000 AXNT20 KNHC 181805 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 205 PM EDT Tue Jul 18 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 Storm Don is centered near 11.5N 58.0W at 18/1500 UTC or about 135 nm southeast of Barbados and 222 nm east of Grenada moving westward or 275 degrees at 17 kt. Estimated minimum central pressure has risen to 1011 mb during the morning, with maximum sustained winds of 35 kt with gusts to 45 kt. Don weakened during the morning according to data collected from an Air Force Reserved reconnaissance aircraft. Latest satellite imagery shows a rather ragged and less symmetrical cloud pattern of the cyclone. Scattered moderate to isolated strong convection is noted within 90 nm of cyclone in the western semicircle. Scattered moderate convection marks an outer rainband within 30 nm of line from 14N56W to 13N58W and within 30 nm of 08N58W. On the current forecast track based on the latest NHC advisory, the center of Don will move across the across the Windward Islands later today or tonight and across the southeastern Caribbean Sea on Tuesday. Don is forecast to weaken further to a trough late Wednesday or Wednesday night. Don is expected to produce total rain accumulations of 3 to 6 inches across Barbados, Trinidad and Tobago and the southern Windward Islands through Wednesday These rains could produce life- threatening flash floods and mudslides, especially in mountainous areas. Please refer to the latest NHC forecast/advisory under AWIPS/WMO headers MIATCPAT5/WTNT35 KNHC for more details. ...TROPICAL WAVES... A tropical wave over the central Atlantic has its axis extending from near 18N38W to a 1013 mb low near 10N39W to 05N39W, moving westward at 10-15 kt. An 1154 UTC Ascat pass suggested the low to be located near this position, and latest visible satellite imagery shows a mostly exposed low center. The SSMI Total Precipitable Water loop animation depicts this system to be embedded within a very deep atmospheric moisture column. The METEOSAT-9 imagery shows a very dry and stable environment to the north of the tropical wave and low as evidenced by the stratocumulus cloud and dust areas to the north of 17N. Scattered moderate isolated strong convection from 10N-11N between 40W-41W. Scattered moderate convection is within 30 nm of 12N38W, and within 60 nm west of the wave axis from 06N-08N. According to the latest NHC Tropical Weather Outlook, this system has the possibly for some gradual development during the next couple of days while it moves west-northwestward or to the northwest near 12 kt. A tropical wave is over the eastern Caribbean Sea with its axis extending from the Atlantic waters just north of the northeastern Caribbean to 15N64.5W to inland Venezuela at 10N65W, moving westward at 20 kt. The wave is clearly evident in the SSMI Total Precipitable Water animation, and in the visible satellite imagery. This wave is racing out ahead of Tropical Storm Don. Scattered showers and thunderstorms are quickly moving westward within 60 nm either side of the wave axis from 12N-15N. Scattered showers and isolated thunderstorms are behind the wave from 11.5N-15N. This activity is capable of producing strong gusty winds. The wave is forecast to move across the rest of the eastern Caribbean through this evening, and the central Caribbean Tuesday through Wednesday and over much of the western Caribbean Wednesday night. The northern portion of an eastern Pacific tropical wave extends northward across southwestern Guatemala and far eastern Mexico along 91W north to near 21N. It is moving westward near 15 kt. This wave is entrenched within a very deep pool of atmospheric moisture as observed in the CIRA low to mid-level atmospheric moisture profile. In addition, latest water vapor imagery shows upper level moisture advecting northward from deep convection along the eastern Pacific monsoon trough to across the eastern and central portions of the SW Gulf. Deep convection noted earlier over western Guatemala far southeastern Mexico has weakened during the morning and early afternoon hours. Only isolated showers and thunderstorms are occurring over the far eastern Bay of Campeche. ...ITCZ/MONSOON TROUGH... The monsoon trough axis extends from 12N16W to 09N22W to 12N31W to the 1013 mb low centered near 10N39W and to 08N45W. The ITCZ then extends from 08N45W to 09N54W. Aside from the convection associated with the tropical wave and attendant low, scattered moderate isolated strong convection is within 180 nm south of trough axis between 20W-25W. Scattered moderate convection is within 120 nm south of the ITCZ between 14W-18W. ...DISCUSSION... GULF OF MEXICO... In the upper levels, a small upper level low is centered just west of Tampico Mexico near 21N98W, with a trough reaching NNE to near 28N96W. Another small upper level low is over the south- central Gulf near 24N87W. A mid to upper level trough stretches from the eastern United States southwestward to across the southeast United States. The upper flow pattern is somewhat diffluent just to the south of the trough. At the surface, a trough is analyzed over the central Bay of Campeche, while the northern portion of an eastern Pacific tropical wave reaches northward to the far eastern Bay of Campeche. A 1019 mb high center is over the eastern Gulf near 26N85W, with a ridge extending southwestward to 24N94W. The associated anticyclonic flow around the high consists of light to gentle winds as a weak pressure pattern persists across the area. The atmosphere across the Gulf remains very moist and unstable. Latest mosaic NWS radar imagery shows scattered showers and thunderstorms developing over the far northern Gulf. Scattered showers and thunderstorms over the SW Gulf from 20N-23N between 94W-96W are quickly moving northward. This activity is associated with the upper low just west of Tampico. This activity will reach the far western Gulf this afternoon. Little change is expected in the shower and thunderstorm activity over the next couple of days as moist and unstable conditions continue, and the mid to upper trough just north of the basin begins to shift southward in response to an upper level over the mid-Atlantic region that is forecast to drop to the south-southwest to offshore the southeastern United States coast. CARIBBEAN SEA... A tropical wave quickly moving across the eastern Caribbean. Tropical Storm is rapidly approaching the far southeastern Caribbean, and is expected to move across the Windward Islands later this afternoon or tonight, and across the southeastern Caribbean on Tuesday. Scattered squalls and thunderstorms with strong gusty winds are beginning to move across portions of the Windward Islands. This activity will increase through the remainder of the afternoon and into tonight. See above for further details on both of these features. In the upper levels, a small upper level low is noted on water vapor imagery to be just west of Jamaica, with a cyclonic shear axis stretching west- northwestward to just east of the northeastern Yucatan Peninsula. A small upper level anticyclone is just east of northeastern Nicaragua. The upper flow between both of these features is northwesterly. Upper debris clouds from deep convection over the eastern Pacific are being transported east- southeastward over the far western Caribbean from 14N-18N. The extreme tail-end of a central Atlantic upper trough extends southwestward to the central Caribbean. Water vapor imagery shows an area of dry air between 67W-77W. Aside from convection related to the eastern Caribbean tropical wave, scattered showers and thunderstorms are over the waters between eastern Cuba and Jamaica. Deep convection, in the form of the scattered moderate to isolated strong type intensity, is confined to the far SW Caribbean. This activity is being supported by the combination of diffluence aloft, found between the upper low and the upper anticyclone, along with low- level forcing from low-level wind speed convergence. In addition, the presence of the the eastern segment of the eastern Pacific monsoon trough across much of Costa Rica and Panama is providing added instability to this convective activity. This activity will remain quite active through the next 24-48 hours. ...HISPANIOLA... Scattered to locally low clouds are developing over the interior of Hispaniola early this afternoon. Scattered showers and isolated thunderstorms will likely form from these clouds later this afternoon as a result of daytime heating combining with local effects and an already present very moist southeasterly flow. Some of this activity is likely to persist into the evening hours. A fast moving tropical wave over the eastern Caribbean Sea will move into the vicinity of the island tonight and into Wednesday morning. Shower and thunderstorm activity associated with the wave are to be mainly of the scattered nature south of the island as dry air aloft that has advected westward across much of the eastern Caribbean should limit the shower and thunderstorm activity north of about 16N. However, any moisture from the wave that does mix in with local effects may result in a bit more shower and thunderstorm activity in some locations of the island through Wednesday. ATLANTIC OCEAN... In the upper levels, an upper level low is near 32N68W, with a trough extending southwest to near 24N69W and to vicinity of the Windward Passage. A large upper level low is to its northeast at 33N50W, with a trough extending southward to 25N48W, and southwestward to just east of the NE Caribbean Sea. An elongated east to west anticyclone is near 33N27.5W. A mid to upper level trough is across the far eastern United States. To its south- southeast, a small upper level low is over the NW Bahamas. At the surface, a A 1030 mb high located north of the area near 34N46W and moving eastward at 10 kt extends a broad ridge southwestward to 32N62W and to near 31N76W. The northern portion of an eastern Caribbean Sea tropical wave extends along 64W north to 21N. The atmosphere remains very moist and unstable over much of the western portion of the basin. Scattered moderate to isolated strong convection is noted north of 29N between 77W-80W, and over much of the central Bahamas and vicinity. Scattered showers and isolated thunderstorms are elsewhere west of about 61W. Scattered showers and thunderstorms ahead of the tropical wave are from 19N-21.5N between 65.5W-69W. This activity may contain gusty winds as it races westward. Over the remainder of the basin, conditions are stable under the influence of the ridging and upper level anticyclone. An extensive area of Saharan Air layer dust is observed on the METEOSAT-9 imagery over much of the central and eastern Atlantic areas. The dust is migrating westward with time, and is following in the wake of the tropical wave along 64W. The shower and thunderstorm activity over the western half of the basin is forecast to remain quite active over the next couple of days. For additional information please visit http://www.hurricanes.gov/marine $$ Aguirre