000 AXNT20 KNHC 071127 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 727 AM 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 0600 UTC surface analysis and satellite imagery through 1115 UTC. ...SPECIAL FEATURES... Tropical Depression Four is centered near 15.0N 48.8W at 07/0900 UTC or about 725 nm east of the Lesser Antilles moving WNW at 19 kt. Estimated minimum central pressure is 1011 mb. The cloud and convective structure of the depression appears more symmetrical compared to yesterday and last night as observed on latest satellite imagery. The imagery reveals scattered moderate to strong convection within 90 nm of the center in the north and northeast quadrants. Scattered moderate convection is within 75 nm of the center in the southeast and south quadrants. The depression is expected to maintain intensity through Saturday, then weaken to a post-tropical/remnant low late Saturday night near 20.1N 61.9W as westerly vertical wind shear increases over it. 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 16N21W to 11N22W to 05N22W, moving westward at 10 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 continues to presently suppress deep convection N of 08N. Observed convection is within and near the monsoon trough, and consists of the scattered moderate to isolated strong type within 180 nm east and 120 nm west of the axis from 06N-08N. A tropical wave is over the eastern Caribbean Sea with axis extending from 20N63W to 15N63W to inland northeastern Venezuela at 07N63W, moving westward near 18 kt. The wave crossed the Lesser Antilles on Thursday evening as noted in the surface observations. The CIRA mid-level satellite moisture animation suggests that mid-level dry air is present across the wave's environment, and the Metosat-9 imagery confirms the presence of the dry air in showing a broken swath of eastern Atlantic Saharan African dust that protrudes into the eastern Caribbean Sea. The moisture present at the lower levels is shallow, and latest satellite imagery shows scattered showers and thunderstorms within 60 nm either side of the axis south of 13N. Only isolated showers and thunderstorms are within 30-60 nm of the axis north of 13N. The wave is forecast to move across the eastern Caribbean through Saturday night, and across the eastern portion of the central Caribbean on Saturday. Expect increasing moisture along with some pockets of scattered showers and thunderstorms behind the wave. A tropical wave is analyzed over the far western Caribbean Sea with axis extending from near 22N86W south to inland the northwest portion of Honduras at 16N87W, and continues southward to the eastern Pacific waters just west of northern Costa Rica. It has moved westward at about 20-25 kt during the past 24 hours, but should begin to soon slow down. The wave is in a region of unfavorable wind shear. However, shallow moisture and upper level diffluent flow between an upper level low over the eastern Gulf of Mexico and a broad ridge noted over the western Caribbean support scattered moderate to isolated strong convection west of the wave axis south of 18N. Similar activity is east of the wave from along the northeastern coast of Honduras north to 18N. Isolated showers and thunderstorms are west of the axis north of 18N, and elsewhere east of the axis to 84W as the atmospheric environment remains very moist and unstable over the much of the western Caribbean Sea. A tropical wave has moved inland eastern Mexico near 99W. The flow pattern between an upper level low over central Mexico and broad upper level ridging over the SW Gulf of Mexico supports scattered moderate to isolated strong convection south of 24N and west of 91W, including the Bay of Campeche. ...ITCZ/MONSOON TROUGH... The Monsoon Trough axis extends from 13N17W to 06N23W to 04N30W. The Intertropical Convergence Zone extends from 04N30W to 03N40W to 07N47W to 09N53W to 10N58W. Aside from the convection related to the tropical wave off the coast of Africa, scattered moderate to isolated strong convection is within 120 nm north of the axis between 37W-43W. Similar activity is within 60 nm south of the axis between 37W-44W. Scattered moderate convection is within 120 nm north of the axis between 24W-26W, within 60 nm north of the axis between 17W-19W and within 60 nm south of the axis between 34W-37W. ...DISCUSSION... GULF OF MEXICO... A tropical wave has moved inland eastern Mexico. A very moist and unstable environment east of the wave in combination with the the flow pattern set-up between an upper level low over the central Gulf and broad upper ridging over the SW Gulf is supporting scattered moderate to isolated strong convection south of 24N and west of 91W, including the Bay of Campeche. This activity is expected to continue into Saturday. A 1019 mb high is centered over the NE Gulf near 29N85W as of 0900 UTC with associated light to moderate anticyclonic flow covering the eastern Gulf north of 28N and east of 87W. Isolated showers and thunderstorms are indicated on mosaic national radar imagery to be present over the eastern Gulf east of 85W and south 28N. This activity is moving in a west-northwestward direction. Model guidance indicates that moisture will be on the increase through the weekend allowing for more convection to develop over much of the basin. CARIBBEAN SEA... A tropical wave is about to move inland as it extends from along the Yucatan Peninsula southward to inland the northwestern section of Honduras. Another tropical wave is over the eastern Caribbean. See above for further details on these tropical waves. Scattered moderate convection is within 30 nm either side of a line that extends from just inland southeastern Nicaragua to near 10N80W. This activity is within close proximity of the eastern Pacific monsoon trough that stretches into northwestern Colombia. In the upper levels, broad anticyclonic flow covers the western Caribbean, while a small upper level low is centered over central Caribbean near 14N72W. Upper southerly flow over the eastern portion of the low is transporting upper moisture from northeastern Venezuela to across the eastern Caribbean, but most of it dissipates in the dry air that has moved in from the central Atlantic. 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, possibly related to a low-level trough, is moving westward to the northwest of Tropical Depression Four. This activity is observed from 19N-21N between 49W-53W. The upper flow related to an upper low over the eastern Gulf of Mexico is drawing abundant moisture northward across the much of the 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 near 35N36W and 37N29W, respectively. The associated high pressure ridge extends into the area through 32N47W and to central 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 $$ Aguirre