Tropical Weather Discussion
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AXNT20 KNHC 181805

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 PM EDT Tue Jul 18 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.


Tropical Storm Don is centered near 11.5N 58.0W at 18/1500 UTC or
about 135 nm southeast of Barbados and 222 nm east of Grenada
moving westward or 275 degrees at 17 kt. Estimated minimum central
pressure has risen to 1011 mb during the morning, with maximum
sustained winds of 35 kt with gusts to 45 kt. Don weakened during
the morning according to data collected from an Air Force Reserved
reconnaissance aircraft. Latest satellite imagery shows a rather
ragged and less symmetrical cloud pattern of the cyclone.
Scattered moderate to isolated strong convection is noted within
90 nm of cyclone in the western semicircle. Scattered moderate
convection marks an outer rainband within 30 nm of line from
14N56W to 13N58W and within 30 nm of 08N58W. On the current
forecast track based on the latest NHC advisory, the center of Don
will move across the across the Windward Islands later today or
tonight and across the southeastern Caribbean Sea on Tuesday. Don
is forecast to weaken further to a trough late Wednesday or
Wednesday night. Don is expected to produce total rain
accumulations of 3 to 6 inches across Barbados, Trinidad and
Tobago and the southern Windward Islands through Wednesday These
rains could produce life- threatening flash floods and mudslides,
especially in mountainous areas. Please refer to the latest NHC
forecast/advisory under AWIPS/WMO headers MIATCPAT5/WTNT35 KNHC
for more details.


A tropical wave over the central Atlantic has its axis extending
from near 18N38W to a 1013 mb low near 10N39W to 05N39W, moving
westward at 10-15 kt. An 1154 UTC Ascat pass suggested the low to
be located near this position, and latest visible satellite
imagery shows a mostly exposed low center. The SSMI Total
Precipitable Water loop animation depicts this system to be
embedded within a very deep atmospheric moisture column. The
METEOSAT-9 imagery shows a very dry and stable environment to
the north of the tropical wave and low as evidenced by the
stratocumulus cloud and dust areas to the north of 17N. Scattered
moderate isolated strong convection from 10N-11N between 40W-41W.
Scattered moderate convection is within 30 nm of 12N38W, and
within 60 nm west of the wave axis from 06N-08N. According to the
latest NHC Tropical Weather Outlook, this system has the possibly
for some gradual development during the next couple of days while
it moves west-northwestward or to the northwest near 12 kt.

A tropical wave is over the eastern Caribbean Sea with its
axis extending from the Atlantic waters just north of the
northeastern Caribbean to 15N64.5W to inland Venezuela at
10N65W, moving westward at 20 kt. The wave is clearly evident
in the SSMI Total Precipitable Water animation, and in the
visible satellite imagery. This wave is racing out ahead
of Tropical Storm Don. Scattered showers and thunderstorms
are quickly moving westward within 60 nm either side of
the wave axis from 12N-15N. Scattered showers and isolated
thunderstorms are behind the wave from 11.5N-15N. This activity
is capable of producing strong gusty winds. The wave is forecast
to move across the rest of the eastern Caribbean through this
evening, and the central Caribbean Tuesday through Wednesday and
over much of the western Caribbean Wednesday night.

The northern portion of an eastern Pacific tropical wave extends
northward across southwestern Guatemala and far eastern Mexico
along 91W north to near 21N. It is moving westward near 15 kt.
This wave is entrenched within a very deep pool of atmospheric
moisture as observed in the CIRA low to mid-level atmospheric
moisture profile. In addition, latest water vapor imagery shows
upper level moisture advecting northward from deep convection
along the eastern Pacific monsoon trough to across the eastern and
central portions of the SW Gulf. Deep convection noted earlier
over western Guatemala far southeastern Mexico has weakened during
the morning and early afternoon hours. Only isolated showers and
thunderstorms are occurring over the far eastern Bay of Campeche.


The monsoon trough axis extends from 12N16W to 09N22W to 12N31W
to the 1013 mb low centered near 10N39W and to 08N45W. The ITCZ
then extends from 08N45W to 09N54W. Aside from the convection
associated with the tropical wave and attendant low, scattered
moderate isolated strong convection is within 180 nm south of
trough axis between 20W-25W. Scattered moderate convection is
within 120 nm south of the ITCZ between 14W-18W.



In the upper levels, a small upper level low is centered just
west of Tampico Mexico near 21N98W, with a trough reaching NNE to
near 28N96W. Another small upper level low is over the south-
central Gulf near 24N87W. A mid to upper level trough stretches
from the eastern United States southwestward to across the
southeast United States. The upper flow pattern is somewhat
diffluent just to the south of the trough. At the surface, a
trough is analyzed over the central Bay of Campeche, while the
northern portion of an eastern Pacific tropical wave reaches
northward to the far eastern Bay of Campeche. A 1019 mb high
center is over the eastern Gulf near 26N85W, with a ridge
extending southwestward to 24N94W. The associated anticyclonic
flow around the high consists of light to gentle winds as a weak
pressure pattern persists across the area. The atmosphere across
the Gulf remains very moist and unstable. Latest mosaic NWS radar
imagery shows scattered showers and thunderstorms developing over
the far northern Gulf. Scattered showers and thunderstorms over
the SW Gulf from 20N-23N between 94W-96W are quickly moving
northward. This activity is associated with the upper low just
west of Tampico. This activity will reach the far western Gulf
this afternoon. Little change is expected in the shower and
thunderstorm activity over the next couple of days as moist and
unstable conditions continue, and the mid to upper trough just
north of the basin begins to shift southward in response to an
upper level over the mid-Atlantic region that is forecast to
drop to the south-southwest to offshore the southeastern United
States coast.


A tropical wave quickly moving across the eastern Caribbean.
Tropical Storm is rapidly approaching the far southeastern
Caribbean, and is expected to move across the Windward Islands
later this afternoon or tonight, and across the southeastern
Caribbean on Tuesday. Scattered squalls and thunderstorms
with strong gusty winds are beginning to move across portions of
the Windward Islands. This activity will increase through the
remainder of the afternoon and into tonight. See above for
further details on both of these features. In the upper levels, a
small upper level low is noted on water vapor imagery to be just
west of Jamaica, with a cyclonic shear axis stretching west-
northwestward to just east of the northeastern Yucatan Peninsula.
A small upper level anticyclone is just east of northeastern
Nicaragua. The upper flow between both of these features is
northwesterly. Upper debris clouds from deep convection over the
eastern Pacific are being transported east- southeastward over the
far western Caribbean from 14N-18N. The extreme tail-end of a
central Atlantic upper trough extends southwestward to the central
Caribbean. Water vapor imagery shows an area of dry air between
67W-77W. Aside from convection related to the eastern Caribbean
tropical wave, scattered showers and thunderstorms are over the
waters between eastern Cuba and Jamaica. Deep convection, in the
form of the scattered moderate to isolated strong type intensity,
is confined to the far SW Caribbean. This activity is being
supported by the combination of diffluence aloft, found between
the upper low and the upper anticyclone, along with low- level
forcing from low-level wind speed convergence. In addition, the
presence of the the eastern segment of the eastern Pacific monsoon
trough across much of Costa Rica and Panama is providing added
instability to this convective activity. This activity will remain
quite active through the next 24-48 hours.


Scattered to locally low clouds are developing over the interior
of Hispaniola early this afternoon. Scattered showers and isolated
thunderstorms will likely form from these clouds later this
afternoon as a result of daytime heating combining with local
effects and an already present very moist southeasterly flow.
Some of this activity is likely to persist into the evening
hours. A fast moving tropical wave over the eastern Caribbean Sea
will move into the vicinity of the island tonight and into
Wednesday morning. Shower and thunderstorm activity associated
with the wave are to be mainly of the scattered nature south of
the island as dry air aloft that has advected westward across much
of the eastern Caribbean should limit the shower and thunderstorm
activity north of about 16N. However, any moisture from the wave
that does mix in with local effects may result in a bit more
shower and thunderstorm activity in some locations of the island
through Wednesday.


In the upper levels, an upper level low is near 32N68W, with a
trough extending southwest to near 24N69W and to vicinity of
the Windward Passage. A large upper level low is to its northeast
at 33N50W, with a trough extending southward to 25N48W, and
southwestward to just east of the NE Caribbean Sea. An elongated
east to west anticyclone is near 33N27.5W. A mid to upper level
trough is across the far eastern United States. To its south-
southeast, a small upper level low is over the NW Bahamas. At the
surface, a A 1030 mb high located north of the area near 34N46W
and moving eastward at 10 kt extends a broad ridge southwestward
to 32N62W and to near 31N76W. The northern portion of an eastern
Caribbean Sea tropical wave extends along 64W north to 21N. The
atmosphere remains very moist and unstable over much of the
western portion of the basin. Scattered moderate to isolated
strong convection is noted north of 29N between 77W-80W, and
over much of the central Bahamas and vicinity. Scattered showers
and isolated thunderstorms are elsewhere west of about 61W.
Scattered showers and thunderstorms ahead of the tropical wave are
from 19N-21.5N between 65.5W-69W. This activity may contain gusty
winds as it races westward. Over the remainder of the basin,
conditions are stable under the influence of the ridging and upper
level anticyclone. An extensive area of Saharan Air layer dust is
observed on the METEOSAT-9 imagery over much of the central and
eastern Atlantic areas. The dust is migrating westward with time,
and is following in the wake of the tropical wave along 64W. The
shower and thunderstorm activity over the western half of the
basin is forecast to remain quite active over the next couple of

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