[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 121755
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
154 PM EDT Wed Sep 12 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 1200 UTC surface analysis and satellite imagery through
1745  UTC.

...SPECIAL FEATURES...

Hurricane Florence is centered near 29.8N 71.3W at 12/1500 UTC or
about 420 nm SE of Wilmington North Carolina and 450 nm ESE of
Myrtle Beach South Carolina, moving NW or 305 degrees at 13 kt.
Estimated minimum central pressure is 943 mb. Maximum sustained
winds are 115 kt with gusts to 140 kt. Latest satellite imagery
continues to show a rather distinct eye feature. The imagery
shows numerous moderate to strong convection within 75 nm of the
center in the NE quadrant, within 60 nm of the center in the SE
and NW quadrants and within 30 nm of the center in the SW
quadrant. Scattered moderate to isolated strong convection is
elsewhere within 90 nm of center in the E semicircle and within
90 nm of the center in the W semicircle. The present NW motion
of Florida is forecast to continue through Sat with a gradual
decrease in forward speed. On the forecast track, the center of
Florence will move over the southwestern Atlantic Ocean between
Bermuda and the Bahamas today, and approach the coast of North
Carolina or South Carolina in the hurricane warning area on Thu
and Fri, then move slowly near the coastline through Sat. Florence
is a category 4 hurricane on the Saffir-Simpson Hurricane Wind
Scale. Further strengthening is forecast through tonight. While
some weakening is expected on Thu, Florence is still forecast to
be an extremely dangerous major hurricane when it nears
the U.S. coast on Friday. Public Advisories for Florence are
issued under WMO header WTNT31 KNHC and under AWIPS header
MIATCPAT1. Forecast/Advisories for Florence are issued under WMO
header WTNT21 KNHC and under AWIPS header MIATCMAT1.

Tropical Storm Isaac is centered near 15.0N 54.7W at 12/1500 UTC
or 365 nm E of Martinique or 395 nm E of Guadeloupe moving W or
275 degrees at 15 kt. Estimated minimum central pressure is 1000
mb. Maximum sustained winds have diminished some to 50 kt with
gusts to 60 kt. Latest satellite imagery shows scattered moderate
convection removed to the SE of Isaac as it undergoes upper
northerly shear. The cyclone is exhibiting a fully exposed center
on visible imagery. The observed convection is located from 12N
to 15N between 49W and 55W. On the present forecast track, Isaac
is forecast to move across the central Lesser Antilles and into
the eastern and central Caribbean Sea on Thu, and move into the
eastern and central Caribbean Sea Thu night through Sat. Heavy
rainfall, with the possibility of life-threatening flash flood
flooding, is expected across Martinique, Dominica, Guadeloupe,
Puerto Rico and the southern United States Virgin Islands. Gradual
weakening is forecast during the next few days. Public Advisories
for Isaac are issued under WMO header WTNT34 KNHC and under AWIPS
header MIATCPAT4. Forecast/ Advisories for Isaac are issued under
WMO header WTNT24 KNHC and under AWIPS header MIATCMAT4.

Hurricane Helene is centered near 20.3N 36.5W at 12/1500 UTC,
which is about 755 nm WNW of the Cabo Verde Islands and about
1170 nm SSW of the Azores. It is moving WNW at 12 kt. Estimated
minimum central pressure is 977 mb. Maximum sustained winds
are 80 kt with gusts to 100 kt. Latest satellite imagery shows
that the eye has become less distinct, but the overall cloud
pattern remains well organized and symmetric. Scattered moderate
to strong convection is within 90 nm of the center, except 240 nm
of the center in the NE quadrant. Helene is forecast to turn
to the north and northeast with an increase in forward speed
during the next few days. The cyclone is forecast to weaken to a
tropical storm by late Thu. Public Advisories for Helene are
issued under WMO header WTNT33 KNHC and under AWIPS header
MIATCPAT3. Forecast/Advisories for Helene are issued under WMO
header WTNT23 KNHC and under AWIPS header MIATCMAT3.

Satellite images and surface observations indicate that a broad
area of low pressure has formed in the central Gulf of Mexico.
Although the shower activity is still disorganized, upper-level
winds are forecast to become more conducive for development, and
it is likely that a tropical depression will form on Thu, before
the system reaches the western Gulf coast. Another reconnaissance
plane is scheduled to investigate the disturbance tomorrow.
Regardless of development, heavy rainfall and gusty winds are
expected across portions of northeastern Mexico, Texas, and
Louisiana late this week, and interests there should monitor the
progress of this system, and refer to products from their local
weather office. This system has a high chance of tropical cyclone
development over the next couple of days. Please read the latest
Tropical Weather Outlook under WMO header ABNT20 KNHC, and under
the AWIPS header MIATWOAT for more details on this evolving
system.

Satellite images indicate that a strong area of low pressure
located a little more than 600 miles west-southwest of the Azores
is rapidly becoming better organized and a subtropical or tropical
storm appears to be forming. If this trend continues, advisories
will be initiated later today or tonight. The low is forecast to
move toward the southwest for the next couple of days, and then
turn to the northeast behind Hurricane Helene. For more
information, see High Seas Forecasts issued by the National
Weather Service. The satellite imagery shows scattered moderate
convection within 60 nm of the center, except 30 nm in the NW
quadrant. Similar convection is within 30 nm of a line from
37.5N38W to 38N40W, and from 32W to 37N between 35W and 40W. This
system has a medium chance for tropical or subtropical cyclone
development over the next 48 hours. For more information, see High
Seas Forecasts issued by the National Weather Service. Please
read the latest Tropical Weather Outlook under WMO header ABNT20
KNHC, and under the AWIPS header MIATWOAT for updates on the
chances for development of this system.

...TROPICAL WAVES...

A new tropical wave has its axis just offshore the coast of
Africa along 18W from 03N to 20N, moving westward at 10 to 15 kt.
The wave is supported by well pronounced 700 mb troughing as
depicted in the GFS model guidance. Scattered moderate convection
is seen within 120 nm west and 60 nm east of the wave axis from
15N to 19N. Scattered moderate convection is occurring within 120
nm west of the wave axis from 08N to 10N. Similar activity is
within 180 nm inland the coast of Africa from 10N to 18N.

...MONSOON TROUGH/ITCZ...

The monsoon trough extends from W Africa near 11N15W to 06N17W to
04N22W. Scattered moderate convection is within 30 nm of the
trough between 17W and 22W. Similar convection is north of the
monsoon trough within 60 nm of a line from 09N19W to 09N23W, and
within 30 nm of a line from 08N23W to 08N29W.

...DISCUSSION...

GULF OF MEXICO...

See Special Features above for details on an area of disturbed
weather over the southeastern Gulf of Mexico.

Within the area of disturbed area of weather described above under
Special Features, an upper-level low moving S to SW just north of
the Yucatan Peninsula is providing further instability over and
near the surface trough related to the aforementioned disturbance.
The trough is analyzed from near 26N90W to a 1010 mb low at 23.5N
92.5W and to inland southeastern Mexico. Deep convection has
increased over the south-central gulf waters and Yucatan Channel
and vicinity waters during the morning and early afternoon hours.
This convection consists of the scattered moderate to strong type
intensity from 22N to 25N between 88W and 93W. Scattered moderate
isolated strong convection is noted south of 24N between 85W and
88W, and is lifting northward with time. Scattered showers and
isolated thunderstorms are elsewhere south of 27N east of 92W.

A stationary front is just inland the Texas and SW Louisiana
coasts to a 1012 mb low over far southern Texas, and continues
to far NE Mexico. In the upper levels, a narrow trough stretches
from northeastern Texas southward to near 24N96W. Plenty of
available atmospheric moisture is being tapped by these features
leading to ample instability to exist over the far western gulf.
Scattered showers and thunderstorms are present north of 20N and
west of 94W.

CARIBBEAN SEA...

Widely scattered moderate convection is over the NW Caribbean and
Central America W of 85W. Elsewhere, scattered moderate convection
is over S Haiti. The eastern extent of the eastern Pacific
monsoon trough is over Costa Rica and Panama along 09N. Scattered
moderate convection is within 120 nm S of the monsoon trough. A
1009 mb low is centered over N Colombia near 09N74W.

In the upper-levels, the upper-level low just north of the Yucatan
Peninsula extends a trough to inland the eastern Yucatan
Peninsula. The trough energy is acting on a very moist and
unstable atmosphere over the far western Caribbean. This has
resulted in scattered moderate to isolated strong convection
north of 17N and west of 85W. This activity is capable of
producing very heavy rainfall and strong gusty winds over these
waters as well within about 60 nm inland the Yucatan Peninsula.
Isolated showers and thunderstorms are elsewhere north of
15N west of 71W and east of 71W.

Elsewhere, an upper level-low is centered just along the northwest
coast of Colombia near 11N75W. An upper-level anticyclone is
centered over the northern Leeward Islands near 17N63W.

The eastern extent of the eastern Pacific monsoon trough is over
Costa Rica and Panama along 09N. Isolated showers and thunderstorms
are within 60 nm S of the monsoon trough. A 1010 mb low is
centered over northern Colombia near 10N75W. Isolated showers and
thunderstorms are within 60 nm S of the monsoon trough.

Tropical Storm Isaac near 15.0N 54.7W 1000 mb at 11 AM EDT moving
W at 15 kt. Maximum sustained winds 50 kt gusts 60 kt. Isaac will
move to 15.1N 56.7W this evening, 15.3N 59.4W Thu morning, 15.3N
62.6W Thu evening, 15.4N 65.6W Fri morning, and 15.5N 71.0W Sat
morning.

ATLANTIC OCEAN...

Please refer to the Special Features section above for details
on Hurricanes Florence and Helene and on Tropical Storm Isaac.

Scattered showers are over the central Atlantic N of 29N between
40W and due to a cold front N of the area.

In the upper-levels, a small upper level low is centered N of the
Bahamas near 29N78W. Isolated showers and thunderstorms are over
and near the NW Bahamas.

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

$$
Aguirre
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