[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 171004
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1205 UTC Fri Sep 17 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 0600 UTC surface analysis and satellite imagery through
0830 UTC.

...SPECIAL FEATURES...

Post-Tropical Cyclone Nicholas is centered near 31.1N 93.1W at
0900 UTC or 35 nm WSW of Alexandria Louisiana and moving N at 6
kt. Estimated minimum central pressure is 1010 mb. Maximum
sustained wind speed is 10 to 15 kt with higher gusts. Gentle to moderate
breezes and mostly slight seas are noted over the coastal waters
off the north-central Gulf coast. A surface trough extends west-
southwestward from Nicholas to the central Texas coast. Local
weather radar shows a few showers west of Alexandria. Nicholas is
expected to produce additional rainfall of 1 to 2 inches across
the central Gulf coast through the end of the week, with isolated
storm total amounts of 14 inches possible. Flash flooding impacts,
especially in urban areas, are possible across these regions.
Farther east, scattered showers and a few thunderstorms are active
within 120 nm off the eastern shore of Alabama and the western
Florida Panhandle. The Weather Prediction Center is issuing
bulletins on Nicholas and they can be found at
http://www.wpc.ncep.noaa.gov/tropical/tropical_advisories.php?storm=NICHOLAS


A broad area of low pressure is located about 100 miles southeast
of the Outer Banks of North Carolina. Although the circulation of
this system has become a little better defined since yesterday,
the associated showers and thunderstorms remain disorganized and
located mainly well to the east of the center. Environmental
conditions are becoming more conducive for development, and a
tropical depression is likely to form during the next day or so
while the system moves northward to north-northeastward off the
southeast and mid-Atlantic U.S. coasts. Regardless of development,
this system could bring high surf to portions of the southeast
and mid-Atlantic U.S. coasts through this weekend. Additional
information on this system, including gale warnings, can be found
in High Seas Forecasts issued by the NOAA Ocean Prediction
Center. Please read the latest Tropical Weather Outlook at
www.hurricanes.gov for more details.

Showers and thunderstorms remain disorganized in association with
a tropical wave and broad area of low pressure located about 1100
miles west-southwest of the Cabo Verde Islands. However,
environmental conditions are expected to be conducive for
development during the next few days, and a tropical depression
could still form over the weekend. This system is expected to move
westward to west-northwestward at 15 to 20 mph across the
tropical Atlantic during the next several days and has a medium
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...

A tropical wave is near 18W/19W from N Mauritania southward
into the Atlantic Ocean, moving W at 5 to 10 kt. Scattered
moderate to isolated strong convection is found from 09N to 14N
between 22W and 26W.

An Atlantic tropical wave is along 41W from 16N southward, moving
W near 15 to 20 kt. A 1010 mb low near 09N40W is associated with
this wave. Scattered moderate convection is evident from 12N to
16N between 40W and 45W. Earlier scatterometer and altimeter data
show moderate to fresh with locally strong S to SE winds and seas
at 5 to 6 ft to the SE of the low. Refer to the Special Features
section above for more information on development potential of
this low.

A Caribbean tropical wave is near 83W from 19N southward to
western Panama, and moving west near 10 kt. Scattered moderate
thunderstorms are found over the SW Caribbean Basin in a line from
central Panama to central Nicaragua.

...MONSOON TROUGH/ITCZ...

A monsoon trough extends across the coast of Guinea-Bissau near
11N16W through a low near 11N22W to another low near 09N40W. The
ITCZ reaches from near 09N40W to 06N50W. No significant convection
is noted other than the convection mentioned in the Tropical Wave
section above.

...GULF OF MEXICO...

For information on Post Tropical Cyclone Nicholas over SW
Louisiana, refer to the Special Features section above.

Modest convergent SSE to SW winds are coupling with upper-level
wind shear to trigger a few showers and thunderstorms within 120
nm off the western Florida Panhandle, and over the far
southeastern Gulf south of 25N and east of 90W. A weak pressure
pattern continues across the Gulf. Recent buoy observations and
scatterometer satellite data show mostly light to gentle breezes
accordingly across the Gulf, with moderate to fresh off the
northwest coast of the Yucatan Peninsula. Seas are generally 2 to
4 ft across the basin.

For the forecast, weak high pressure will extend across the
eastern and central Gulf of Mexico through early next week. This
pattern will support continued gentle winds and mostly slight seas
across the Gulf through the next several days.

...CARIBBEAN SEA...

A recent data from a scatterometer satellite pass and reports
from buoy 42058 near 14N75W indicated a large area of fresh winds
across the south-central Caribbean. Stronger winds are likely off
the coast of northeast Colombia. Trade wind convergence is
responsible for the line of showers and thunderstorms from central
Panama to central Nicaragua. A few showers are embedded in the
trade wind flow across the eastern Caribbean, but no significant
convection is noted otherwise. Roatan in the Bay Islands of
Honduras reported moderate to fresh winds, indicative of fresh to
strong winds pulsing across the Gulf of Honduras. Buoy and
altimeter satellite data confirm 6 to 7 ft seas over the south-
central Caribbean and 8 ft is plausible off Colombia. Moderate
seas are in place elsewhere except for slight seas in parts of the
far northwest Caribbean.

For the forecast, the Atlantic ridge will build into the
Caribbean through Sat, supporting fresh to strong winds over the
south- central Caribbean and over the Gulf of Honduras into early
Sun. The ridge will weaken starting Sun as an area of low pressure
currently well east of the area moves to the northeast of the
Leeward Islands early next week. There is a medium chance this low
pressure will form into a tropical cyclone over the next couple
of days. Regardless of further development, expect increase winds,
seas and thunderstorms east of the Leewards early next week.

...ATLANTIC OCEAN...

Ship and buoy data show generally light to gentle breezes north
of 25N and west of 60W, the result of a looser pressure gradient
brought about by the broad low pressure found well to the north of
the area. Seas are 3 to 5 ft in open waters. A few showers and
maybe thunderstorms are ongoing north of 30N between 65W and 70W,
on the southern periphery of a larger area of convection related
to the broad low north of the region. No significant convection is
noted elsewhere west of 60W. Farther east, a pair of surface
troughs are analyzed along 53W north of 25N and along 45W from 22N
to 27N. Scattered moderate to isolated strong convection is note
within 90 nm east of each trough. These surface troughs are
supported by a large mid/upper level trough extending from the
central Atlantic near 40N45W to 20N53W. Farther to the southeast
is the tropical wave and associated low pressure along 40W. So far
convection has been minimal near this low pressure area, but it is
moving into a more favorable environment once it reaches the
southeast side of this upper trough. A recent altimeter satellite
pass captured seas to 8 ft north of this low from 10N to 15N
between 35W and 40W, in an area of fresh trade winds. Broad
ridging dominates the subtropical waters farther east to the coast
of Africa, supporting fresh to strong NW winds off Western Sahara
and Mauritania with seas to 9 ft. Gentle to moderate trade winds
persist elsewhere east of 60W with 5 to 7 ft seas.

For the forecast west of 60W, weak ridging through the central
Bahamas will support mostly light breezes and slight seas north of
22N through early next week, and moderate trade winds and
moderate seas south of 22N at least through Sat. Meanwhile, an
area of low pressure currently well to the southeast over the
tropical Atlantic will approach the waters northeast of the
Leeward Islands Sun and Mon. There is a medium chance this low
pressure will form into a tropical cyclone over the next couple of
days. Regardless of development, expect increased winds, seas,
and thunderstorms northeast of the Leeward Islands Sun into Mon.

$$
Christensen
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