[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 241805
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1805 UTC Fri Sep 24 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
1745 UTC.

...SPECIAL FEATURES...

Hurricane Sam is centered near 11.8N 43.7W at 24/1500 UTC or
about 1280 nm ESE of the Northern Leeward Islands moving
W at 12 kt. Estimated minimum central pressure is 993 mb. Maximum
sustained wind speed is 65 kt with gusts to 80 kt. Satellite
imagery shows quite a healthy system with well pronounced tightly
coiled outer rain bands. Scattered moderate to strong convection
within the central dense overcast feature are within 45 nm of
center of Sam. Scattered moderate to strong convection are
with the outer rain bands within 30 nm of a line from 11N42W
to 13N43W and to 12N45W. Similar convection in another outer
rain band is within 30 nm of a line from 12N46W to 10N44W. Peak
seas are currently near 21 ft, but are forecast to reach near 34
ft Sun. A decrease in forward speed and a motion toward the west-
northwest are expected over the weekend. Rapid intensification is
forecast to continue, and Sam is likely to become a major
hurricane on Sat. Please read the latest HIGH SEAS FORECAST issued
by the National Hurricane Center at website-
https://www.nhc.noaa.gov/text/MIAHSFAT2.shtml and the latest Sam
NHC Forecast/Advisory and Public Advisory at www.hurricanes.gov
for more details.

Surface observations and recent satellite wind data indicate that a
surface low located a couple hundred miles north-northeast of
Bermuda is gradually becoming better defined as it interacts with an
upper-level trough. This system is producing winds of 40 to 45 mph
north of the low center and additional development into a
subtropical or tropical storm appears likely. Advisories could be
initiated as soon as this afternoon as the system moves slowly
toward the north-northwest over the next day or so. This system
has a high chance of tropical cyclone formation during the next
48 hours. Additional information on this system, including gale
warnings, can be found in High Seas Forecasts issued by the Ocean
Prediction Center ocean.weather.gov. Also see the latest Tropical
Weather Outlook at www.hurricanes.gov for more details.

...TROPICAL WAVES...

A far eastern Atlantic tropical wave has its axis along 26W from
04N to 19N. It is moving westward at 10 kt. Scattered moderate
to isolated strong convection is within 180 nm east of the wave
from 09N to 12N. Per latest GOES-16 SAL imagery, the northern
part of the wave remains embedded in dry Saharan air, inhibiting
the development of shower and thunderstorm activity.

A central Atlantic tropical wave has its axis near 58W from 07N
to 19N. It is moving westward at 10-15 kt. Scattered showers
and isolated thunderstorms are seen from 11N to 13N between 56W-
59W. This is where the TPW animation imagery indicates a maximum
in moisture content.

...MONSOON TROUGH/ITCZ...

The monsoon trough axis passes through coast of Senegal near
14N17W to 06N28W. A segment of the ITCZ extends from 06N28W to
08N36W and another segment continues from 07N46W to 06N55W. Aside
from convection associated to the tropical waves and Sam
described above, scattered moderate convection is within
120 nm north of the ITCZ between 29W-32W and within 60 nm
north of the ITCZ between 32W-34W.

GULF OF MEXICO...

As of 15Z, a quasi-stationary frontal boundary extends from
southwestern Florida to 24N90W and to Veracruz. Satellite imagery
shows scattered showers and thunderstorms along and within 30 nm
either side of the boundary west of 85W. This activity is being
aided by a mid to upper-level shortwave trough. The air mass
behind the front is quite dry, allowing fairly tranquil weather
conditions front Texas to Florida in the N Gulf. Recent
scatterometer satellite data and current buoy observations
reveal gentle to moderate northeast winds behind the front east
of 94W, and gentle northeast to east winds west of 94W. Light and
variable winds are south of the frontal boundary. Seas across the
basin are 2-4 ft, except for slightly lower seas of 1-3 ft over
the southeastern Gulf.

For the forecast, the quasi-stationary frontal boundary will
dissipate by Sat. Fresh winds prevail within 90 nm north of the
front. Winds and seas will diminish across basin from Sat night
and into early next week as high pressure builds over the northern
Gulf.

CARIBBEAN SEA...

A small upper-level low near the southeastern Bahamas has an
associated trough that reaches to the eastern tip of Cuba and
to the southwestern Caribbean. A surface trough is analyzed over
the western Caribbean near 85W from 17N-22N. Some reflection of
this feature is noted at 700 mb. Scattered showers and a few
thunderstorms are occurring near the southern part of the trough
and along and just inland the northeast section of Honduras.
Isolated thunderstorms are west of the trough. Increased scattered
moderate to isolated strong convection is over the southwestern
Caribbean and south-central Caribbean sections, especially near
the northern coast of Colombia. This activity is being further
enhanced by the eastern segment of the eastern Pacific monsoon
trough that stretches east to northern Colombia and to some extent
by instability created by the southern part of the aforementioned
upper-level trough. Expect for this activity to remain quite
active through early Sat. Partial ASCAT data passed from this
morning indicate generally gentle to moderate trades across the
basin, with the majority of the gentle trades confined to the NW
Caribbean and to the northern sections of the central and eastern
Caribbean. Seas across the area remain relatively low, in the 2-3
ft range, except for slightly higher seas of 3-4 ft over the
southwestern Caribbean.

For the forecast, Hurricane Sam is near 11.8N 43.7W 993 mb at 11
AM EDT moving W at 12 kt. Maximum sustained winds are 65 kt with
gusts to 80 kt. Sam will strengthen to a major hurricane Sat eve
near 13N48W and move to near 15N52W Mon morning. Sam is forecast
to enter the offshore waters east of Leeward Islands Tue near
17N55W and move to near 18N58W Wed morning. Meanwhile, moderate
trades will persist over much of the Caribbean into early next
week, becoming fresh over the south-central Caribbean starting
tonight as high pressure builds east of the Bahamas. Fresh to
locally strong winds are forecast in the Gulf of Honduras Sun
night.

ATLANTIC OCEAN...

Please read the sections above for details on Hurricane Sam and
tropical waves moving across the basin.

The Cumbre Vieja volcano on the island of La Palma in the Canary
Islands has been erupting since 19/1410 UTC. Pilots have reported
volcanic ash near Tenerife and over La Gomera. The height of the
ash plume cloud is difficult to estimate due to cloud cover.
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 issue no later than 24/2100 UTC.

As of 15Z, a cold front extends from near 31N76W to 27N79W, and
transitions to a stationary frontal boundary to Stuart, Florida
and to southwest Florida. A pre-frontal trough is within 30-60 nm
ahead of the cold front. Scattered moderate convection is from 27N
to 31N between the front/pre-frontal trough and 74W. Moderate to
fresh southerly winds are east of the trough and front to near 75W
and north of 26N. Seas with these winds are in the 4-6 ft range.

Another area of interest is a sharp surface trough extending from
31N60W to 25N65W and to a 1010 mb low near 24N66W. The
interaction of these features with a large upper-level low near
Bermuda is inducing a large area of showers and thunderstorms,
especially from N of 20N and between 56W and 64W. Latest
ASCAT wind data indicate fresh to strong cyclonic winds near the
northern end of the surface trough north of 28N. Seas in this
region are 5-8 ft, with the highest seas noted near the highest
winds.

Farther east, a 1010 mb low is analyzed near 29N40W. A large area
of scattered moderate to strong convection is present north
of 27N between 33W-40W. This convection is shifting eastward.
A stationary front stretches from 31N43W to 29N47W. Scattered
showers and thunderstorms have recently developed along this
boundary. Moderate to fresh northerly winds were captured by
ASCAT off the coast of Mauritania, mainly east of 20W. A large
area of northerly swell that is producing 8-11 ft seas N of 22N
and between 35W and 60W. Moderate to fresh trade winds are noted
elsewhere across the basin along with 4 to 6 ft seas in open
waters.

For the forecast west of 65W, the cold front extending from
31N76W to 27N79W will stall early this afternoon then dissipate
through early Sun just before a second weak front possibly moves
into the region late Sun. Farther east, Hurricane Sam is near
11.8N 43.7W 993 mb at 11 AM EDT moving W at 12 kt. Maximum
sustained winds are 65 kt with gusts to 80 kt. Sam will strengthen
to a major hurricane Sat evening near 13N48W and move to near
15N52W Mon morning. Sam is forecast to move to near 17N55W Tue and
to near 18N58W Wed morning.

$$

Aguirre
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