000 AXNT20 KNHC 071805 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 205 PM EDT Fri Jul 7 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 1745 UTC. ...SPECIAL FEATURES... Tropical Depression Four is centered near 15.6N 50.9W at 07/1500 UTC or about 602 nm east of the Lesser Antilles, moving WNW at 18 kt. Estimated minimum central pressure is 1011 mb. The depression has weakened considerably in the last several hours and is forecast to degenerate into a low pressure system later today. Scattered moderate convection and isolated tstms remain in the NW quadrant within 165 nm of the depression center. Please see the latest NHC Public Advisory under AWIPS/WMO headers MIATCPAT4/WTNT34 KNHC and the full Forecast/Advisory under the AWIPS/WMO headers MIATCMAT4/WTNT24 KNHC for more details. ...TROPICAL WAVES... A tropical wave is over the far eastern Atlantic with its axis extending from 16N20W to 04N22W, moving westward at 5 kt. The wave is in a region of favorable to neutral wind shear. However, extensive intrusion of Saharan dry air and dust into the wave's environment continue to limit deep convection N of 09N. Upper level diffluence along with abundant moisture associated with the monsoon trough support numerous heavy showers from 05N to 09N between 20W and 24W. A tropical wave is over the eastern Caribbean Sea with axis extending from 18N65W to 15N63W to inland Venezuela near 09N66W, moving westward at 20 kt. The wave is in a region of favorable wind shear. However, dry air subsidence from aloft as indicated by water vapor imagery hinder the development of deep convection. Shallow moisture shown in CIRA LPW imagery allows for scattered to isolated showers across the E Caribbean E of 67W. A tropical wave is moving across northern Central America. Its axis extends from near 21N88W south to 12N88W and has been moving west at about 20-25 kt during the past 24 hours. The wave is in a region of unfavorable wind shear. However, shallow moisture and upper level diffluent flow between an upper level low over NE Mexico and a broad ridge noted over the SW Gulf extending to the western Caribbean support scattered to isolated showers over the far NW Caribbean S of 20N W of 84W and in the E Bay of Campeche. ...ITCZ/MONSOON TROUGH... The Monsoon Trough axis extends from 11N16W to 07N25W to 02N36W. The Intertropical Convergence Zone extends from 02N37W to 03N45W to 04N51W. Aside from the convection related to the tropical wave E of the Cape Verde Islands, scattered moderate convection is within 120 nm either side of the monsoon trough and ITCZ. ...DISCUSSION... GULF OF MEXICO... Trailing moisture associated with the passage of a tropical wave earlier today along with the flow pattern set-up between an upper level low over NE Mexico and broad upper ridge over the SW Gulf continue to support scattered heavy showers and tstms S of 27N W of 91.5W, including the Bay of Campeche. A second tropical wave is moving over the Yucatan Peninsula and northern Central America, which will allow the continuation of showers in the SW Gulf during the weekend. Shallow moisture and another area of diffluence aloft support isolated showers and tstms over the E basin N of 24N. Otherwise, a weak surface ridge extends across the northern Gulf anchored by a 1021 mb high near 29N86W, which is forecast to dissipate by Saturday morning. Light to gentle variable flow covers the Gulf, except for possible moderate winds in the SW basin associated with the convection around a surface trough that extends from 25N95W to 17N95W. CARIBBEAN SEA... Trailing moisture associated with the passage of a tropical wave over the far NW Caribbean supports isolated showers in that region. A second tropical wave is over E Caribbean waters generating similar convection E of 68W. See above for further details on these tropical waves. In the SW basin, the EPAC monsoon trough supports numerous heavy showers and tstms S of 12N W of 77W, including inland SE Nicaragua, E Costa Rica and Panama. Otherwise, the pressure gradient throughout supports 10-20 kt trades with the strongest in the 20-25 kt range along the coast of NW Colombia where the gradient is the tightest. ...HISPANIOLA... Latest water vapor imagery shows dry air aloft advecting westward from the eastern Atlantic to across portions of the eastern and central Caribbean Sea, including Hispaniola. This is presently keeping weather conditions on the quite side over and near the island. The tropical wave currently over the eastern Caribbean is forecast to pass through the area Saturday, and may bring an increase of moisture to the island. The amount of moisture will depend on just how prevalent the dry air will be at that time. At most, brief passing low-topped showers are possible through the weekend mainly near the southern portions of the island. If some of the moisture were to spread more northward over the interior of the island, then this may combine with local effects to bring an increase of scattered showers and thunderstorms to interior portions of the island through the weekend. ATLANTIC OCEAN... Tropical Depression Four and a tropical wave are analyzed over the tropical Atlantic. See above for further details. An area of scattered showers and isolated thunderstorms related to a low- level trough is moving westward to the northwest of Tropical Depression Four. This activity is observed from 18N-23N between 50W-55W. The upper flow related to an upper low over the eastern Gulf of Mexico is drawing abundant moisture northward across the northern Bahamas and waters between the Bahamas and southern Florida. This has resulted in scattered showers and isolated thunderstorms across these waters. Dual surface highs, one of 1029 mb and the other of 1030 mb, are centered north and northeast of the discussion area. The associated high pressure ridge extends into the area N of 22N and across Florida, and is the main feature in control of the wind regime throughout. An extensive area of African Saharan dust is situated over the eastern and central Atlantic, and is gradually shifting westward as strong easterly winds aloft are present to the south of the upper level anticyclone located over those portions of the Atlantic. This synoptic pattern will change little through the next couple of days. For additional information please visit http://www.hurricanes.gov/marine $$ Ramos