000 AXNT20 KNHC 131737 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 137 PM EDT Sun Aug 13 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... Tropical Depression Eight is centered near 27.4N 71.5W at 13/1500 UTC, or about 455 nm SW of Bermuda moving toward the north- northwest, or 335 degrees, at 11 kt. A turn toward the north is forecast to occur tonight, followed by a turn toward the north- northeast on Monday. Estimated minimum central pressure is 1011 mb. Maximum sustained wind speed is 30 kt with gusts to 40 kt. Numerous moderate and scattered strong convection is noted within 150 nm eastern semicircle of the low center. The system is expected to remain east of the United States, reaching tropical storm intensity later today or tonight. See latest NHC forecast/ advisory under AWIPS/WMO headers MIATCMAT3/WTNT23 KNHC for more details. ...TROPICAL WAVES... A tropical wave that has emerged off the west coast of Africa is along 19W/20W from 09N-21N. A 1010 mb low pres is analyzed along the wave axis near 13.5N19.5W. A cluster of moderate to isolated strong convection is within about 180 nm west quadrant of low center. Scattered moderate convection is elsewhere from 10N-16N between 17W-23W. The low is forecast to move westward over the next couple of days and merge with a broad area of low pressure located southwest of the Cabo Verde Islands. Environmental conditions are expected to become more conducive for development of this system later this week while it moves westward at about 10-15 kt over the tropical Atlantic. Currently, there is a medium chance for tropical cyclone formation through 5 days. A large tropical wave is in the eastern Atlantic moving west at 15-20 kt. It axis extends from 21N33W to 08N34W. 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-12N between 34W-40W. A broad low pres area is noted along the wave axis. African dust surrounds the wave. The axis of a tropical wave is along near 52W/53W from 09N to 21N. The wave is moving W at 15-20 kt. It coincides with a 700 mb trough, and is well depicted in the moisture product. There is limited convection associated to this wave. It is forecast to move across the waters between 55W and the Lesser Antilles this afternoon and tonight, and across the eastern Caribbean on Monday. A tropical wave is in the western Caribbean and extends from the westernmost tip of Cuba across Honduras, Nicaragua, and Costa Rica into the Eastern Pacific region. Scattered moderate convection is noted in the wake of the wave axis and mainly from 10N-17N between 82W-85W. ...ITCZ/MONSOON TROUGH... The monsoon trough extends from the coast of Africa near 13N17W to 12N30W to 09N45W. The Intertropical Convergence Zone continues from 09N45W to 08.5N50W. Aside from convection associated with the tropical waves, scattered moderate isolated strong convection is noted from 10N-15N between 23W-30W. This convective activity is ahead of the next tropical wave located along 19W/20W. ...DISCUSSION... GULF OF MEXICO... As of 1500 UTC, a surface trough is analyzed over the NE Gulf and extends from the Florida Panhandle at 30N85W to 28N90W. A recent scatterometer pass indicate the wind shift associated with this trough. Scattered showers with embedded thunderstorms are near the trough axis, affecting the Gulf waters N of 26N between 85W-90W. The trough will lift northward on Monday, but abundant moisture is expected to persist over the NE Gulf in an southerly wind flow between the trough inland and a ridge to the south that currently extends west across the central Gulf. The ridge is producing a light to gentle anticyclonic flow across the remainder of the Gulf. The one exception is an area of moderate winds west of a thermal trough that moved off the Yucatan Peninsula into the southwest Gulf. The ridge will remain across the Gulf waters the next several days. Southerly return flow will freshen over western Gulf by midweek. 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... Please see above for more on the tropical wave moving through the western Caribbean. The tropical wave will continue to propagate westward, reaching the Yucatan Peninsula tonight. Latest scatterometer data provide observations of moderate to fresh winds across the east and central Caribbean, while mainly light and variable winds are noted over the NW Caribbean. The pressure gradient is forecast to tighten early this week as the Atlantic ridge builds westward in the wake of currently T.D. Eight. 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. These marine conditions will persist across the basin through midweek. Convection continues to flare up across the SW Caribbean. A cluster of moderate convection is noted from 10N-13N between 76W-79W. An ASCAT pass shows an area of fresh to locally strong winds over the SW Caribbean likely associated with this convective activity. A diffluent pattern aloft, associated with an upper-level anticyclonic circulation situated near 13N76W continues to enhance this convective activity. Shallow moisture embedded in the trade wind flow is noted elsewhere. ...HISPANIOLA... Daytime heating, local sea breezes and mountain upslope lifting will combined with available moisture to produce isolated showers and thunderstorms mainly during the afternnon and evening hours. The next tropical wave is forecast by the computer model to affect the island on Tuesday. ATLANTIC OCEAN... Please see the Special Features section above for more on Tropical Depression Eight, and the Tropical Wave section above for more on the tropical waves propagating across the tropical Atlantic waters. The remainder of the forecast area is under the influence of the Bermuda-Azores high that is a subtropical high located over the Atlantic Ocean. This high pressure system will build westward across the forecast region as currently T.D. Eight moves northward. Winds are forecast to increase between the eastern tropical wave and strengthening subtropical high to the north. The fresh winds will shift along with the tropical wave as it propagates westward the next couple of days. The Saharan Air Layer tracking product from CIMSS continues to indicate abundant african dust N of 20N E of 30W and between 30W and 50W. For additional information please visit http://www.hurricanes.gov/marine $$ GR