000 AXNT20 KNHC 121736 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 136 PM EDT Sat Aug 12 2017 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 1200 UTC surface analysis and satellite imagery through 1715 UTC. ...SPECIAL FEATURES... A low pressure of 1012 mb is centered near 23N68.5W. Scattered moderate convection is noted from 21N-25N between 64W-68W. Dry air aloft and unfavorable upper-level winds have limited significant development during the past few days. This morning, the low has become a little better defined and the shower activity has increased somewhat since yesterday. This system continues to show some signs of organization, but there is no evidence of a closed surface circulation. Conditions remain conducive for development, and a tropical depression could form during the next day or two. The low is forecast to move northwestward and northward through the weekend, and then turn to the northeast away from the United States early next week. There is a medium chance for tropical cyclone formation during the next 48 hours. ...TROPICAL WAVES... A tropical wave is in the far eastern Atlantic just west of the Cabo Verde Islands moving west at about 20 kt. It axis extends from 20N25W to 08N26W. The wave shows up very well on the TPW animation and 700 mb streamline analysis. Scattered moderate convection is noted where the wave meets the monsoon trough from 08N-13N between 25W-28W. Another tropical wave in the central Atlantic is near 46W from 08N to 20N, moving W at 15-20 kt. The wave coincides with a well defined 700 mb trough, and a high amplitud northward bulge of moisture is noted on the TPW product. African dust surrounds the wave, which is limiting the associated convective activity. Only isolated moderate convection is seen near the southern end of the wave axis. A tropical wave is in the central Caribbean and extends from eastern Cuba through Jamaica to near 10N77W. The wave coincides with a 700 mb trough, and is well depicted in the moisture product. Scattered showers are noted over the southwest Caribbean in the vicinity of this wave. ...ITCZ/MONSOON TROUGH... The monsoon trough extends from the coast of African near 14N17W to 13N20W to 11N30W to 09N40W to 08N47W. The Intertropical Convergence Zone continues from 08N47W to the coast of South America near 08N59W. Aside from convection associated with the tropical waves, scattered moderate convection is noted from 10N- 15N E of 21W, and from 07N to 10N between 28W-35W. Isolated moderate convection is from 07N to 10N between 38W and 50W. ...DISCUSSION... GULF OF MEXICO... A 1018 mb high pressure is centered over the NE Gulf of Mexico near 26N88W. Light to gentle anticyclonic flow is observed over the Gulf waters, with the exception of moderate winds noted west of a thermal trough moving off the Yucatan into the southwest Gulf. The high pressure center will be nearly stationary across the NE and north-central Gulf waters through the weekend. Then, the ridge is forecast to shift early next week allowing fresh southerly return flow to set up over the western Gulf Tue and Wed. A thermal trough will develop each evening over the Yucatan Peninsula, shifting westward into the southwest Gulf during the overnight hours, and dissipating over this area each morning. Locally higher winds can be expected west of the trough axis. CARIBBEAN SEA... A tropical wave is moving through the central Caribbean. See Tropical Waves section for details. The tropical wave will continue to propagate westward, moving west of the area late Sunday night. Fresh to locally strong winds prevail over the south central Caribbean, while gentle to moderate winds are blowing elsewhere. These general conditions will persist through the weekend. The pressure gradient is forecast to tighten early next week as the Atlantic ridge builds westward. This will allow for the trade winds to expand northward across the central Caribbean to near 17N by Monday morning. Expect building seas to 8-10 ft with the strongest winds across the south-central Caribbean. Clusters of moderate to isolated strong convection are noted over the SW Caribbean, just west of a 1012 mb low pressure located within the monsoon trough near 10N79W. A diffluent pattern aloft, associated with an upper-level anticyclonic circulation situated near 12N79W supports this convective activity. Shallow moisture embedded in the trade wind flow is noted elsewhere. ...HISPANIOLA... An upper-level trough extending just north of Hispaniola and through Windward Passage to Jamaica will combine with the local effects to favor the development of scattered showers and tstms mainly during the afternoon and evening hours today. Moisture is forecast to diminish across the island on Sunday. Expect isolated showers and tstms due to local effects as daytime heating, local sea breezes and mountain upslope lifting. ATLANTIC OCEAN... Please see the tropical wave section above for more on the tropical waves moving across the Atlantic waters. Also, please see above in the Special features section for more on low pressure, and the potential for tropical cyclogenesis, over the southwest north Atlantic waters. A weak surface trough persists across the central Atlantic and extends from 31N46W to 27N53W. Isolated showers are near the trough axis. The Bermuda-Azores high will strengthen over the central Atlantic this weekend. The pressure gradient will tighten between the area of high pressure and the tropical wave currently over the eastern Atlantic this weekend. This will freshen winds north of the tropical wave by Sunday as the wave propagates westward. The Saharan Air Layer tracking product from CIMSS indicates abundant african dust between the two tropical waves currently located across the tropical Atlantic. For additional information please visit http://www.hurricanes.gov/marine $$ GR