000 AXNT20 KNHC 152107 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 0005 UTC Thu Sep 16 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 2040 UTC. ..SPECIAL FEATURES... Tropical Depression Nicholas is centered near 30.2N 92.4W at 15/2100 UTC or 70 nm S of Alexandria Louisiana moving E at 2 kt. Estimated minimum central pressure is 1006 mb. Maximum sustained wind speed is 25 kt with gusts to 35 kt. The center of Nicholas is largely devoid of deep convection, with most of the activity located over the Mississippi Valley spreading into the remainder of the southeastern U.S. A trailing trough is over the N-central and western Gulf from 30N90W to 24N97W. Scattered moderate and isolated strong convection is noted within 180 nm E-SE of the trough over the basin, where winds are also moderate to fresh and seas are in the 3 to 5 ft range. The depression is moving very slowly to the east, and the center will continue to make only slow/erratic progress overnight and Thu before a gradual turn north. The center of the tropical depression may become ill- defined in the next 36 to 48 hours as it becomes a remnant low with post-tropical character. Even so, heavy rainfall will continue as tropical moisture persists across the Gulf Coast region. Nicholas is expected to produce additional rainfall amounts of 2 to 5 inches across the central Gulf coast in southeastern Louisiana, southern Mississippi, southern Alabama and the Florida Panhandle through Fri, with isolated storm totals of 8 inches possible. Considerable flash flooding impacts, especially in urban areas, are possible across these regions. Widespread minor river flooding is expected, while scattered moderate river flooding is possible, across portions of southeastern Louisiana, southern Mississippi, southern Alabama, and the Florida Panhandle. The Weather Prediction Center is using the advisories on Nicholas and they can be found at http://www.wpc.ncep.noaa.gov. A surface trough extends from 31N72W to broad 1010 mb low pressure near 18.5N73W, or a few hundred nm northeast of the central Bahamas, to 23N76W. Current associated winds are 20 to 30 kt from 27N to 31N east of the low and trough to 70W. Satellite images indicate that the low is gradually becoming better defined, however the associated showers and thunderstorms are still disorganized. Scattered moderate and isolated strong convection is noted across a wide area from 20N to 31N between 63W and the trough. Environmental conditions are expected to become more conducive for development, and a tropical depression is likely to form during the next day or two while the system moves north- northwestward to northward off the southeast U.S. coast. Regardless of development, this system could bring high surf to portions of the southeast and mid-Atlantic U.S. coast later this week, and increasing winds and building seas are expected north of 27N between 68W and 75W overnight. This system has a high chance of tropical cyclone formation in the next 48 hours. Please read the latest Tropical Weather Outlook at www.hurricanes.gov for more details. A tropical wave in the Atlantic Ocean along 27W/28W from 03N to 17N, with 1009 mb low pressure along the wave near 10.5N27.5W, or a few hundred nm southwest of the Cabo Verde Islands, moving west at around 15 kt. Current associated winds with the low are 20 to 30 kt, with seas of 5 to 7 ft. Showers and thunderstorms have changed little in organization, with a broad area of scattered moderate and isolated strong convection noted from 08N to 15N between the tropical wave and 34W. Environmental conditions are expected to remain conducive for development, and a tropical depression is likely to form within the next couple of days. This system is expected to move westward to west-northwestward across the tropical Atlantic during the next several days. This system has a high chance of tropical cyclone formation in the next 48 hours. Please read the latest Tropical Weather Outlook at www.hurricanes.gov for more details. ...TROPICAL WAVES... Refer to the Special Features section above for details on a tropical wave and associated area of low pressure in the Atlantic Ocean along 27W/28W. Another tropical wave is expected to emerge off of the west coast of Africa in a day or two. Thereafter, environmental conditions are forecast to be conducive for development while the system moves generally west-northwestward to northwestward over the far eastern Atlantic. A tropical wave is in the Atlantic Ocean along 45W, from 04N to 17N, moving west at around 10 kt. Scattered moderate and isolated strong convection is noted near the wave and the nearby monsoon trough from 08N to 13N between 37W and 45W. A tropical wave is in the Caribbean Sea along 73W from 20N southward across Haiti to the Colombia-Venezuela border, moving west at around 15 kt. The tropical wave combined with a mid to upper level low and daytime heating are helping to induce scattered showers and thunderstorms across the greater Antilles and adjacent waters. A tropical wave is in the NW Caribbean Sea along 86W from 18N southward across Honduras and western Nicaragua and into the eastern tropical Pacific Ocean, moving west at around 15 kt. No significant convection is noted over the NW Caribbean Sea, however the tropical wave is helping to support isolated to scattered showers and thunderstorms over portions of Central America. ...MONSOON TROUGH/ITCZ... The monsoon trough emerges off the coast of Africa near the Mauritania-Senegal border 16.5N16W to 1009 mb low pressure near 10.5N27.5W to 08N33W to 10N39W to 07N51W. The ITCZ continues from 07N51W to 08.5N59W. Aside from the convection mentioned above, scattered moderate and isolated strong convection is noted from 07N to 10N between 14W and 19W, and from 06N to 07N between 36W and 39W. Scattered moderate convection is noted from 02N to 05N between 21W and 24W, and from 01N to 06N between 31W and 37W. GULF OF MEXICO... Refer to the Special Features section above for details on Tropical Depression Nicholas. Gentle to moderate SE-S winds are noted elsewhere in the basin east of the trough mentioned south of Nicholas, with gentle to moderate NW-N winds west of the trough toward the Texas coast. Seas are 3 ft or less east of 87W, and 2 to 4 ft west of 87W outside of the influence of Nicholas. Isolated to widely scattered showers and thunderstorms are noted over the eastern Gulf. For the forecast, Tropical Depression Nicholas inland north of the area is forecast to become a remnant low Thu. Conditions will continue to gradually improve offshore the Louisiana and upper Texas coasts, but moderate to fresh winds and seas of 3 to 5 ft can be expected into tonight. For the remainder of the week, gentle to moderate mainly southerly flow will dominate the basin. CARIBBEAN SEA... Refer to the Tropical Waves section above for details on the tropical waves moving across the Caribbean Sea. Moisture in the SW Caribbean Sea associated with the east Pacific monsoon trough results in scattered moderate convection south of 11N between the coast of Colombia and Costa Rica. The rest of the basin experiences fairly tranquil weather conditions outside of some isolated thunderstorms between 76W and 85W. A relatively weak pressure gradient exists across the region resulting in moderate to fresh trades, except light to gentle in the SW Caribbean as well as in the lee of Cuba outside of any convection. Seas are in the 2 to 4 ft range in the eastern and western Caribbean, and 3 ft or less elsewhere outside of any convection. For the forecast, gentle to moderate winds will increase to moderate to fresh by Thu, with strong winds pulsing during the overnights into the weekend offshore Colombia and in the Gulf of Honduras. ATLANTIC OCEAN... Refer to the Special Features section above for details on the disturbance northeast of the Bahamas, and the system in the far eastern Atlantic. Otherwise, the subtropical ridge extending from near the Azores to near Bermuda dominates most of the tropical Atlantic. A surface trough is located in the central Atlantic from 31N43W to 25N45W and there are a few showers near the trough axis. Another surface trough is located from 23N35W to 14N36W and it is devoid of any significant convection. Scattered moderate and isolated strong convection is noted northeast of the NE Caribbean islands from 16N to 20N between 57W and 60W, associated with a mid to upper level feature. The rest of the basin is fairly quiet. Fresh to strong N-NE winds are present off the coasts of Morrocco and Mauritania, mainly east of 20W, due to the pressure gradient caused by the ridge and lower pressures over western Africa. Seas are in the 6 to 9 ft range across this area. Moderate to fresh trades are noted elsewhere north of the convergence zone and east of 35W, along with seas in the 5 to 7 ft range. Gentle to moderate winds and seas of 3 to 6 ft can be found in the rest of the basin. For the forecast west of 55W, outside of the area of low pressure northeast of the Bahamas, weak high pressure will lead to gentle to moderate mainly easterly winds across the basin. $$ Lewitsky