000 AXNT20 KNHC 172313 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 0005 UTC Sat Dec 18 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 31N. The following information is based on satellite imagery, weather observations, radar and meteorological analysis. Based on 1800 UTC surface analysis and satellite imagery through 2300 UTC. ...MONSOON TROUGH/ITCZ... The monsoon trough is confined to the African continent. The ITCZ begins near the coast of Sierra Leone near 07N13W and continues to 05N25W to 07N40W to the coast of NE Brazil near 03N51W. A surface trough is within the ITCZ and extends from 12N35W to 05N38W. Fresh NE to E winds are near the northern end of the trough axis. Scattered moderate convection is noted from 03N-06N between 28W and 33W. GULF OF MEXICO... High pressure of 1027 mb located E of Bermuda near 31N59W extends a ridge across Florida into the Gulf of Mexico producing a gentle to moderate E to SE wind flow across much of the basin. Moderate to locally fresh SE winds are noted over the western Gulf with mainly fresh easterly winds in the Straits of Florida. Seas are generally 3-5 ft across the Gulf waters, except for seas up to 6 ft in the SE Gulf and Florida Straits. A diffluent pattern aloft supports scattered showers and isolated thunderstorms over the NE Gulf, particularly N of 25N and E of 88W. For the forecast, a ridge will prevail through Sat, supporting moderate to locally fresh SE to S winds. A cold front is forecast to enter the basin Sat night. Fresh to strong N to NE winds and building seas will prevail west of the front on Sun. By Mon, low pressure should develop along the cold front in the central Gulf and steadily progress east-northeastward, pulling the front across the Gulf by Tue. Winds and seas across the basin should slowly subside on Wed. CARIBBEAN SEA... The tight pressure gradient between high pressure located E of Bermuda and the Colombian/Panamanian low is resulting in strong to near gale-force trades across the central Caribbean Sea, with the strongest winds occurring offshore NW Colombia. Satellite derived wind data show near gale-force trades within about 120 nm of NW Colombia while an altimeter pass indicates seas of up to 12 ft. Fresh to locally strong winds are present across the remainder of the E and central Caribbean, particularly near Cabo Beata, Dominican Republic. Moderate to fresh NE to E winds are seen over the NW Caribbean, including the lee of Cuba. Seas of 9-12 ft are present in the central Caribbean, with seas of 6-8 ft in the eastern Caribbean, and 4-6 ft in the NW Caribbean. Low-level convergence continues to enhance the development of scattered showers and isolated thunderstorms in the SW Caribbean Sea, mainly W of 80W, including the Caribbean plains of Nicaragua. A few showers are also noticed in the Gulf of Honduras, affecting the Bay Islands. Patches of low level moisture, embedded in the trade wind flow, are observed elsewhere producing isolated to scattered passing showers. For the forecast, the strong high pressure located E of Bermuda will prevail over W Atlantic through late Sun. This will support fresh to strong tradewinds over the central and E Caribbean and tropical N Atlantic waters. From Mon through Wed, a developing low pressure along a frontal boundary off of the SE United States coast will act to weaken the trade winds across the area. Large N to NE swell reaching the tropical N Atlantic as well as Mona and Anegada Passages tonight will continue through Mon. ATLANTIC OCEAN... The above mentioned high pressure located E of Bermuda dominates the western Atlantic, the Bahamas and the State of Florida. Farther E, a cold front enters the forecast waters near 31N26W then continues SW to near 20N47W. A shearline begins at 20N47W and stretches westward to near 20N64W. A belt of fresh to strong NE to E winds is noted between the high pressure and the shearline, mainly from 20N-25N between 40W-75W. Seas are 8-12 ft within this area of winds. An altimeter pass indicates seas of up to 15-16 ft in the wake of the front, roughly between 40W-50W. Low level clouds with possible showers are associated with the frontal boundary. The pressure gradient between the strong high and lower pressures in the vicinity of the ITCZ also supports fresh to locally strong NE-E winds from 10N-20N between 40W and the Lesser Antilles. Seas of 8-11 ft in NE swell are in this region. For the forecast, the Bermuda High will build southward, bringing an increase in winds and seas during the weekend. On Mon, a strong cold front will move off of SE United States coast ahead of a developing low pressure on Tue. Winds on both sides of the cold front are anticipated to reach strong or near gale force north of 27N on Wed. Large N swell impacting the Atlantic zone from 19N-22N between 55W-65W will diminish on Sun. $$ GR