[Tropical] Tropical Weather Discussion

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AXNT20 KNHC 150712 AAA
TWDAT

Tropical Weather Discussion...Updated
NWS National Hurricane Center Miami FL
312 AM EDT Sat Sep 15 2018

Updated to include latest information on Tropical Storm Helene

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
0700 UTC.

...SPECIAL FEATURES...Updated

Tropical Storm Florence is centered near 33.8N 79.1W at 15/0300
UTC, or about 10 nm NNE of Myrtle Beach South Carolina moving WSW
at 4 kt. Estimated minimum central pressure has risen slightly
to 980 mb. Maximum sustained winds are 55 kt with gusts to 65 kt.
Latest satellite imagery shows that Florence is being affected by
drier air into its S and SW quadrants as noted by the lack of deep
convection there. Scattered moderate to isolated strong convection
is present within 300 nm of the center in the NE quadrant and
within 180 nm of the center in the NW quadrant. Scattered
moderate convection within an outer rainband is within 30 nm of a
line from 32N78W to 34N77W and to 34.5N77.5W. Latest NHC advisory
has Florence to continue with its slow motion through early today,
then is forecast to turn westward and then northward through the
Carolinas and to the Ohio Valley by Mon. Gradual weakening is
forecast while Florence moves farther inland during the next couple
of days, and it is likely to weaken to a tropical depression by
tonight. The PUBLIC ADVISORIES for Florence are transmitted via
the WMO header WTNT31 KNHC, and via the AWIPS header MIATCPAT1.
The FORECAST/ADVISORIES for Florence are transmitted via the WMO
header WTNT21 KNHC, and via the AWIPS header MIATCMAT1.

Tropical Storm Helene is centered near 36.6N 35.4W at 15/0600
UTC, which is about 420 nm WSW of Lajes Air Base In The Azores
moving NNE at 16 kt. Estimated minimum central pressure is 988
mb. Maximum sustained winds are 60 kt with gusts to 75 kt.
Latest satellite imagery shows scattered moderate convection
from 35N to 39N between 31W and 39W. Helene is under strong
vertical as indicated by its center being located just to the
south of its related deep convection. This convection consists
of the scattered moderate isolated strong type from 36N to 39N
between 34W and 38W. Scattered moderate convection is seen from
38N to 42N between 32W and 38W. Helene is forecast by the latest
NHC advisory to turn toward the north-northeast expected by
tonight or on Sun. On the forecast track, Helene will pass near
or over the Azores late Sat or Sun. The PUBLIC ADVISORIES for
Helene are transmitted via the WMO header WTNT33 KNHC, and via
the AWIPS header MIATCPAT3. The FORECAST/ADVISORIES for Helene
are transmitted via the WMO header WTNT23 KNHC, and via the
AWIPS header MIATCMAT3.

Tropical Storm Joyce is centered near 31.6N 43.6W at 15/0300 UTC,
or about 900 nm WSW of the Azores moving E at 5 kt. Estimated
minimum central pressure is 997 mb. Maximum sustained winds are
45 kt with gusts to 55 kt. Joyce is being hampered by strong
southwesterly wind shear. Latest satellite imagery shows
scattered moderate convection within 60 nm of the center, and to
the northeast of the cyclone from 32N to 34N, and also within
30 nm of a line from 30N44W to 32N42W. Joyce is forecast by the
latest NHC advisory to turn northeastward with an increase in
forward speed today, with some slight strengthening possible.
Gradual weakening is forecast to begin by Sun. The PUBLIC
ADVISORIES for Joyce are transmitted via the WMO header WTNT3S
KNHC, and via the AWIPS header MIATCPAT5. The FORECAST/ADVISORIES
for Joyce are transmitted via the WMO header WTNT25 KNHC, and via
the AWIPS header MIATCMAT5.

Tropical Storm Isaac is centered near 15.4N 69.5W at 15/0300 UTC,
or about 190 nm S of Santo Domingo Dominican Republic moving W at
13 kt. Estimated minimum central pressure is 1002 mb. Maximum
sustained winds are 35 kt with gusts to 45 kt. Latest satellite
imagery shows that the overall structure of the cyclone is rather
ill-defined and not too symmetrical in nature. The imagery shows
scattered moderate to strong convection to the southwest of the
center within 30nm either side of a line 14N70W to 15.5N70.5W.
Scattered moderate convection is to the east of Isaac from 17N to
17.5N between 64W and 67W. Isaac is forecast by the Latest NHC
advisory to decrease its forward speed over the next couple of
days with a turn to the west-northwest possible by the end of the
weekend, if Isaac has not degenerated into a trough of low
pressure by then. The PUBLIC ADVISORIES for Isaac are transmitted
via the WMO header WTNT34 KNHC, and via the AWIPS header
MIATCPAT4. The FORECAST/ADVISORIES for Isaac are transmitted via
the WMO header WTNT24 KNHC, and via the AWIPS header MIATCMAT4.

...TROPICAL WAVES...

A far eastern Atlantic tropical wave has its axis along 31W from
05N to 21N, moving westward at 10 to 15 kt. The wave is
surrounded by a rather dry environment, which is inhibiting deep
convection from forming along or near it. Only isolated showers
and weak isolated thunderstorms are within 120 nm either side of
the wave axis where it cross the monsoon trough.

...MONSOON TROUGH/ITCZ...

The monsoon trough axis extends from off the coast of Guinea
Bissau, Africa to 09N25W to 08N32W. The ITCZ continues from
08N32W to 09N41W to 10N46W to 12N57W. Aside from the shower and
thunderstorm activity associated with the tropical wave along 30W,
scattered moderate isolated strong convection is within 180 nm
south of the ITCZ from 41W and 54W. Scattered moderate convection
is within 180 nm south of the monsoon trough from 21W to 25W, and
also well south of the monsoon trough from 04N to 09N between
15W and 18W.

...DISCUSSION...

GULF OF MEXICO...

An upper-level cyclonic circulation center is over the north-
central gulf near 26N90W, with an inverted trough to an upper low
over the northwestern Caribbean at 20N84W that is moving
westward. Upper-level cyclonic wind flow covers the central part
of the gulf. A surface trough extends from just inland central
near Freeport southwestward to a 1009 mb low near Baffin Bay and
to Ciudad Camargo and to northeastern Mexico near Ciudad Mante.
Both satellite imagery and NWS doppler radar imagery depict
increasing scattered moderate to strong convection that is
becoming more concentrated over and just inland the Texas coast
from Brownsville to just south of Corpus Christi, and eastward
from these two locations to near 95W. This activity is rotating
in a cyclonic fashion around the low as the trough/low system
slowly moves further inland through today and this evening. The
activity will likely produce heavy rainfall and possible strong
gusty winds as well. Scattered showers and thunderstorms are
over northeastern Mexico. Scattered showers and isolated
thunderstorms are elsewhere south of 28N and west of 92W.
Isolated showers are possible east of 92W.

A weak ridge extends northwest to southeast from SE Louisiana to
Florida Bay will shift slightly south on Sun with the ridge
extending southeast to the Yucatan Basin. A weak surface trough,
extending southwest from the remnants of tropical cyclone
Florence, will pass slowly east across the north-central gulf
waters through early next week.

CARIBBEAN SEA...

Please refer to the Special Feature section above for details on
Tropical Depression Isaac.

An upper level cyclonic circulation center is about 100 nm to the
west of Jamaica, in the NW corner of the Caribbean Sea. Isolated
showers and thunderstorms are north of 15N and west of 80W.

Tropical Depression Isaac near 15.4N 69.5W 1002 mb at
0300 UTC is moving W at 13 kt. Maximum sustained winds 35 kt
gusts 45 kt. Isaac will move more WNW and reach near 15.7N 73.3W
Sat evening, then gradually weaken to a tropical depression near
16.7N 77.4W Sun evening, before shifting NW and dissipating by
late Mon. Otherwise, gentle to moderate trades are forecast west
of Isaac, while moderate to locally fresh trades are forecast to
the east of Isaac. Strong nocturnal trades are forecast off the
northwest coast of Colombia on Mon and Tue nights.

ATLANTIC OCEAN...

Please read the Special Features section, for details on
Tropical Storm Florence, Tropical Storm Helene, and Tropical
Storm Joyce.

An upper-level trough extends from near 32N45W to 26N48W to
20N54W, and to 16N59W. Upper-level cyclonic wind flow is within
360 nm on either side of the trough. Scattered showers and
isolated thunderstorms are within 30 nm ether side of a line
from 22N45W to 22N51W to 22N55W. Isolated showers and
thunderstorms are seen elsewhere to the west of 56W.

A cyclonic shear axis north of the area near 34W from 50W to 63W
is dropping southward. Multilayer clouds with embedded scattered
showers and isolated thunderstorms are noted north of 30N
between 53W and 63W.

A surface ridge will remain about stationary from near 32N68W to
southeastern Florida through Sun. The ridge will shift
northward, to a position from near 32N74W to central Florida by
the middle of the next week.

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

$$
Aguirre
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