AXNT20 KNHC 270006
Tropical Weather Discussion...UPDATED
NWS National Hurricane Center Miami FL
0005 UTC Sun Sep 26 2021
Updated to include the remnants of Peter in the Special Features
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
Hurricane Sam, a category 4 hurricane on the Saffir-Simpson Wind
Scale is centered near 14.2N 50.5W at 26/2100 UTC, or 765 nm ESE
of the Northern Leeward Islands, moving WNW at 6 kt. Estimated
minimum central pressure is 938 mb. Maximum sustained winds are
130 kt with gusts to 160 kt. Peak seas are estimated at 35 to 40
ft just north of the center. Satellite imagery throughout the
afternoon has shown Sam attempting to become an annular hurricane
with a 15 nm wide eye. Numerous moderate to strong convection is
seen within 60 nm of the center. Scattered moderate convection is
found elsewhere within 150 nm of the center. A WNW motion should
continue through tonight, then a turn toward the NW with little
change in speed will begin on Mon. Sam is expected to pass well NE
of the Leeward Islands on Wed and Thu while accelerate toward the
NW. Some slight weakening is possible starting Mon afternoon but
Sam should remain a major hurricane through midweek. Swells
generated by Sam will begin to impact the Lesser Antilles on Mon.
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://hurricanes.gov
for more details.
A surface trough - the remnants of Peter - located several
hundred miles SE of Bermuda is producing numerous moderate
isolated strong convection is present from 26N to 30N between
58W and 62W. Scattered moderate convection is evident farther W
from 25N to 27N between 62W and 66W. The disturbance has become
a little better organized today. Upper-level winds are
marginally conducive for further development, and Peter could
become a tropical depression again during the next couple of
days while it moves northeastward at about 10 mph. By midweek,
environmental conditions are expected to become unfavorable for
further development. The disturbance has a medium chance of
development over the next 48 hours. Please visit hurricanes.gov
for more details.
An Atlantic tropical wave is near the Cabo Verde Islands along
26W from 17N southward, moving west near 15 kt. Associated
convection is described below.
Another Atlantic tropical wave is along 37W from 16N southward
and moving W at 10 to 15 kt. Associated
convection is described below.
A Caribbean tropical wave is near 71W from the central Dominican
Republic southward into NW Venezuela, and moving W near 10 kt.
Scattered showers and isolated thunderstorms are occurring over NW
Venezuela and northern Colombia, while no associated convection is
seen over water.
The monsoon trough exits the coast along the Liberia-Sierra Leone
border near 07N11.5W to 10N23W to low pres 1011 mb near 08N35W to
08.5N39W. The ITCZ then continues from 04.5N40W to 04N48W.
Scattered moderate to strong convection is flaring up near and S
of the monsoon trough off the African coast from 03N to 12N
extending westward to 22W, and is also seen from 04.5N to 09.5N
between 24W and 38W, and from 05N to 08.5N between 42W and 46W.
Scattered heavy showers and thunderstorms are occurring near the
coast of Honduras, Nicaragua, Costa Rica and Panama, and adjacent
waters between the eastern end of the E Pacific monsoon trough and
...GULF OF MEXICO...
A persistent surface trough curves northeastward from the central
Bay of Campeche to the N central Gulf near 27N91W. Scattered
showers and isolated thunderstorms are seen over both the central
Bay of Campeche and Gulf. Otherwise modest surface ridging along
with drier air at mid levels are supporting light to gentle winds
and seas of 2 to 4 ft across much of Gulf.
The aforementioned surface trough will drift westward toward the
Texas coast through Mon night. Otherwise, moderate winds prevail
north of the trough. High pressure presently over the area will
remain in place trough Thu, then be replaced by slighter stronger
high pressure that will build southward over the area Fri and Fri
night. This will lead to freshening of the winds over most sections
of the Gulf Fri and Fri night.
Refer to the Monsoon Trough/ITCZ section for convection in the
Caribbean Basin. Increasing surface pressure gradient between the
eastern end of the Pacific monsoon trough and the Atlantic ridge
is causing moderate to locally fresh trades with seas at 5 to 7 ft
across the central and S central basin. Light to gentle trades
and seas of 2 to 4 ft prevail for the rest of the basin.
Major Hurricane Sam is going to pass well E of the Windward
Islands Mon, and then well NE of the Leeward Islands during
midweek. Swells generated by Sam will cause seas to build across
the waters of the Windward Islands Mon, and the Leeward Islands
Tue. Meanwhile, moderate trades will continue over much of the
Caribbean into early next week. Fresh trades over the S central
Caribbean will diminish to gentle to moderate speeds Tue. Mainly
gentle trades will be over the rest of the Caribbean through Thu,
while fresh trades will follow in the wake of Sam across the
Tropical N Atlantic waters.
Please read the Special Features section above for details on
The Cumbre Vieja volcano on the island of La Palma in the Canary
Islands has been erupting since 19/1410 UTC. Significant
volcanic ash is mainly in the vicinity of the volcano below 5000
ft. Residual SO2 is also drifting to the NE. 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 issued no later than 27/0300 UTC.
A surface trough reaches northeastward from the central Bahamas
to SW of Bermuda near 29N72W. Scattered showers and thunderstorms
are occurring within 60 nm W and 180 nm SE of this feature,
including the NW and central Bahamas. A cold front curves
southwestward from a low pressure SW of the Azores near 31N31W
to 24N34W to near 23N44W. Scattered showers and isolated
thunderstorms are present up to 215 nm S and E of this boundary.
An area of moderate to fresh with locally strong SE to S winds
and seas at 5 to 7 ft are found SE of the low, N of 25N between
25W and 31W. Refer to the Tropical Waves and Monsoon Trough/ITCZ
sections for additional convection across the basin.
Other than the winds and seas SE of the aforementioned low, light
to gentle with locally moderate winds and seas of 4 to 5 ft are
noted N of 20N between 20W and the Georgia-Florida coast. Moderate
NE trades and seas at 5 to 6 ft are occurring near the Canary
Islands N of 20N between the NW African coast and 20W. Outside the
influence of Hurricane Sam, light to gentle with locally moderate
trades and seas of 4 to 6 ft exist from 10N to 20N between the
African coast and Lesser Antilles. Gentle to moderate monsoon
winds and seas at 5 to 7 ft prevail for the remainder of the
The surface trough extends from NE from the central Bahamas will
gradually shift eastward through Tue night. Farther east, major
Hurricane Sam located over the tropical N Atlantic waters is
forecast to turn NW on Mon and this motion should continue through
midweek, passing well NE of the Leeward Islands. It is anticipated
to slowly weaken as it reaches near 22N60.5W by Thu afternoon with
maximum sustained winds 110 kt gusts 135 kt.