam described above, scattered moderate convection is within 120 nm north of the ITCZ between 29W-32W and within 60 nm north of the ITCZ between 32W-34W.

Discussion: Gulf Of Mexico
As of 15Z, a quasi-stationary frontal boundary extends from southwestern Florida to 24N90W and to Veracruz. Satellite imagery shows scattered showers and thunderstorms along and within 30 nm either side of the boundary west of 85W. This activity is being aided by a mid to upper-level shortwave trough. The air mass behind the front is quite dry, allowing fairly tranquil weather conditions front Texas to Florida in the N Gulf. Recent scatterometer satellite data and current buoy observations reveal gentle to moderate northeast winds behind the front east of 94W, and gentle northeast to east winds west of 94W. Light and variable winds are south of the frontal boundary. Seas across the basin are 2-4 ft, except for slightly lower seas of 1-3 ft over the southeastern Gulf.

For the forecast, the quasi-stationary frontal boundary will dissipate by Sat. Fresh winds prevail within 90 nm north of the front. Winds and seas will diminish across basin from Saturday night and into early next week as high pressure builds over the northern Gulf.

Discussion: Caribbean Sea
A small upper-level low near the southeastern Bahamas has an associated trough that reaches to the eastern tip of Cuba and to the southwestern Caribbean. A surface trough is analyzed over the western Caribbean near 85W from 17N-22N. Some reflection of this feature is noted at 700 mb. Scattered showers and a few thunderstorms are occurring near the southern part of the trough and along and just inland the northeast section of Honduras. Isolated thunderstorms are west of the trough. Increased scattered moderate to isolated strong convection is over the southwestern Caribbean and south-central Caribbean sections, especially near the northern coast of Colombia. This activity is being further enhanced by the eastern segment of the eastern Pacific monsoon trough that stretches east to northern Colombia and to some extent by instability created by the southern part of the aforementioned upper-level trough. Expect for this activity to remain quite active through early Sat. Partial ASCAT data passed from this morning indicate generally gentle to moderate trades across the basin, with the majority of the gentle trades confined to the NW Caribbean and to the northern sections of the central and eastern Caribbean. Seas across the area remain relatively low, in the 2-3 ft range, except for slightly higher seas of 3-4 ft over the southwestern Caribbean.

For the forecast, Hurricane Sam is near 11.8N 43.7W 993 mb at 11 AM EDT moving W at 12 kt. Maximum sustained winds are 65 kt with gusts to 80 kt. Sam will strengthen to a major hurricane Saturday eve near 13N48W and move to near 15N52W Monday morning. Sam is forecast to enter the offshore waters east of Leeward Islands Tuesday near 17N55W and move to near 18N58W Wednesday morning. Meanwhile, moderate trades will persist over much of the Caribbean into early next week, becoming fresh over the south-central Caribbean starting tonight as high pressure builds east of the Bahamas. Fresh to locally strong winds are forecast in the Gulf of Honduras Sunday night.

Discussion: Atlantic Ocean
Please read the sections above for details on Hurricane Sam and tropical waves moving across the basin.

The Cumbre Vieja volcano on the island of La Palma in the Canary Islands has been erupting since 19/1410 UTC. Pilots have reported volcanic ash near Tenerife and over La Gomera. The height of the ash plume cloud is difficult to estimate due to cloud cover. Marine and aviation interests should monitor this on-going situation by reading Volcanic Ash Advisory issued by Meteo-France at http://vaac.meteo.fr/volcanoes/la-palma/. The next advisory will be issue no later than 24/2100 UTC.

As of 15Z, a cold front extends from near 31N76W to 27N79W, and transitions to a stationary frontal boundary to Stuart, Florida and to southwest Florida. A pre-frontal trough is within 30-60 nm ahead of the cold front. Scattered moderate convection is from 27N to 31N between the front/pre-frontal trough and 74W. Moderate to fresh southerly winds are east of the trough and front to near 75W and north of 26N. Seas with these winds are in the 4-6 ft range.

Another area of interest is a sharp surface trough extending from 31N60W to 25N65W and to a 1010 mb low near 24N66W. The interaction of these features with a large upper-level low near Bermuda is inducing a large area of showers and thunderstorms, especially from N of 20N and between 56W and 64W. Latest ASCAT wind data indicate fresh to strong cyclonic winds near the northern end of the surface trough north of 28N. Seas in this region are 5-8 ft, with the highest seas noted near the highest winds.

Farther east, a 1010 mb low is analyzed near 29N40W. A large area of scattered moderate to strong convection is present north of 27N between 33W-40W. This convection is shifting eastward. A stationary front stretches from 31N43W to 29N47W. Scattered showers and thunderstorms have recently developed along this boundary. Moderate to fresh northerly winds were captured by ASCAT off the coast of Mauritania, mainly east of 20W. A large area of northerly swell that is producing 8-11 ft seas N of 22N and between 35W and 60W. Moderate to fresh trade winds are noted elsewhere across the basin along with 4 to 6 ft seas in open waters.

For the forecast west of 65W, the cold front extending from 31N76W to 27N79W will stall early this afternoon then dissipate through early Sunday just before a second weak front possibly moves into the region late Sun. Farther east, Hurricane Sam is near 11.8N 43.7W 993 mb at 11 AM EDT moving W at 12 kt. Maximum sustained winds are 65 kt with gusts to 80 kt. Sam will strengthen to a major hurricane Saturday evening near 13N48W and move to near 15N52W Monday morning. Sam is forecast to move to near 17N55W Tuesday and to near 18N58W Wednesday morning.

Forecaster: Jorge Aguirre-Echevarria, National Hurricane Center


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