[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 270554
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
0605 UTC Mon Sep 27 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 0000 UTC surface analysis and satellite imagery through
0535 UTC.

...SPECIAL FEATURES...

Hurricane Sam remains a powerful tropical cyclone, but the
anticipated fluctuations in intensity have started. It is
centered near 14.7N 50.8W at 27/0300 UTC, or 740 nm ESE of the
Northern Leeward Islands, moving NW at 6 kt. Estimated minimum
central pressure is 940 mb. Maximum sustained winds are 125 kt
with gusts to 150 kt. Peak seas are estimated at 35 to 40 ft just
north of the center and similar seas are expected over the next
day or two. Satellite imagery this evening indicate that the eye
has become covered by convection and there has been an erosion of
the western eyewall. Indications are that Sam is undergoing an
eyewall replacement, which will cause the intensity to fluctuate.
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. Sam is moving toward the northwest,
and this motion is expected to continue for the next few days,
along with a gradual increase in forward speed beginning around
midweek. Sam is expected to pass well NE of the Leeward Islands on
Wed and Thu while accelerating toward the NW. Some fluctuations in
intensity are expected during the next day or so. Thereafter,
slow weakening is forecast. However, Sam is still expected to
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, associated with the remnants of Peter, is located
several hundred miles SE of Bermuda. Recent scatterometer satellite
data show an elongated area of low pressure producing fresh to strong
southerly winds. Scattered showers and a few thunderstorms are present
within 150 nm of the low pressure, mainly in the E quadrant due to
strong wind shear. The disturbance has become a little better organized
since last night, and environmental conditions are marginally conducive
for some further development of this system. Peter could briefly become
a tropical depression again during the next day or two 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.

...TROPICAL WAVES...

An Atlantic tropical wave has its axis along 28W, south of 18N,
and it is moving W near 15 kt. Interaction between the wave and
the monsoon trough allows for scattered moderate to isolated
strong convection from 03N to 09N and between 25W and 30W. The
northern portion of the wave is devoid of deep convection.

Another Atlantic tropical wave has its axis along 38W, south of
17N and it is moving W at 10-15 kt. Interaction between the wave
and the monsoon trough results in scattered moderate convection E
of the wave axis to 30W and between 04N and 11N. The northern
portion of the wave is devoid of deep convection.

A Caribbean tropical wave has its axis along 72W, south of 20N,
extending from Haiti to W Venezuela and it is moving W near 10
kt. The wave is enhancing the thunderstorm activity over
Hispaniola. No deep convection is noted near the wave axis over
the Caribbean Sea.

...MONSOON TROUGH/ITCZ...

The monsoon trough enters the Atlantic through the coast of The
Gambia near 13N17W to 08N25W to 09N32W and to 08N39W. The ITCZ
then continues from 08N40W to 09N45W. Scattered moderate to
strong convection is observed south of the monsoon trough from 03N
to 08N and between 23W and the coast of Africa.

The E Pacific monsoon trough enters the Caribbean Sea through the
coast of Costa Rica near 11N84W and continues to the coast of NW
Colombia. Scattered moderate to isolated strong is seen within 120
nm to the north of the monsoon trough.

...GULF OF MEXICO...

A tenacious surface trough remains in the W Gulf, extending from
28N91W to 21N94W, and a few showers continue to flare up near the
trough axis. A few showers are also noted over the E Bay of
Campeche and off W Florida. However, this activity is quickly
losing strength and should dissipate soon. The rest of the basin
is under the dominance of dry air, leading to fairly tranquil
weather conditions. A recent scatterometer satellite pass indicate
fresh NE winds in the Florida Straits, while gentle to moderate
anticyclonic winds prevail in the remainder of the Gulf. Seas in
the SE Gulf are 2-4 ft, with the highest seas occurring in the
Florida Straits. Elsewhere, seas are 1-3 ft.

The aforementioned surface trough will drift westward toward the
Texas coast through Mon night. Moderate winds will prevail north
of the trough through Mon morning. 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 bring moderate to fresh winds
over most of the Gulf on Fri through Sat.

...CARIBBEAN SEA...

Favorable upper-level atmospheric dynamics and diurnal heating in
the northern Caribbean allowed for the development of afternoon
and evening thunderstorms across the Greater Antilles. The storm
activity is decreasing at this hour, but some of the storms have
moved south and offshore and are affecting the waters within 60
nm of Puerto Rico and the Dominican Republic. A few showers are
also seen in the E Caribbean, near the Lesser Antilles, while
fairly tranquil conditions prevail in the rest of the basin.
Recent satellite-derived wind data show fresh trades within 100 nm
of the coast of Colombia and NW Venezuela, including the waters
surrounding the ABC islands. Moderate to locally fresh trades are
present elsewhere in the Caribbean Sea. Sea of 3-6 ft are
occurring in the central Caribbean, with the highest seas
materializing in the south-central and SW Caribbean. Elsewhere,
seas are 1-3 ft.

Hurricane Sam near 14.7N 50.8W 943 mb at 11 PM EDT moving NW at 6
kt. Maximum sustained winds 125 kt gusts 150 kt. Sam will move to
15.3N 51.6W Mon morning, 16.2N 52.8W Mon evening, 17.0N 53.9W Tue
morning, 17.8N 55.0W Tue evening, 18.7N 56.3W Wed morning, and
20.0N 58.0W Wed evening. Sam will change little in intensity as it
moves near 23.4N 61.0W late Thu. Meanwhile, moderate trades will
continue over much of the Caribbean through Tue. Fresh trades over
the south- central Caribbean will diminish to gentle to moderate
speeds by 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. Swells generated by
Sam are forecast to reach the Lesser Antilles by late Mon into
Tue.

...ATLANTIC OCEAN...

Please read the Special Features section above for details on
Hurricane Sam.

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. ANother cloud mainly composed of SO2 is 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 27/0900 UTC.

A cold front traversing the NW Atlantic enters the tropical
Atlantic near 31N69W and extends to 28N77W, about 100 nm NE of
the NW Bahamas. A few showers are observed near the frontal
boundary. Nearby, a surface trough extends from 30N69W to 24N77W
in the central Bahamas. Moderate scattered convection is seen on
satellite imagery within 100 nm of the trough axis. Satellite-
derived wind data depict moderate to fresh cyclonic winds
surrounding the trough, with the highest winds occurring near the
convection.

Farther east, a cold front extends from a 1012 mb low pressure SW
of the Azores and enters the tropical Atlantic near 31N29W and
transitions into a stationary front that extends to 23N34W and to
22N45W. The convection near the boundary is quite meager,
consisting of a few showers and isolated thunderstorms. However,
satellite-derived wind data indicate that fresh to strong
southerly winds are occurring N of 25N and within 150 nm to the E
of the frontal boundary. The scatterometer satellite data also
depict fresh to locally strong NE winds N of 22N and E of 20W,
with the strongest winds occurring near the coast of NW Africa.
Outside of the influence of Hurricane Sam, the rest of the basin
experiences moderate or weaker winds. Seas are 4-8 ft, with the
highest seas occurring near the areas described above.

A cold front is pressing southward off the Florida coast with a
pre-frontal trough stretching from 30N69W to 24N77W. This front
will weaken by late Mon into early Tue. Expect showers and
thunderstorms along this boundary through Mon night. Gentle to
moderate winds will prevail through most of the week. Stronger
high pressure will build southward over the NW and north- central
offshore waters late in the week, bringing moderate to fresh winds
especially off the Florida coast. Hurricane Sam near 14.7N 50.8W
943 mb at 11 PM EDT moving NW at 6 kt. Maximum sustained winds 125
kt gusts 150 kt. Sam is forecast to continue on its current
motion over the next few days as it slowly weakens reaching near
28N63W by late Fri, with maximum sustained winds 105 kt gusts 130
kt. Swells generated by Sam are expected to spread westward across
the forecast waters late in the week.

$$
DELGADO
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