[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 170548
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
147 AM EDT Mon Sep 17 2018

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 0000 UTC surface analysis and satellite imagery through
0545 UTC.

...SPECIAL FEATURES...

Tropical Depression Joyce is centered near 34.0N 30.3W at 17/0300
UTC or 290 nm SW of the Azores moving E at 18 kt. Estimated
minimum central pressure is 1008 mb. Maximum sustained wind speed
is 30 kt with gusts to 40 kt. Latest satellite imagery shows that
Joyce is a sheared system under strong westerly winds as noted by
its deep convection being sheared off to the northeast of its
center. The convection is defined as the moderate type intensity
within 30 nm either side of a line from 37N25W to 36N29W. Overcast
to broken low and mid-level clouds with embedded scattered
showers and isolated thunderstorms are well to the east and
northeast of Joyce from 34N to 38N between 21W and 25W, and also
north of 34N east of 21W. Strong southwest to west winds aloft
and dry air in its surrounding environment will continue to hamper
Joyce over the next few days causing it to gradually weaken.
Joyce is forecast by the latest NHC advisory to slow its forward
motion while turning toward the southeast today, south on
Tue, and southwest on Wed. See the latest NHC forecast/advisory
under AWIPS/WMO headers MIATCPAT5/WTNT35 KNHC for more details.

...TROPICAL WAVES...

A central Atlantic tropical wave has its axis extending along 43W
from 04N and 20N, moving westward at 15-20 kt. Model guidance
indicates that the wave is moving through a pretty well pronounced
700 mb jet stream branch accompanied by a jet core of 30-45 kt
east winds to the east of the wave axis. The wave marks the
leading edge of a quite an extensive area of Saharan Air (SAL) for
this time during the season. The African dust extends from 10N to
25N east of the wave to Africa. Earlier dust that surrounded the
wave has thinned out since yesterday as now a surge of low-level
moisture is surging westward on either side of the wave south of
15N to the region of the ITCZ. This moisture surge consists of
scattered showers and thunderstorms moving quickly westward from
10N to 15N between 38W and 44W, and within 120 nm either side of
the wave in the vicinity of the ITCZ region from 04N to 09N.

...MONSOON TROUGH/ITCZ...

The monsoon trough axis extends from well inland Africa southwestward
to the coastline of northwest Senegal near 17N16W and continues
to 09N22W. The ITCZ extends from 09N22W to 06N30W to just west of
the tropical wave along 43W. It resumes at 08N43W to 08N58W.
Aside from the shower and thunderstorms activity associated with
the tropical wave described above, scattered moderate isolated
strong convection is seen within 210 nm north of the ITCZ between
53W and 58W, within 180 nm south of the ITCZ between 45W and 49W,
and within 60 nm south of the ITCZ between 51W and 53W. Similar
convection is northwest of the ITCZ within 60 nm of a line from
09N58W to the coast of S America at 08N60W. Scattered moderate
convection is within 150 nm north of the ITCZ between 48W and 51W.

...DISCUSSION...

GULF OF MEXICO...

In the upper-levels, and elongated upper low is over the western
gulf near 25N93W, with a trough extending northeastward to
inland southeastern Louisiana and continues well northeastward of
the area. Another trough extends from the low southeastward
to the eastern Bay of Campeche and over the southwest section of
the Yucatan Peninsula. Upper-level cyclonic flow covers the basin
west of 86W. Anticyclonic flow aloft associated with an upper-
level anticyclone over the northwestern Caribbean near 20N84W is
over the gulf east of 86W. At the surface, a trough is just inland
the coast of Mexico from 20N to near San Fernando. Plenty of
atmospheric instability along with deep moisture in place is
leading to scattered showers and isolated thunderstorms south of
25N and west of 94W to inland Mexico, where the activity increases
to scattered in coverage. At the surface, high pressure of 1015
mb is centered near 27N93W. Relatively weak high pressure is
across the basin. A surface trough extends southwestward from what
is now Tropical Depression Florence located over the border of
eastern Georgia, southwestward to over he Florida Panhandle.
Scatterometer data from this afternoon highlighted light to gentle
anticyclonic flow west of 87W, and light to moderate south to
southwest flow east of 87W. Little change in these winds is
expected through the middle part of the upcoming week.

CARIBBEAN SEA...

A surface trough extends across the central Caribbean from just
west of Haiti to a 1009 mb low near 17.5N76W, and southeastward
to near 13N74W. These features and related convection denote a
broad area of low pressure associated with former tropical cyclone
Isaac. Abundant moisture and atmospheric instability, that is
Father enhanced by the tail-end of a central Atlantic deep layer
trough that extends to Haiti and to near 17N85W, remains
entrenched across the central Caribbean and much of the eastern
Caribbean. This is resulting in scattered moderate to strong
convection from 16N to 18N between 75W and 76W, including the
eastern portion of Jamaica. Scattered showers and isolated
thunderstorms are elsewhere between 65W and 80W, while isolated
showers and thunderstorms are east of 65W. The broad area low
pressure remains disorganized. Only slow development will be
possible during the next day or so due to proximity to dry air and
land interaction with Jamaica. By Wed, environmental conditions
are forecast to become less conducive for re-development to
occur. Regardless of development, locally heavy rains and gusty
winds will be possible over portions of Hispaniola, Jamaica, and
eastern Cuba during the next couple of days while the system moves
west-northwestward to westward toward the northwestern Caribbean
Sea. Gentle trades are west of the trough and low, while gentle to
moderate trades are elsewhere east of the trough. The central
Atlantic tropical wave will move across the tropical waters east
of the Windward Islands Tue and Tue night, and through the east
Caribbean on Wed and Thu accompanied by fresh to strong trades and
building seas.

ATLANTIC OCEAN...

Please refer to the Special Features section above, for details
on now Tropical Depression Joyce.

A rather strong and broad deep-layer trough extends across the
central Atlantic from a large upper-level low (TUTT), that is
observed to be near 27N65W, southwestward to Haiti. Plenty of
instability exists with this trough as it acts on a very moist
atmosphere. The upper-level low is forecast to gradually move
westward today, then in a northward direction tonight through
Tue. At the surface, a nearly stationary 1013 mb low is near
24N65W, with a trough extending northeastward to near 30N59W,
and southwestward to near 20N70W. To the east of this system,
another trough extends from 29N50W to near 24N61W. All of these
features along with the present very moist and unstable
atmosphere are leading to the development of numerous showers and
thunderstorms over the waters from 19N to 26N between 59W and 65W,
and from 18N to 23N between 65W and 74W, including the Windward
Passage and vicinity waters. Scattered showers and thunderstorms
are also evident from 18N to 23N between 54W and 59W, and from 26N
to 32N between 50W and 56W. Any of this activity is capable of
producing very heavy rainfall as well as frequent lightning
strikes. Drier air is present to the west of the upper trough and
TUTT low, where isolated showers and thunderstorms are noted.

A 1023 mb high center is near 28N39W, with a ridge extending
southwestward to near 20N60W. Rather dry and stable conditions
are present east of 50W, except near the tropical wave along 43W
as described above. Also as sated above, a rather extensive batch
of Saharan Air (SAL) is observed over the eastern Atlantic from
10N to 25N and east to Africa. These conditions are contributing
to the already dry and stable conditions supported by the high
pressure that in place over that part of the Atlantic. The dust
will continue to migrate westward during the next few days.

For additional information please visit
http://www.hurricanes.gov/marine

$$
Aguirre
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