Atlantic Tropical Weather Discussion (Text)

AXNT20 KNHC 180927 AAA

Tropical Weather Discussion...Updated
NWS National Hurricane Center Miami FL
427 AM EST Sun Feb 18 2018

Updated Monsoon Trough/ITCZ Section

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 
0530 UTC. 


Caribbean Sea Gale Warning: A tight surface pressure gradient,
that exists between surface high pressure in the western Atlantic
Ocean, and lower surface pressure in northern sections of South 
America, should continue to support winds pulsing to minimal 
gale force near the coast of Colombia through early next week. 
The resultant wave heights with the gale force NE to E winds are 
forecast to range from 12 to 16 feet, except building to 12 to 18
ft Mon. Please read the latest NHC High Seas Forecast under 
AWIPS/WMO headers MIAHSFAT2/FZNT02 KNHC for more details.


The monsoon trough axis extends from the southern coastal 
sections of Liberia near 05N08W southwestward dipping to below 
the Equator at 18W as the ITCZ axis to 03S27W and to the coast 
of Brazil near 04S38W. Scattered moderate isolated convection is 
within 60 nm S of the ITCZ axis between 24W and 28W. Similar 
convection is just to the N of the Equator within 30 nm of line 
from 01N27W to 02N31W to 02N34W.



A surface ridge extends from an Atlantic Ocean 1026 mb high
pressure center located near 28N64W, to across central Florida, 
to a 1023 mb nearly stationary high centered at 27N83W and
continues west-northwestward to the NW Gulf near 28N94W. A weak
surface trough is analyzed over the NE Gulf from near 
Apalachicola to 28N87W to 27N89W. Isolated weak showers are 
possible along the trough. Similar showers are over the extreme 
southeastern gulf near NW Cuba. The ridge will continue to 
maintain mainly gentle to moderate southeast winds in the western 
Gulf, and light to gentle east to southeast elsewhere through 
Sun night with the exception of moderate winds over the SE 
waters and the Straits of Florida. Areas of dense fog producing 
reduced visibility to below 1 nm are again expected through this 
morning mainly N of 27N, persisting into this afternoon over 
some locations. Isolated showers are over some portions of the 
eastern gulf and far SE waters near the Straits of Florida. 
Expect increasing winds and building seas by early next week as 
high pressure strengthens in the western Atlantic Ocean. A 
thermal trough will develop over the western Yucatan Peninsula 
during each afternoon. The trough will drift westward across the 
eastern Bay of Campeche during the overnight hours, and then 
dissipate in the SW Gulf waters by late each morning.


Please read the SPECIAL FEATURES section for details regarding 
the Gale Warning near the coast of Colombia. 

Patches of broken to at times overcast low clouds with isolated 
showers are quickly moving westward across the western and central
Caribbean N of about 14N, including waters adjacent to the
southwest tip of Haiti and waters just east and west of Jamaica.
Similar clouds with brief passing isolated showers are noted
over portions of the eastern Caribbean. Over the eastern 
Caribbean, the moisture coverage has decreased since yesterday as
very dry air aloft has moved into that portion of the sea.

A tight pressure gradient between a high pressure system that is 
in the western Atlantic Ocean, and lower pressure in northern 
sections of South America, will continue to support pulsing winds 
of minimal gale force near the coast of Colombia through early 
next week. Fresh to locally strong trade winds are expected 
elsewhere across the Caribbean Sea. The high pressure will be 
reinforced by a stronger high pressure system on Mon. The stronger
system will bring increasing winds and building seas across the 
eastern and central Caribbean Sea, and the Tropical N Atlantic 
waters possibly into the middle of next week.


A central Atlantic upper level trough enters the area through
32N25W, and stretches southwestward to an elongated cyclonic 
circulation that is dropping southward near 24N43W, and continues 
southwestward to 15N51W where it becomes a cyclonic shear axis 
westward to 14N58W and to the eastern Caribbean Sea near 14N66W.
A surface trough extends from 31N50W to 26N58W, and another one 
extends from near 29N43W to 24N44W to 19N44W. Isolated showers and
thunderstorms are noted within 240 nm to the E of the second 
trough, and within 60 nm W of the same trough from 20N to 23N. 
Isolated showers are near the first trough. Other than these 
troughs, the Atlantic is dominated by high pressure as a nearly 
1026 mb high center is near 28N64.5W, and another 1026 mb high
center is over the eastern Atlantic near 29N38W. A ridge extends 
from the 1026 mb high at 28N64.5W westward to across central 
Florida. In the upper levels, an anticyclone is near 22N66W, with 
its broad anticyclonic flow covering just about the entire western
half of the area. An upper level trough has entered the far NW 
corner of the area. Subsidence and dry air aloft associated with 
the upper anticyclone is suppressing any deep convection from 
developing. The upper trough is void of any shower activity as it 
nudges into a very stable atmospheric environment. Isolated 
showers are seen over the waters between Andros Island and the 
Straits of Florida. The aforementioned high pressure ridge will 
retreat eastward today allowing for a weak cold front to move off 
the southeastern United States coast. The front will quickly 
become stationary and dissipate across the northwest portion on 
Mon. Gentle to moderate south to southwest winds will prevail to 
the north of the ridge, with moderate to fresh winds expected 
generally to the south of 25N, except becoming strong along the 
north coast of Hispaniola and the northern approach to the 
Windward Passage each evening. The ridge will be reinforced by a 
stronger high pressure system on Mon through Tue.

Over the eastern portion of the Atlantic, extensive mid and upper
level moisture in the form of overcast to broken mid and high
level clouds are observed streaming northeastward from 
northeastern S America to within 300 nm either side of a line 
from 05N51W to 13N37W to 17N28W to 18N23W where the moisture thins
out allowing for the clouds to transition to mainly scattered high 
clouds to inland the coast of Africa at 19N16W. This area of 
moisture and clouds are driven by a rather strong jet stream 
branch that is along the southeast sector of the upper level 
trough described above. Scattered moderate convection is along 
and just inland the coast of S America between 43W-47W. Isolated 
showers are possible elsewhere from 02N-11N between 41W-51W, and 
also from 10N-19N between 30W-41W.

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Page last modified: Sunday, 18-Feb-2018 09:27:29 UTC