Tropical Weather Discussion
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AXNT20 KNHC 201805

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 PM EDT Thu Jul 20 2017

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 1200 UTC surface analysis and satellite imagery through
1745 UTC.


A tropical wave over the far eastern Atlc has its axis extending
from near 18N27W to 12N28W to 04N28W, moving westward at 20 kt.
This wave is along the leading edge of a deep pool of moisture
associate with a well pronounced monsoon trough off the coast
of Africa. The GFS guidance continues to show well-defined 700 mb
over the vicinity of the wave. The METEOSAT-9 imagery along with
the Total Precipitable Water (TPW) imagery animation both show an
extensive area of Saharan dry air and dust northwest through
northeast of the wave.

A tropical wave with a large amplitude is over clearly identified
over the central Atlantic along a position from near 24N48W to
weak low pressure near 14N49W 1013 mb and to near 08N50W. This
system is moving west-northwestward around 15 kt. The Total
Precipitable Water (TPW) imagery animation depicts very deep
moisture south of 20N between 48W-54W and from 20N-24N between
44W-51W. Latest satellite imagery shows an increasing convection
of the scattered moderate to strong intensity within 240 nm of the
low in its northwest quadrant. Scattered moderate convection is
within 30 NM of 18N50W, and within 30 nm of a line from 08N50W
to 07N54W. The METEOSAT-9 and GOES-E satellite imagery show
Saharan dry air and dust north of 19N, but appears to be thinning
out with time. A portion of this wave is forecast to move across
the eastern Caribbean during the upcoming weekend bringing
increasing moisture and probabilities for scattered showers
and thunderstorms with gusty winds.

A tropical wave crossed the Lesser Antilles this morning, and
produced wind gusts up to 31 kt at Martinique at 1230Z. The wave
is analyzed along 61W south of 20N, moving westward around 20 kt.
The TPW imagery only shows a small area of moisture within 120 nm
east of the wave from 13N-17N. The 700 mb streamline model field
from the GFS clearly indicates troughing just to the east of this
wave. Scattered showers and possible isolated thunderstorms are
within 210 nm west and 180 nm east of the wave. This activity is
capable of producing strong gusty winds as it moves across the
eastern Caribbean through late tonight and into Friday morning,
and across the central Caribbean on Saturday and Saturday night.

A tropical wave is over the central Caribbean Sea, the remnants
of former Tropical Storm Don. The wave axis extends from near
18N74W to inland NW Colombia at 11N74W. The wave is under the
southern portion of an upper level trough. A small upper level
low forming along this portion of the upper trough is helping to
generate scattered showers and thunderstorms west of the wave
from 10N-15N between 75W-79W. This activity is preceded by outflow
boundaries racing westward with strong gusty winds. In addition,
diffluence aloft on the southern periphery of the upper low is
helping to support this activity. The wave will quickly move
across the rest of the central Caribbean through this evening, and
across the western Caribbean on Friday. Strong gusty winds will
continue to be possible with the associated shower and
thunderstorm activity.

A western Caribbean tropical wave has its axis extending from
near 22N82W to inland northeastern Honduras and to over eastern
Nicaragua. It is moving westward at 15-20 kt. The TPW imagery
animation shows low/mid-level moisture content within 240 nm east
of the wave south of 19N. In the upper levels dry air and Saharan
dust are evident in the latest satellite imagery. Scattered
moderate isolated strong convection is west of the wave across
Honduras from 14N-16N. Scattered showers and isolated
thunderstorms are east of the wave to 79W and south of 15N.
The remainder of the wave will move inland central America
on Friday.


The monsoon trough axis extends from 14N17W to 11N27W to 10N32W.
The ITCZ then extends from 10N32W to 07N38W to 09N45W. The ITCZ
resumes W of a tropical wave near 10N50W and extends to the coast
of South America near 09N61W. Aside from the convection associated
with the tropical waves, scattered moderate isolated convection
exists within 180 nm north of the ITCZ axis between 33W-42W.



In the upper levels, an upper level low is located over central
Mexico near 24N100W. This feature continues to enhance scattered
showers and thunderstorms along the coast of Mexico as of early
this afternoon. A small upper level low is centered just
northeast of the Yucatan Peninsula moving westward. A larger upper
level low is moving southwestward along the southeastern coast of
Georgia. This low will shift southwest to the NE Gulf Friday
night, and retrograde westward to the western Gulf by Saturday
while weakening to a trough. Instability associated with this
feature should help set-off scattered showers and thunderstorms
over some areas of the eastern and central Gulf in addition to the
isolated showers and thunderstorms that are presently occurring
over much of the central, eastern and SW Gulf. In addition, expect
scattered showers and thunderstorms to remain active over the Bay
of Campeche through Saturday. Otherwise, high pressure of
1019-1020 mb will remain over the central to eastern Gulf, with
its associated gradient maintaining generally gentle to moderate
anticyclonic flow throughout.


The main features in the basin are the now three tropical waves
moving through the basin. See above for discussions on these
features which mentions any associated shower/thunderstorm
activity. Scattered showers and isolated thunderstorms are
seen over the far SW Caribbean within 120 nm of the coasts
of Costa Rica and Panama. Elsewhere, mostly fair weather
conditions are observed, with areas of Saharan dust spreading
westward across much of the waters north of about 15N.


A fast moving tropical wave recently passed just to the south of
the island, and is along 74W. In its wake, dry air and dust aloft
have moved in over the Caribbean north of about 15N including
the area of Hispaniola. Latest satellite imagery is showing
sea breeze low clouds developing just inland the southwest side
of Haiti, and over some inland sections of the northern and
eastern portions of the island. Isolated showers may be developing
with some of these clouds. With dry air in place over the island
and nearby regional waters, shower and thunderstorm activity is
expected to be limited to some extent this afternoon. A tropical
wave over the far eastern Caribbean just west of the Lesser
Antilles early this afternoon will pass just to the south of the
island late Friday afternoon and evening. The wave should be just
to the southwest of the island on Saturday. Moisture associated
with this wave may help erode some of the dry air in place as it
advects towards the island Friday into early on Saturday leading
to increases chances for scattered showers and isolated
thunderstorms to affect some portions of the island during those


The northern portions of three tropical wave are moving through
the far southern waters. See Tropical Waves section above for
discussions on these features. In the upper levels, a large upper
level low is identified on water vapor imagery to be located
along the southeastern coast of Georgia moving southwestward.
Another upper level low is to its southeast near 26N72W, with a
trough extending southwestward to eastern Cuba. Well to the east
and northeast of these features, another upper level low moving to
the east-northeast is near 30N49W, with a trough extending
southwest to near 23N51W, where it becomes a cyclonic shear axis
to 23N60W and to northern Hispaniola. At the surface, a nearly
stationary 1026 mb high is centered over the central Atlantic near
31N56W, with a ridge stretching southwestward to just east of
east-central Florida. A surface trough extends from near 32N77W to
just north of the NW Bahamas. The combination of the upper level
low along the southeastern coast of Georgia along with the surface
trough acting on plenty of deep moisture and instability in place
over the far western and northwestern portions of the basin is
resulting in scattered to numerous showers and thunderstorms north
of 23N and west of about 75W. Scattered showers and isolated
thunderstorms are noted within 150 nm of the low in southeast,
south and southwest quadrants. An area of numerous showers with
embedded scattered thunderstorms is noted within 180 nm of the
upper level low near 30N49W in the southeast quadrant. Isolated
showers and isolated thunderstorms are north of 27N between 49W-
55W. Little change is expected with the ongoing convective
activity through at least Friday night.

Weather conditions are very stable elsewhere as an extensive
area of Saharan dust continues to migrate westward over the
basin. It appears, based on latest satellite data, that the
leading edge of the dust has reached as far west as the Bahamas.

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