AXNT20 KNHC 261748
Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1805 UTC Sun Sep 26 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
Hurricane Sam, a category 4 hurricane on the Saffir-Simpson Wind
Scale is centered near 13.9N 50.2W at 26/1500 UTC or 785 nm ESE
of the Northern Leeward Islands, and moving WNW at 7 kt. Estimated
minimum central pressure is 943 mb. Maximum sustained winds are
125 kt with gusts to 150 kt. Seas are peaking at 25 to 28 ft near
the center. An area of strong to gale winds with seas at 15 to 20
ft is seen SE to NE to N of the center from 12N to 16N between 47W
and 49W. Numerous moderate to strong convection is seen within 80
nm of the center. Scattered moderate convection is found farther E
and SE from 11N to 14N between 47W and 49W. 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 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.
An Atlantic tropical wave is near the Cabo Verde Islands along
25W from 17N southward, and moving W near 15 kt. Scattered
moderate convection is noted from 05N to 12N between 20W and 30W.
Another Atlantic tropical wave is along 36W from 16N southward
and moving W at 10 to 15 kt. Scattered moderate convection is
observed from 05N to 11N between 30W and 38W.
A Caribbean tropical wave is near 70W from the central Dominican
Republic southward into NW Venezuela, and moving W near 10 kt.
Scattered showers and isolated thunderstorms are occurring over NW
A monsoon trough passes through the coast of Guinea-Bissau near
12N16W to 07N38W. The ITCZ then continues from 07N38W to 06N46W.
Scattered moderate to isolated strong convection is flaring up
near and S of the monsoon trough along the African coast from 03N
to 16N. Scattered moderate convection is found near the ITCZ from
04N to 09N between 38W and 47W.
The eastern end of the E Pacific monsoon trough is triggering
scattered heavy showers and thunderstorms near the coast of
Honduras, Nicaragua, Costa Rica and Panama, and adjacent waters.
...GULF OF MEXICO...
A persistent surface trough curves northeastward from the central
Bay of Campeche to the N central Gulf. 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.
For the forecast, the aforementioned surface trough is expected to
maintain scattered showers and a few thunderstorms for the next
couple of days. The trough will drift westward toward the Texas
coast through Mon night. Otherwise, moderate winds prevail N of
the trough. Winds and seas will diminish across the basin through
Mon as high pressure builds over the N Gulf. The high pressure
will change little through Thu night.
Refer to the Monsoon Trough/ITCZ section for convection in the
Caribbean Basin. Increasing 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.
For the forecast, 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 rise
near the Windward Islands Mon, and near 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 26/2100 UTC.
A surface trough reaches northeastward from the central Bahamas
to SW of Bermuda near 29N72W. Scattered showers and thunderstorms
are occurring within 90 nm along neither side of this feature,
including the NW and central Bahamas. Another surface trough
curves north-northeastward from well N of Puerto Rico at 24N65W
through a low pressure centered at 27N62W to beyond 31N at 62W.
Numerous moderate 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. 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
For the forecast, a surface trough extends from near 30N72W to
the central Bahamas and to 22N77W. Scattered showers and
thunderstorms are along and near this trough. The trough will move
little today, then gradually shift eastward through Tue night.
Farther east, major Hurricane Sam located over the tropical N
Atlantic waters near 13.9N 50.2W 943 mb at 26/1500 UTC is moving
WNW at 7 kt. Maximum sustained winds are 125 kt with gusts 150
kt. Sam is forecast to turn NW on Mon and this motion should
continue through midweek. It is anticipated to slowly weaken
as it reaches near 21N60W by early on Thu with maximum sustained
winds 110 kt gusts 135 kt.