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Atlantic Tropical Weather Discussion

AXNT20 KNHC 121126

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1205 UTC Mon Apr 12 2021

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 0600 UTC surface analysis and satellite imagery through 
1115 UTC.


Windward Islands Ashfall Advisory: La Soufriere volcano on the 
island of Saint Vincent near 13.3N 61.2W is currently undergoing 
frequent eruptions. Volcanic ash may be reaching the ocean 
surface and low level visibility may be reduced. Mariners 
traveling within the area 18N39W to 08N40W to 10N50W to 15N62W
to 17N45W to 18N39W are urged to exercise caution, especially 
west of 50W where the densest ash is being observed. If mariners 
encounter volcanic ash or floating volcanic debris, you are urged 
to report the observation to the National Hurricane Center by 
calling 305-229-4424.


The monsoon trough axis extends southward from the coast of 
Senegal near 13N17W to 05N21W, where overnight scatterometer data 
indicates that it transitions to the ITCZ to 00N30W and to 
00N45W. Scattered moderate to isolated strong convection is noted
from 05S-03N between 18W-26W and from 03S to 00N between 26W-30W.


A weak cold front extends from northern Florida south-southwest
to near 25N92W, where it transitions to a dissipating stationary
front to inland Mexico at 25N98W. An overnight ASCAT pass
indicated gentle wind speeds on both sides of the front. The 
front will reach from the southeastern Gulf to the central Gulf 
this afternoon and evening and to just SE of the Gulf tonight.
As of 1100 UTC, an E to W oriented squall line extends from 
extreme southwest Florida to 26N85W. The squall line, in 
combination with forcing aloft from a shortwave trough, is 
generating a large convective complex that consists of numerous 
moderate to isolated strong type convection from 23N-26N between 
81W-85W as 11Z. The convective complex is moving slowly 
southeastward. It has a history of producing frequent lightning 
and strong wind gusts. Scattered moderate showers extend eastward 
from the squall line over South Florida. Elsewhere, a weak surface
trough is located in the SW Gulf of Mexico with its axis analyzed
from 24N90W to inland Mexico at 19N97W. Wave heights are in the 
range of 3-5 ft across the Gulf of Mexico, except for lower wave 
heights of 1-3 ft in the southern Bay of Campeche.


See the Special Features section above for details on an Ashfall 
Advisory issued for ongoing volcanic eruptions on St. Vincent.

The latest surface analysis depicts a rather weak pressure 
gradient across the basin. Overnight ASCAT data highlighted
moderate to fresh trade winds over the central Caribbean and
fresh to strong southeast winds in the Gulf of Honduras. It 
revealed moderate trade winds elsewhere across the basin. Mainly 
fair weather conditions are in place across the basin, except for 
isolated shower and thunderstorm activity that is occurring over 
the far southwestern Caribbean along the coast of Panama. Wave 
heights are in the 4-6 ft range across most of the basin, except 
up to 7 ft in some locations in the south-central Caribbean.

For the forecast, fresh southeast winds will continue over the 
northwestern Caribbean through the period, while fresh to strong 
east to southeast winds continue in the Gulf of Honduras. Winds 
will pulse to strong speeds over a small area near the coast of 
Colombia during the afternoons and at night through Thu. Elsewhere,
generally moderate to fresh east winds will change little through
Fri night. 


See the Special Features section above for details on an Ashfall 
Advisory issued for ongoing volcanic eruptions on St. Vincent.

Upper-level diffluence to the east of Florida is sustaining 
weaker convection than what was noted earlier over the central
Bahamas. The latest satellite imagery shows scattered moderate 
convection moving offshore South Florida to the NW Bahamas, and 
extending from there southwestward to just east of South Florida
and to over sections of the middle and upper Florida keys. Scattered
showers and isolated thunderstorms are seen elsewhere west of 70W
from 23N-32N. Expected for additional convective activity to move
offshore South Florida this morning. An overnight ASCAT data pass
detects a large area of fresh to locally strong south to 
southwest winds to the east of northern and central Florida, 
mainly north of 26N extending eastward to about 72W. A 1015 mb 
high pressure center is analyzed near 25N57W. A ridge extends from
the high westward to the southeastern Bahamas. The ASCAT data 
pass also depicts moderate anticyclonic winds to the east of the 
southeastern Bahamas. Wave heights are 6-8 ft north of the Bahamas
between 70W-77W.

For the forecast west of 65W, fresh to strong southwest winds 
east of northern and central Florida will shift eastward today 
ahead of a cold front that will move off the southeastern U.S. 
coast this morning. The front will reach from near 31N73W to the 
northern Bahamas and to South Florida early this afternoon, from 
near 31N67W to the southeastern Bahamas and to eastern Cuba by 
late tonight and the southeast part of the area on Tue. The front 
will move east of the forecast waters on Wed as weak high pressure
builds in behind it, and as another cold front approaches the far
northwest waters. This front will move across the water N and NE 
of the Bahamas from late Wed night through Fri night. Fresh to 
strong southwest winds are expected ahead of this front.

Farther east, a surface trough is analyzed from 26N45W to 22N56W. 
Isolated showers are possible near the trough between 50W-56W. 
Another surface trough is analyzed from 27N30W to 22N38W and
to near 18N49W. A rather potent upper sub-tropical jet stream
branch passes to its southeast as it stretches northeastward
around the western periphery of a broad upper anticyclonic located
over the far eastern Atlantic. The jet stream branch is advecting 
ample mid and upper-level moisture northeastward across and to the 
east of the trough. There is enough jet dynamics and lift for 
scattered showers and possible isolated thunderstorms to exist
underneath the mid and upper-level moisture, namely from 20N
to 28N between 27W-32W, also from 27N to 31N between 22W-27W
and from 20N-25N between 32W-36W. An ASCAT pass from last night 
shows a swath of southwest to west 20-25 kt winds over the area 
north of 30N between 36W-40W. These winds are on the outer 
periphery of a tight gradient that surrounds a deep low pressure 
system located north of the discussion area. Wave heights are in 
the range of 10-12 ft with these winds. Northerly swell over the 
central Atlantic will diminish today as the low moves toward the 
northeast and weakens. A 1019 mb high pressure center is over the 
Canary Islands. A ridge extends from this high center to 24N25W 
and to 21N34W. The overnight ASCAT pass highlights light to gentle
winds in anticyclonic fashion across the ridge axis.