[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 171759
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1805 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 1200 UTC surface analysis and satellite imagery through
1700 UTC.

...SPECIAL FEATURES...

Recent satellite images indicate that a new and better-defined
center of circulation has developed in association with a low
pressure area located about 215 nm east of Norfolk Virginia. In
addition, shower and thunderstorm activity is becoming more
organized near this new center. If these development trends
continue, then a tropical depression or tropical storm is likely
to form later today or tonight while the low moves toward the
northeast or east-northeast at 10-15 kt, away from the United
States Mid-Atlantic and Northeast coasts. The low is expected to
transform into a non-tropical gale-force low Saturday or Saturday
night while it is located south of Atlantic Canada, and it is
likely to bring strong winds and heavy rains to portions of
Newfoundland by Sunday and Sunday night. This system is also
expected to bring high surf to portions of the Mid-Atlantic and
Northeast U.S. coasts and Atlantic Canada through this weekend.
The system has a high chance of tropical cyclone formation within
48 hours. Additional information on this system, including gale
warnings, can be found in High Seas Forecasts issued by the NOAA
Ocean Prediction Center. Refer to the latest NHC Tropical Weather
Outlook at www.hurricanes.gov for more details.

A tropical wave located midway between the Lesser Antilles and
Cabo Verde Islands has an axis that extends from 18N46W to a 1011
mb low pressure near 12N44W to 03N42W, moving W at 15-20 kt.
Scattered moderate convection from 12.5N-16N between 44W-47W has
become a little better organized since yesterday. Environmental
conditions are conducive for development during the next couple of
days, and a tropical depression is likely to form over the weekend
or early next week while the system moves west-northwestward at
15 kt across the central tropical Atlantic and then near the
northern Leeward Islands by Monday and Tuesday. Interests in the
Leeward Islands should monitor the progress of this system, which
has a high chance of tropical cyclone formation during the next
48 hours. Refer to the latest NHC Tropical Weather Outlook at
www.hurricanes.gov for more details.

...TROPICAL WAVES...

A tropical wave is along 21W from 19N southward, moving W at 15
kt. A broad 1012 mb low pressure is along the wave near 11N21W.
Disorganized scattered moderate convection is found from 05N-14N
between 20W-30W. Some gradual development is possible over the
weekend before the system encounters less favorable environmental
conditions next week. The system is expected to move toward the
west and then northwest at 5-10 kt over the far eastern Atlantic.
This system has a low chance of tropical cyclone formation within
the next 48 hr. Refer to the latest NHC Tropical Weather Outlook
at www.hurricanes.gov for more details.

A tropical wave is along 44W. See the Special Features section for
details.

A western Caribbean tropical wave is near 83W from 18N southward
to the Costa Rica/Panama border, moving W near 15 kt. Scattered
moderate to strong convection is found near where the tropical
wave intersects the monsoon trough, from 08.5N to 13.5N between
80W-85W, including near the coasts of western Panama, Costa Rica
and southern Nicaragua.

...MONSOON TROUGH/ITCZ...

The monsoon trough extends across the coast of Mauritania near
17N16W to 1011 mb low pressure near 11N21W to 11N34W to 08N43W.
The ITCZ is from 07N45W to 05N52W. Aside from the convection
mentioned in the sections above, scattered moderate convection is
east of 17W along the coast of Africa, from 11N-19N.

GULF OF MEXICO...

The weak 1012 mb remnant low of Nicholas is drifting northward
over west-central Louisiana, about 45 nm NW of Alexandria, at
1500 UTC. A surface trough extends from the low pressure to 30N94W
to 28N96W. Another surface trough extends from near Tallahassee to
near New Orleans. A mid to upper-level low centered near Dallas
Texas is helping to induce scattered to numerous moderate with
isolated strong convection over the north-central and NE Gulf of
Mexico north of 24.5N and east of 94W. A few additional showers
and tstorms are noted near the coast of northeastern Mexico from
22.5N-25N west of 96W. A recent ASCAT pass shows generally gentle
southerly winds across the eastern and western thirds of the
basin, with moderate S winds over the central Gulf. Seas are 2 to
4 ft, except near 5 ft in the Yucatan Channel.

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...

Convection associated with a tropical wave approaching Central
America is described above in the Tropical Waves section. Some
additional moderate convection is noted in the SW Caribbean south
of 10N between 76W-78W in association with the East Pacific
monsoon trough. An upper-level trough axis extending from the
central Bahamas to eastern Nicaragua could be helping to enhance
convection in the SW Caribbean. The remainder of the basin is
free of any significant areas of showers or thunderstorms, except
for isolated showers seen over the NE Caribbean, moving with the
trade wind flow. A recent ASCAT pass shows strong trades in the
south-central Caribbean from 11N-14N between 72W-77W, where seas
are 7 to 9 ft. Fresh trades are elsewhere over the central
Caribbean south of 17.5N between 70W-80W, where seas are likely 5
to 7 ft. NOAA buoy 42058 located near 14.4N 74.8W reported E wind
19 kt gusting to 23 kt with seas of 7.5 ft at 1700 UTC. Mainly
moderate trades prevail elsewhere with 3 to 5 ft seas, except for
moderate to fresh winds from the Gulf of Honduras northward to the
Yucatan Channel.

For the forecast, the Atlantic ridge will build modestly westward
through Sat, as low pressure offshore of the Carolinas lifts
northward. This will support fresh to strong winds over the south-
central Caribbean and Gulf of Honduras into early Sun. The ridge
will weaken starting Sun as a tropical wave and area of low
pressure currently across the central tropical Atlantic moves to a
position near the northernmost Leeward Islands early next week.
There is a chance this system could become a tropical cyclone
before it passes near the northern Leeward Islands.

ATLANTIC OCEAN...

An upper-level low is centered just east of the northwest Bahamas.
Some weak surface troughing is present offshore NE Florida, where
scattered moderate thunderstorms are noted from 29N-31N between
78W-80.5W. Some locally fresh to strong winds are possible within
the tstorms. Weak surface ridging prevails elsewhere across the
western Atlantic, leading to mostly gentle SE winds from 23N-30N
west of 65W. Moderate showers are noted north of 29N between
65W-68W, due to a tail of convection extending well SE from the
low pressure that is currently offshore of North Carolina. Seas
are 3 to 5 ft across the western Atlantic.

Farther east, an upper-level trough axis extends from 31N50W to
14N60W. An associated surface trough extends from 31N51W to 29N53W
to 25N53W. Scattered moderate convection is east of the trough
axis between 46W-53W, and north of 26N. Another surface trough
extends from 26N46W to 18N46W. A recent ASCAT pass indicates that
this is the same surface trough that is associated with the
tropical wave described above in the Special Features section,
that extends southward from 18N46W. Scattered moderate convection
is from 24N-30N between 41W-46W. Fresh to locally strong winds
are noted within the stronger showers and tstorms associated with
both troughs. Broad surface ridging prevails farther east in the
subtropical Atlantic. Fresh to locally strong NE trades prevail
between Western Sahara and the Cabo Verde Islands. Similar wind
speeds are seen on the latest ASCAT pass from 14N-23N between 25W-
45W, where seas are likely 7 to 9 ft. Moderate NE winds are north
of 23N between the Canary Islands and 40W, where seas are 6 to 7
ft.

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 over the central tropical Atlantic
will approach the waters near, or just northeast of the Leeward
Islands Sun and Mon. There is a high 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.

$$
Hagen
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