[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 152107
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
0005 UTC Thu Sep 16 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
2040 UTC.

..SPECIAL FEATURES...

Tropical Depression Nicholas is centered near 30.2N 92.4W at
15/2100 UTC or 70 nm S of Alexandria Louisiana moving E at 2 kt.
Estimated minimum central pressure is 1006 mb. Maximum sustained
wind speed is 25 kt with gusts to 35 kt. The center of Nicholas
is largely devoid of deep convection, with most of the activity
located over the Mississippi Valley spreading into the remainder
of the southeastern U.S. A trailing trough is over the N-central
and western Gulf from 30N90W to 24N97W. Scattered moderate and
isolated strong convection is noted within 180 nm E-SE of the
trough over the basin, where winds are also moderate to fresh and
seas are in the 3 to 5 ft range. The depression is moving very
slowly to the east, and the center will continue to make only
slow/erratic progress overnight and Thu before a gradual turn
north. The center of the tropical depression may become ill-
defined in the next 36 to 48 hours as it becomes a remnant low
with post-tropical character. Even so, heavy rainfall will
continue as tropical moisture persists across the Gulf Coast
region. Nicholas is expected to produce additional rainfall
amounts of 2 to 5 inches across the central Gulf coast in
southeastern Louisiana, southern Mississippi, southern Alabama and
the Florida Panhandle through Fri, with isolated storm totals of
8 inches possible. Considerable flash flooding impacts, especially
in urban areas, are possible across these regions. Widespread
minor river flooding is expected, while scattered moderate river
flooding is possible, across portions of southeastern Louisiana,
southern Mississippi, southern Alabama, and the Florida Panhandle.
The Weather Prediction Center is using the advisories on Nicholas
and they can be found at http://www.wpc.ncep.noaa.gov.

A surface trough extends from 31N72W to broad 1010 mb low pressure
near 18.5N73W, or a few hundred nm northeast of the central
Bahamas, to 23N76W. Current associated winds are 20 to 30 kt from
27N to 31N east of the low and trough to 70W. Satellite images
indicate that the low is gradually becoming better defined,
however the associated showers and thunderstorms are still
disorganized. Scattered moderate and isolated strong convection is
noted across a wide area from 20N to 31N between 63W and the
trough. Environmental conditions are expected to become more
conducive for development, and a tropical depression is likely to
form during the next day or two while the system moves north-
northwestward to northward off the southeast U.S. coast.
Regardless of development, this system could bring high surf to
portions of the southeast and mid-Atlantic U.S. coast later this
week, and increasing winds and building seas are expected north
of 27N between 68W and 75W overnight. This system has a high
chance of tropical cyclone formation in the next 48 hours.
Please read the latest Tropical Weather Outlook at
www.hurricanes.gov for more details.

A tropical wave in the Atlantic Ocean along 27W/28W from 03N to
17N, with 1009 mb low pressure along the wave near 10.5N27.5W, or
a few hundred nm southwest of the Cabo Verde Islands, moving west
at around 15 kt. Current associated winds with the low are 20 to
30 kt, with seas of 5 to 7 ft. Showers and thunderstorms have
changed little in organization, with a broad area of scattered
moderate and isolated strong convection noted from 08N to 15N
between the tropical wave and 34W. Environmental conditions are
expected to remain conducive for development, and a tropical
depression is likely to form within the next couple of days. This
system is expected to move westward to west-northwestward across
the tropical Atlantic during the next several days. This system
has a high chance of tropical cyclone formation in the next 48
hours. Please read the latest Tropical Weather Outlook at
www.hurricanes.gov for more details.

...TROPICAL WAVES...

Refer to the Special Features section above for details on a
tropical wave and associated area of low pressure in the Atlantic
Ocean along 27W/28W.

Another tropical wave is expected to emerge off of the west coast
of Africa in a day or two. Thereafter, environmental conditions are
forecast to be conducive for development while the system moves
generally west-northwestward to northwestward over the far eastern
Atlantic.

A tropical wave is in the Atlantic Ocean along 45W, from 04N to
17N, moving west at around 10 kt. Scattered moderate and isolated
strong convection is noted near the wave and the nearby monsoon
trough from 08N to 13N between 37W and 45W.

A tropical wave is in the Caribbean Sea along 73W from 20N
southward across Haiti to the Colombia-Venezuela border, moving
west at around 15 kt. The tropical wave combined with a mid to
upper level low and daytime heating are helping to induce
scattered showers and thunderstorms across the greater Antilles
and adjacent waters.

A tropical wave is in the NW Caribbean Sea along 86W from 18N
southward across Honduras and western Nicaragua and into the
eastern tropical Pacific Ocean, moving west at around 15 kt. No
significant convection is noted over the NW Caribbean Sea, however
the tropical wave is helping to support isolated to scattered
showers and thunderstorms over portions of Central America.

...MONSOON TROUGH/ITCZ...

The monsoon trough emerges off the coast of Africa near the
Mauritania-Senegal border 16.5N16W to 1009 mb low pressure near
10.5N27.5W to 08N33W to 10N39W to 07N51W. The ITCZ continues from
07N51W to 08.5N59W. Aside from the convection mentioned above,
scattered moderate and isolated strong convection is noted from
07N to 10N between 14W and 19W, and from 06N to 07N between 36W
and 39W. Scattered moderate convection is noted from 02N to 05N
between 21W and 24W, and from 01N to 06N between 31W and 37W.

GULF OF MEXICO...

Refer to the Special Features section above for details on
Tropical Depression Nicholas.

Gentle to moderate SE-S winds are noted elsewhere in the basin
east of the trough mentioned south of Nicholas, with gentle to
moderate NW-N winds west of the trough toward the Texas coast.
Seas are 3 ft or less east of 87W, and 2 to 4 ft west of 87W
outside of the influence of Nicholas. Isolated to widely scattered
showers and thunderstorms are noted over the eastern Gulf.

For the forecast, Tropical Depression Nicholas inland north of
the area is forecast to become a remnant low Thu. Conditions will
continue to gradually improve offshore the Louisiana and upper
Texas coasts, but moderate to fresh winds and seas of 3 to 5 ft
can be expected into tonight. For the remainder of the week,
gentle to moderate mainly southerly flow will dominate the basin.

CARIBBEAN SEA...

Refer to the Tropical Waves section above for details on the
tropical waves moving across the Caribbean Sea.

Moisture in the SW Caribbean Sea associated with the east Pacific
monsoon trough results in scattered moderate convection south of
11N between the coast of Colombia and Costa Rica. The rest of the
basin experiences fairly tranquil weather conditions outside of
some isolated thunderstorms between 76W and 85W. A relatively weak
pressure gradient exists across the region resulting in moderate
to fresh trades, except light to gentle in the SW Caribbean as
well as in the lee of Cuba outside of any convection. Seas are in
the 2 to 4 ft range in the eastern and western Caribbean, and 3 ft
or less elsewhere outside of any convection.

For the forecast, gentle to moderate winds will increase to
moderate to fresh by Thu, with strong winds pulsing during the
overnights into the weekend offshore Colombia and in the Gulf of
Honduras.

ATLANTIC OCEAN...

Refer to the Special Features section above for details on the
disturbance northeast of the Bahamas, and the system in the far
eastern Atlantic.

Otherwise, the subtropical ridge extending from near the Azores
to near Bermuda dominates most of the tropical Atlantic. A surface
trough is located in the central Atlantic from 31N43W to 25N45W
and there are a few showers near the trough axis. Another surface
trough is located from 23N35W to 14N36W and it is devoid of any
significant convection. Scattered moderate and isolated strong
convection is noted northeast of the NE Caribbean islands from 16N
to 20N between 57W and 60W, associated with a mid to upper level
feature. The rest of the basin is fairly quiet. Fresh to strong
N-NE winds are present off the coasts of Morrocco and Mauritania,
mainly east of 20W, due to the pressure gradient caused by the
ridge and lower pressures over western Africa. Seas are in the 6
to 9 ft range across this area. Moderate to fresh trades are noted
elsewhere north of the convergence zone and east of 35W, along
with seas in the 5 to 7 ft range. Gentle to moderate winds and
seas of 3 to 6 ft can be found in the rest of the basin.

For the forecast west of 55W, outside of the area of low pressure
northeast of the Bahamas, weak high pressure will lead to gentle
to moderate mainly easterly winds across the basin.

$$
Lewitsky
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