000 AXNT20 KNHC 242335 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 0005 UTC Sat Sep 25 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 2220 UTC. ...SPECIAL FEATURES... Hurricane Sam is centered near 12.1N 44.8W at 2100 UTC, or about 1120 nm ESE of the Northern Leeward Islands, moving W at 10 kt. Estimated minimum central pressure is 986 mb. Maximum sustained wind speed is 75 kt with gusts to 90 kt. Satellite imagery shows improving convective structure, with an eye attempting to develop and well pronounced tightly coiled outer rain bands. Scattered strong convection within the central dense overcast feature is within 45 nm of center of Sam. Scattered moderate to strong convection is with the outer rain bands within 30 nm of a line from 11N42W to 13N43W and to 12N45W. Similar convection in another outer rain band is within 30 nm of a line from 12N46W to 10N44W. Peak seas are estimated near 22 ft. A decrease in forward speed and a motion toward the west-northwest is expected over the weekend. Rapid intensification is forecast to continue, and Sam is likely to become a major hurricane on Sat. Swell associated with Sam is expected to reach the Lesser Antilles Sun evening. Please read the latest HIGH SEAS FORECAST issued by the National Hurricane Center at website- https://www.nhc.noaa.gov/text/MIAHSFAT2.shtml and the latest Sam NHC Forecast/Advisory and Public Advisory at www.hurricanes.gov for more details. Surface observations and recent satellite imagery indicates that the low pressure center located 135 nm north of Bermuda has become subtropical, and has been classified as Subtropical Storm Teresa. Teresa is centered near 34.5N 64.5W at 2100 UTC, moving NW at 12 kt. Maximum sustained winds are estimated at 40 kt with gusts to 50 kt, as depicted by late morning ASCAT wind data. Estimated minimum central pressure is 1008 mb. Scattered moderate isolated strong convection is seen removed from the center, and occurring within 180 nm across the N and 360 nm across the NE quadrants. Teresa is expected to turn northward on Sat and then northeastward, ahead of an extratropical storm system that is forecast to develop off of New England this weekend. This will allow limited opportunity for Teresa to strengthen slightly during the next 48 hours before it become absorbed in this developing system. Please see the latest High Seas Forecasts issued by the Ocean Prediction Center at website https://ocean.weather.gov/shtml/NFDHSFAT1.php for more details on Teresa. ...TROPICAL WAVES... A far eastern Atlantic tropical wave has its axis along 27W from 04N to 19N, moving westward at 10 kt. Widely scattered moderate convection is seen from 03.5N to 11.5N between 21W and 28W. Per the latest GOES-16 SAL imagery, the northern part of the wave remains embedded in dry Saharan air, inhibiting the development of shower and thunderstorm activity. A central Atlantic tropical wave has its axis near 60W from 07N to 19N, moving westward at 10-15 kt. Scattered showers and isolated thunderstorms are seen from 11N to 13.5N between 55W and 60W. Very dry and stable middle level atmospheric conditions prevail north of 15N across the region, and is limiting convection. ...MONSOON TROUGH/ITCZ... The monsoon trough axis passes through coast of Senegal near 13N16.5W to 06.5N28W to 07N33W. A segment of the ITCZ extends from 07N46W to 06N53W. Aside from convection associated to the tropical waves and Sam described above, scattered moderate isolated strong convection is seen from 05N to 09N between 30W and 45W. GULF OF MEXICO... A quasi-stationary frontal boundary extends from southwestern Florida to 23.5N90W and to near Veracruz, Mexico. Satellite imagery shows scattered showers and thunderstorms along and within 60 nm either side of the boundary west of 84W. This activity is being aided by a mid to upper-level shortwave trough. The air mass behind the front is quite dry, allowing fairly tranquil weather conditions front Texas to Florida in the N Gulf. Recent scatterometer satellite data and current buoy observations reveal moderate to locally fresh northeast winds behind the front east of 91W, and gentle northeast to east winds west of 91W. Light and variable winds are south of the frontal boundary. Seas across the basin are 3-6 ft, except for slightly lower seas of 1-3 ft over the southeastern Gulf. This quasi-stationary frontal boundary will dissipate by late Sat. Moderate to fresh winds prevail north of the front. Winds and seas will diminish across basin from Sat night and into early next week as high pressure builds over the northern Gulf. CARIBBEAN SEA... A small upper-level low near the southeastern Bahamas has an associated trough that reaches to the eastern tip of Cuba and to the southwestern Caribbean. A surface trough is analyzed over the western Caribbean near 86W from 17N-22N. Scattered to numerous showers and few thunderstorms prevail across much of the SW Caribbean, south of 15N and west of 77W to coastal sections of Nicaragua. Expect for this area to remain convectively active through early Sat. Partial ASCAT data passed from this morning indicate generally gentle to moderate trades across the basin, with the majority of the gentle trades confined to the NW Caribbean and to the northern sections of the central and eastern Caribbean. Seas across the area remain relatively low, in the 2-3 ft range, except for slightly higher seas of 4-5 ft over the waters north of the A-B-C Islands and Gulf of Venezuela. Hurricane Sam across the central tropical Atlantic will strengthen to a major hurricane Sat afternoon near 13N48W and move to near 15N53W Mon afternoon. Sam is forecast to enter the offshore waters east of Leeward Islands Tue near 17N55W and move to near 19N58W Wed. Meanwhile, moderate trade winds will persist over much of the Caribbean into early next week, becoming fresh over the south-central Caribbean starting tonight as high pressure builds east of the Bahamas. ATLANTIC OCEAN... Please read the sections above for details on Hurricane Sam and tropical waves moving across the basin. The Cumbre Vieja volcano on the island of La Palma in the Canary Islands has been erupting since 19/1410 UTC. Pilots have reported volcanic ash near Tenerife and over La Gomera. The height of the ash plume cloud is difficult to estimate due to cloud cover. Marine and aviation interests should monitor this on-going situation by reading Volcanic Ash Advisory issued by Meteo-France at http://vaac.meteo.fr/volcanoes/la-palma/. The next advisory will be issue no later than 24/2100 UTC. A cold front extends from near 31N76.5W to 287N78.5W, and transitions to a stationary frontal boundary to Stuart, Florida and to southwest Florida. A pre-frontal trough is within 30-60 nm ahead of the cold front. Scattered moderate to strong convection is within 240 nm southeast of the front, from 25N across the Bahamas northward to 31N. Moderate to fresh southerly winds are east of the trough and front to near 75W and north of 26N. Seas with these winds are in the 4-6 ft range. Another area of interest is a sharp surface trough extending from 31N61W to a 1011 mb low near 24N66W. The interaction of these features with a large upper-level low near Bermuda is inducing a large area of showers and thunderstorms, especially from N of 20N and between 56W and 64W. Morning ASCAT wind data indicate fresh to strong cyclonic winds near the northern end of the surface trough north of 28N. Seas in this region are 5-8 ft. Farther east, a 1010 mb low is analyzed near 29N40W. A large area of scattered moderate to strong convection is present north of 27N between 33W-40W. This convection is shifting eastward. A stationary front stretches from 31N43W to 29N47W. Scattered showers and thunderstorms are along this boundary. Moderate to fresh northerly winds were captured by ASCAT off the coast of Mauritania, mainly east of 20W. A large area of northerly swell that is producing 8-11 ft seas N of 22N and between 35W and 60W. Moderate to fresh trade winds are noted elsewhere across the basin along with 4 to 6 ft seas in open waters. The front across the NW waters to Stuart, Florida will remain stationary and weaken through early Sun. Then, reinforcing moderate N winds will push the front eastward as a weak cold front, reaching from 31N69W to 27N74W by Mon morning. The front will stall and dissipate Tue. Farther east, Hurricane Sam will strengthen to a major hurricane Sat afternoon near 13N48W and move to near 15N53W Mon afternoon. Sam will move to near 17N55W Tue and to near 19N58W Wed. Swell associated with Sam will reach 55W early Sun, 60W Sun evening and begin to pass north of the Virgin Islands and Puerto Rico Mon morning. $$ Stripling