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
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
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
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
...GULF OF MEXICO...
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
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.