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Atlantic Tropical Weather Discussion

AXNT20 KNHC 161723

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1805 UTC Thu Sep 16 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 
1630 UTC.


Post-Tropical Cyclone Nicholas is centered near 29.8N 91.7W at
1500 UTC or 100 nm SSE of Alexandria Louisiana moving N at 2 kt. 
Estimated minimum central pressure is 1007 mb. Maximum sustained 
wind speed is 20 kt with gusts to 25 kt. A trailing trough is 
analyzed over the Gulf of Mexico and extends SW to near the Texas
coast along 27N97W. Nicholas is currently nearly stationary along
or just offshore of southern Louisiana, and recent visible
satellite imagery suggests the center is attempting to reform just
to the SW. However, this low is expected to gradually move 
northward over the next couple of days. Shallow low to middle
level clouds and minimal convection is noted near the center of
the former Nicholas at this time. However, scattered moderate to 
strong convection persists to the east and south of the trailing 
trough, from the Florida Panhandle to the Mexican coast along 
98W, where fresh southerly low level winds are converging along 
and just ahead of the trough. The convection will shift with and 
ahead of the remnants of Nicholas during the next few days, and is
expected to produce additional rainfall amounts of 2 to 4 inches 
across the central Gulf coast Friday, with isolated storm totals 
of 12 inches possible. Flash flooding impacts, especially in urban
areas, are possible across these regions. Widespread minor river 
flooding is expected, while scattered moderate river flooding is 
possible, across portions of southeastern Louisiana, southern 
Mississippi, southern Alabama, and the Florida Panhandle. The 
Weather Prediction Center is issuing bulletins on Nicholas and 
they can be found at 

A surface trough extends from broad 1011 mb low pressure located 
near 31.5N74W to just north of the Bahamas near 28N77W. Scattered
moderate to strong convective activity associated with this low 
is displaced to the E of the trough and covers the area from 24N 
to 35N between 64W and 73W. Fresh to strong winds associated with
this system are also noted to the E of the low center and mainly 
from 28N to 31N between 68W and 74W. Environmental conditions are 
expected to become more conducive for development, and a tropical 
depression is still likely to form during the next day or two 
while the system moves north- northwestward to northward off the 
southeast U.S. and mid Atlantic coasts. Regardless of 
development, this system could bring high surf to portions of the 
southeast and mid- Atlantic U.S. coasts later this week. This 
system has a high chance of tropical cyclone formation in the next
48 hours. Please read the latest Tropical Weather Outlook at for more details. 

A tropical wave is in the Atlantic Ocean along 34W from 03N 
to 17N, moving west at 15 to 20 kt. Associated showers and 
thunderstorms are slightly less organized with this wave this 
morning. However, environmental conditions are still expected to 
remain conducive for development, and a tropical depression is 
likely to form during the next couple of days. This system is 
expected to move westward to west-northwestward across the 
tropical Atlantic during the next several days. This system has a 
high chance of tropical cyclone formation in the next 48 hours. 
Please read the latest Tropical Weather Outlook at for more details.


Refer to the Special Features section above for details on a
tropical wave and associated area of low pressure in the Atlantic
Ocean along 31W/32W.

Another tropical wave is emerging off of the west coast of Africa
this morning, and will be added to the 1800 UTC surface analysis.  
Satellite imagery and surface observations suggest that there is a
1012 mb low along the African coast near 13.5N17W, shifting
westward. The Dakar, Senegal 1200 UTC sounding also suggests that
the wave is nearing the coast. Morning scatterometer winds showed
a broad NE wind surge of around 20 kt spreading from the coast of
Mauritania southwestward to near 30W. Environmental conditions 
are forecast to be conducive for development while the system 
moves generally west- northwestward to northwestward over the far 
eastern Atlantic. Currently, a large cluster of moderate to 
isolated strong convection is noted from 05N to 13N and E of 25W 
to the coast of Africa. 

A tropical wave is in the Atlantic Ocean along 47W, from 04N to 
14N, moving west at around 10 kt. Scattered showers are noted 
where the wave meets the monsoon trough near 07.5N47W. 

A tropical wave is in the Caribbean Sea along 75W from 18N 
southward to Colombia, moving west at 10 to 15 kt. The tropical 
wave combined with a mid to upper level trough axis across the
western Caribbean is aiding to induce scattered showers and 
thunderstorms along the trough axis from 10N to 12N between
Colombia and far SE Nicaragua. 

A tropical wave is over Central America along 90W from the central
Yucatan Peninsula southward into the eastern tropical Pacific 
Ocean, moving west at 10 to 15 kt. Scattered showers and a few
thunderstorms are noted over the E Pacific.


The monsoon trough emerges off the coast of Africa through 
Senegal to a 1012 mb low along the coast near 13.5N17W to 08N22W
to 1013 mb low pressure located near 08.5N39.5W to 06.5N46W. The 
ITCZ continues from 06N47W to the coasts of French Guiana and
Suriname. Aside from the convection mentioned above near the
tropical waves, scattered showers and moderate thunderstorms are 
noted from 09N to 12N between 54W and 61W. 


The Weather Prediction Center is issuing bulletins on Post Tropical
Nicholas, located along the coast of southern Louisiana. Refer to
the Special Features section above for details.

As previously mentioned, a surface trough extends from the 
cyclonic circulation of Nicholas to near 27N97W. Morning
scatterometer winds showed gentle to moderate SE-S winds east of
88W, while moderate to fresh S winds were detected between 88W and  
the trough. Gentle to moderate NW-N winds prevail west of the 
trough toward the Texas coast. Seas are generally 2 to 3 ft 
across most of the basin, except 3 to 5 ft over the north- 
central Gulf where fresh southerly winds are occurring to the east
of the circulation of Nicholas. Scattered showers and 
thunderstorms, some strong, are noted within 180 nm S and SE of
the trough described above, and most numerous off of the Mexican

Post-Tropical Cyclone Nicholas is nearly stationary along the 
central Louisiana coast. Moderate to fresh winds with seas of up 
to 5 ft may prevail in the north central Gulf into tonight as a 
result. Elsewhere, gentle to moderate southerly winds will 
dominate into the weekend.


Refer to the Tropical Waves section above for details on a 
tropical wave moving across the Caribbean Sea.

Morning scatterometer winds showed fresh easterly winds across the
central Caribbean between the tropical wave along 79W and 70W, and
fresh SE winds across the outer Gulf of Honduras to just offshore
of Cozumel, Mexico. Gentle trade winds prevail elsewhere. Seas 
are generally 2-4 ft, except 5-6 ft in the areas of strongest 
trade winds are blowing.

Showers and thunderstorms continue to flare-up over the NW
Caribbean due to a combination of a tropical wave moving westward
across the basin and an upper-level low trough currently extending
SW across the western basin. 

A tropical wave across the central Caribbean will move across the
western Caribbean into the weekend. Another tropical wave will 
move across the Lesser Antilles this weekend. Strong winds will 
pulse in the overnights into early next week offshore Colombia and
in the Gulf of Honduras. Low pressure in the eastern Atlantic is 
likely to develop into a tropical depression over the next couple 
of days, and this could bring increasing winds and seas to the 
waters near the Leeward Islands for the start of next week. 


Refer to the Special Features section above for details on the
disturbance situated northeast of the Bahamas, and the system in 
the eastern Atlantic. Both systems have a high chance of tropical
cyclone formation through 48 hours.

A ridge along about 36N dominates the Atlantic forecast waters E 
of 65W, with the main high pressure center of 1031 mb just south
of the Azores. A surface trough continues to move westward across
the tropical Atlantic and extends from 26N37W to 17N39W. A cluster
of scattered showers and a few thunderstorms is noted over
northern portions of the trough. Scatterometer data depicted very
well the wind shift associated with the trough axis. Moderate to 
fresh NE-E winds are noted east of the trough to offshore of
Africa. Seas are in the 6 to 8 ft range across this area.

West of 65W, morning scatterometer winds depicted gentle moderate
winds between the Bahamas and 65W, except fresh southerly winds
north of 28N between the trough and 65W. Seas are generally 3 to 5
ft except 5 to 7 ft north of 27N in the fresh southerly wind flow.

Showers and thunderstorms are increasing in association with low 
pressure several hundred miles SSE of the Outer Banks of North 
Carolina. A tropical depression is likely to form, however this 
system is moving northward out of the basin, so any increased 
winds and seas will be north of the area by Fri. Elsewhere, weak 
high pressure will lead to gentle to moderate mainly easterly 
winds across the basin. Another low pressure may impact waters 
north and east of the Leeward Islands early next week.