National Hurricane Center - Tropical Weather Discussion (Atlantic)
AXNT20 KNHC 201452 AAA
Tropical Weather Discussion...Updated
NWS National Hurricane Center Miami FL
1052 AM EDT Thu Sep 20 2018
Updated Caribbean section for SW Caribbean convection
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 32N. The following information is based on satellite
imagery, weather observations, radar and meteorological analysis.
Based on 0600 UTC surface analysis and satellite imagery through
...TROPICAL WAVES/TROPICAL TROUGHS...
A far eastern Atlantic tropical wave has its axis along 21W/23W
from 05N25N to 14N24W to 20N23W, moving westward at 10 to 15 kt.
This wave is supported by a 700 mb inverted-trough from 14N to
20N. om 20N southward. This wave is associated with a distinct 700
mb trough from 13N to 20N. In addition, mid-level satellite
winds show winds veering in direction from NE to SE across the
wave axis. The total precipitable water (TPW) satellite animation
shows a well-depicted plume of moisture trailing the wave to the
coast of Africa.
A far eastern Atlantic surface trough is analyzed from 10N31W to
19N30W, moving westward at about 10 kt. The trough continues
to be reflected at 700 mb as a rather distinct inverted trough.
700 mb trough. No deep convection is seen with the trough, only
isolated showers and thunderstorms within 60 nm either side of the
A 1010 mb low, formerly a tropical wave, is analyzed near 10N48W,
moving westward at 10 to 15 kt. The overall cloud pattern of this
low appears more symmetric than 18 to 24 hours ago. Satellite
imagery shows scattered moderate to strong convection within 120
nm of the low in the northern semicircle. Scattered moderate
convection is within 90 nm of the low in the southern semicircle.
Latest NHC Tropical Weather Outlook calls for some development
of this system today before before upper-level winds become
highly unfavorable for tropical cyclone formation starting tonight
and continuing through the weekend.
An eastern Caribbean Sea tropical wave has its axis along 69W
south of 20N to inland Venezuela at 09N. It is moving westward at
10 to 15 kt. Satellite imagery shows a rather large area of
scattered showers showers and thunderstorms east of the wave north
of 15N and between the wave and 65W. The TPW animation concurs
with this shower and thunderstorm activity in depicting low-level
over the eastern Caribbean Sea. Satellite imagery indicates some
Saharan dust in the wake of the wave over the far eastern
Caribbean Sea and tropical Atlantic between the Lesser Antilles
and 55W. The dust is forecast to continue to spread westward,
across the eastern Caribbean Sea islands, bringing hazy skies and
stable conditions through early Sat.
The monsoon trough axis extends off the coastal sections of
Senegal near 13N17W to 12N21W. The ITCZ axis extends from 10N24W
to 08N33W to 06N44W. Scattered moderate convection is noted from
04N to 10N between 11N and 20W, and within 180 nm south of the
ITCZ between 28W and 31W. Scattered moderate convection is to the
northwest of the ITCZ within 60 nm of line from 09N51W to 09N60W.
GULF OF MEXICO...
A cyclonic shear axis is observed on water vapor imagery to
stretch from the NW Bahamas to S Florida, westward to near
26N85W, and northwestward to near 29N90W. Cyclonic flow covers
the eastern gulf south of this shear axis, while strong easterly
upper-level flow around the southern periphery of an anticylone
anchored over far northwestern Georgia. Between 90W and 94W, the
upper flow is anticyclonic from a weak anticyclone centered at
24N93W. W of 94W, the upper flow becomes broadly cyclconic in
advance of an upper-level trough that is moving eastward across
Texas and extends to the far eastern section of Mexico. The
trough is moving through a very moist and unstable atmosphere.
Scattered showers and isolated thunderstorms are over the far
western gulf from 21N to 28N west of 95W. Mid and upper-level
clouds associated with this activity are streaming northeastward
west of 94W.
A surface trough, remnants of former tropical cyclone Isaac, is
located over the Yucatan Peninsula. No deep convection is noted.
Earlier overnight scattered shower and thunderstorm activity
observed over the eastern Bay of Campeche has weakened as the
typical nocturnal Yucatan Peninsula trough has progressed
westward across the eastern Bay of Campeche this morning.
Isolated showers and thunderstorms are over the Bay of Campeche
and waters adjacent to the Yucatan Peninsula.
A weak surface trough is analyzed over the southeastern Gulf of
Mexico from near Naples to near 22N88W. Isolated showers are
near the trough from 22N to 24N bewteen 83W and 85W.
Isolated showers and thunderstorms are over the remainder of the
The forecast calls for high pressure to build across the eastern
United States through tonight. The associated gradient will
bring gentle to moderate east to southeast winds and 2 to 4 ft
seas across the basin through Sun. The surface trough over the
eastern Yucatan Peninsula will weaken as it gradually moves
westward across the remainder of the Yucatan Peninsula through
A mid to upper-level central Atlantic trough axis stretches
southwestward to a small cyclonic circulation over the Windward
Passage, to 15N76W and to near 13N79W. Isolated showers and
thunderstorms are just northeast of Jamaica from within 60 nm of
18N76W. Isolated showers and thunderstorms are seen from north
of 16N and west of 79W.
An eastern Caribbean Sea tropical wave, with axis along 69W,
will move westward, across the central Caribbean Sea through Fri
and into Central America on Sun. Scattered showers and
thunderstorms east of the wave north of 15N and between the wave
and 65W may be attendant by strong gusty winds and at times
rough seas. This activity is underneath an upper anticyclone
that should help sustain it through Fri even with the wave
axis translating westward. The extreme northern edge of this
activity is approaching the far southwest side of Puerto Rico
and the Mona Passage.
The gradient associated with high pressure building from the
central Atlantic to the central Bahamas will support moderate to
fresh trade winds and building seas across the entire basin from
late on Wed through Fri.
The eastern segment of Pacific monsoon trough protrudes into the
far southwestern Caribbean Sea. With this feature there, and
combined with the already low-level speed convergence in place
there, this presently leading to increasing scattered moderate
to strong convective clusters over the waters southwest of a
line from near 13N83W to 12N80W to 11N76W. This activity is
likely to remain active into this evening.
Scattered showers and thunderstorms are approaching the lower
Windward Islands over the waters just east of Trinidad and
A narrow mid-upper trough is indicated by the models and
observed in water vapor imagery extends from near 32N70W
southwestward to 27N75W, where it transitions to a cyclonic
shear axis to S Florida, and continues well into the
southeastern Gulf of Mexico. To the east and southeast of this
trough, a deep-layer trough extends from near 32N61W to 26N63W
and to a small cyclonic circulation over the Windward Passage.
A surface trough is analyzed from near 32N61W to near 30N63W.
Also at the surface, a cold front extends from a 1004 mb low
(gale center) located well north of the area at 33N68W,
southwestward to 32N71W and to near 29N80W. Surface observations
and satellite imagery suggest that a surface trough may be
forming from near 27N68W to just east of the SE Bahamas.
Isolated showers and thunderstorms are seen from 24N to 28N
between 65W and 71W, while scattered showers and thunderstorms
are along and within 60 nm south of the cold front between 70w
and 79W. Similar convection is within 30 nm either side of a
line from 23N69W to 25N65W to 27N61W, and also north of 29N
between 58W and 63W, where upper dynamics are more robust.
Over the eastern Atlantic, a broad upper-level low is observed
near 28N37W, with a trough axis stretching southwestward to
29N48W and to a base near 13N57W. A cyclonic shear axis extends
east-northeastward from the upper-low to 30N29W and continues
northeastward as an upper trough to an ill-defined small upper-
level low that is seen in water vapor imagery near 32N23W.
A mid and upper-level trough is over the far eastern Atlantic
along a position from near 24N22W to a small upper-level
circulation at 20N23W and to base near 12N29W. Broad upper-level
anticyclone is observed between this trough and the trough
related to the aforementioned broad upper-level low. At the
surface, the remnant low of former tropical cyclone Joyce is
analyzed near 29N31W with a pressure of 1014 mb. Latest
satellite imagery shows broken low and mid-level clouds with
possible isolated showers within 90 nm of the low in the NE
semicircle and 60 nm of the low in the SE semicircle. The recent
trends in the satellite imagery indicate that this low is
weakening further, and most likely will be a trough later today.
A pair of 1021 mb high are analyzed. The first one is anchored
near 30N49W, and the second one is just north of the area near
33N37W. The surface analysis depicts high pressure covering the
area north of 23N from 40W to 60W.
The aforementioned cold front is forecast to move southeastward
to the waters north of 27N through Fri before it dissipates. The
1004 mb gale center north of the area is forecast to weaken
as it moves southeastward to near 34N62.5W by early Fri. The
attendant cold front will also weaken through Fri. The low will
also weaken as it moves to near 31N67W by Fri evening. It is
then forecast to drift westward and slowly weaken Sat through
Sun. NE swell will move into the open waters east of the Bahamas
Fri through early parts next week.
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