[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 242335
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
0005 UTC Sat Sep 25 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
2220 UTC.

...SPECIAL FEATURES...

Hurricane Sam is centered near 12.1N 44.8W at 2100 UTC, or about
1120 nm ESE of the Northern Leeward Islands, moving W at 10 kt.
Estimated minimum central pressure is 986 mb. Maximum sustained
wind speed is 75 kt with gusts to 90 kt. Satellite imagery shows
improving convective structure, with an eye attempting to develop
and well pronounced tightly coiled outer rain bands. Scattered
strong convection within the central dense overcast feature is
within 45 nm of center of Sam. Scattered moderate to strong
convection is 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 estimated near 22 ft. A decrease in forward speed
and a motion toward the west-northwest is expected over the
weekend. Rapid intensification is forecast to continue, and Sam is
likely to become a major hurricane on Sat. Swell associated with
Sam is expected to reach the Lesser Antilles Sun evening. 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 imagery indicates that
the low pressure center located 135 nm north of Bermuda has
become subtropical, and has been classified as Subtropical Storm
Teresa. Teresa is centered near 34.5N 64.5W at 2100 UTC, moving NW
at 12 kt. Maximum sustained winds are estimated at 40 kt with
gusts to 50 kt, as depicted by late morning ASCAT wind data.
Estimated minimum central pressure is 1008 mb. Scattered moderate
isolated strong convection is seen removed from the center, and
occurring within 180 nm across the N and 360 nm across the NE
quadrants. Teresa is expected to turn northward on Sat and then
northeastward, ahead of an extratropical storm system that is
forecast to develop off of New England this weekend. This will
allow limited opportunity for Teresa to strengthen slightly during
the next 48 hours before it become absorbed in this developing
system. Please see the latest High Seas Forecasts issued by the
Ocean Prediction Center at website
https://ocean.weather.gov/shtml/NFDHSFAT1.php for more details on
Teresa.

...TROPICAL WAVES...

A far eastern Atlantic tropical wave has its axis along 27W from
04N to 19N, moving westward at 10 kt. Widely scattered moderate
convection is seen from 03.5N to 11.5N between 21W and 28W. Per
the 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 60W from 07N
to 19N, moving westward at 10-15 kt. Scattered showers and
isolated thunderstorms are seen from 11N to 13.5N between 55W and
60W. Very dry and stable middle level atmospheric conditions
prevail north of 15N across the region, and is limiting
convection.

...MONSOON TROUGH/ITCZ...

The monsoon trough axis passes through coast of Senegal near
13N16.5W to 06.5N28W to 07N33W. A segment of the ITCZ extends
from 07N46W to 06N53W. Aside from convection associated to the
tropical waves and Sam described above, scattered moderate
isolated strong convection is seen from 05N to 09N between 30W
and 45W.

GULF OF MEXICO...

A quasi-stationary frontal boundary extends from southwestern
Florida to 23.5N90W and to near Veracruz, Mexico. Satellite
imagery shows scattered showers and thunderstorms along and within
60 nm either side of the boundary west of 84W. 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
moderate to locally fresh northeast winds behind the front east
of 91W, and gentle northeast to east winds west of 91W. Light and
variable winds are south of the frontal boundary. Seas across the
basin are 3-6 ft, except for slightly lower seas of 1-3 ft over
the southeastern Gulf.

This quasi-stationary frontal boundary will dissipate by late
Sat. Moderate to fresh winds prevail 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 86W from 17N-22N. Scattered to numerous
showers and few thunderstorms prevail across much of the SW
Caribbean, south of 15N and west of 77W to coastal sections of
Nicaragua. Expect for this area to remain convectively 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 4-5 ft over the
waters north of the A-B-C Islands and Gulf of Venezuela.

Hurricane Sam across the central tropical Atlantic will
strengthen to a major hurricane Sat afternoon near 13N48W and move
to near 15N53W Mon afternoon. Sam is forecast to enter the
offshore waters east of Leeward Islands Tue near 17N55W and move
to near 19N58W Wed. Meanwhile, moderate trade winds 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.

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.

A cold front extends from near 31N76.5W to 287N78.5W, 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 to strong convection
is within 240 nm southeast of the front, from 25N across the
Bahamas northward to 31N. 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
31N61W to a 1011 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. Morning 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.

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 are 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.

The front across the NW waters to Stuart, Florida will remain
stationary and weaken through early Sun. Then, reinforcing
moderate N winds will push the front eastward as a weak cold
front, reaching from 31N69W to 27N74W by Mon morning. The front
will stall and dissipate Tue. Farther east, Hurricane Sam will
strengthen to a major hurricane Sat afternoon near 13N48W and move
to near 15N53W Mon afternoon. Sam will move to near 17N55W Tue
and to near 19N58W Wed. Swell associated with Sam will reach 55W
early Sun, 60W Sun evening and begin to pass north of the Virgin
Islands and Puerto Rico Mon morning.

$$
Stripling
This weather bulletin was brought to you by:
www.nashvilleweather.net