[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 130002
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
802 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 1800 UTC surface analysis and satellite imagery through
2315 UTC.

...SPECIAL FEATURES...

Hurricane Florence is centered near 30.9N 72.5W at 12/2100 UTC or
330 nm SE of Wilmington North Carolina moving NW at 14 kt.
Estimated minimum central pressure is 949 mb. Maximum sustained
wind speed is 105 kt with gusts to 130 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 tonight, 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 3 hurricane on the Saffir-Simpson Hurricane Wind
Scale. Some fluctuations in strength will be possible through
Thursday morning. Although slow weakening is expected to begin by
late Thursday, Florence is forecast to be an extremely dangerous
major hurricane when it nears the U.S. coast late Thursday and
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 centered near 15.4N 56.6W at 12/2100 UTC or
260 nm E of Martinique moving W at 17 kt. Estimated minimum
central pressure is 1003 mb. Maximum sustained wind speed is 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 satellite 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 centered near 21.4N 36.7W at 12/2100 UTC or 790
nm WNW of the Cabo Verde Islands moving N at 12 kt. Estimated
minimum central pressure is 980 mb. Maximum sustained wind speed
is 75 kt with gusts to 90 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
northeast with an increase in forward speed during the next few
days. The cyclone is forecast to weaken to a tropical storm on
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.

Subtropical Storm Joyce centered near 34.3N 41.9W at 12/2100 UTC
or 760 nm WSW of the Azores moving SW at 5 kt. Estimated minimum
central pressure is 1006 mb. Maximum sustained wind speed is 40
kt with gusts to 50 kt. Convection is not near the center, but,
over the eastern and southern quadrants. Widely scattered moderate
convection is from 31N-33N between 41W-44W, and from 34N-36N
between 39W-41W. Joyce is forecast to move toward the southwest
for the next couple of days, and then turn to the northeast behind
Hurricane Helene. See latest NHC forecast/advisory under
AWIPS/WMO headers MIATCPAT5/WTNT35 KNHC for more details.

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.

...TROPICAL WAVES...

A new tropical wave has its axis just offshore the coast of
Africa along 20W 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 14N16W to 06N30W.
Scattered moderate convection is within 90 nm of the trough.

...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. 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. 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 09N75W.

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.

ATLANTIC OCEAN...

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

An upper-level trough stretches from an upper-level low north of
the area at 35N40W southwestward to 25N43W, to a small upper-
level low at 22N45W and to 15N50W, where it becomes a cyclonic
shear axis to 13N50W to 09N54W.

In the upper-levels, a small upper level low is centered N of the
Bahamas near 28.5N79W. Isolated showers and thunderstorms are
seen west of 78W as well as over and near the NW Bahamas.

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

$$
Formosa
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