AXNT20 KNHC 201805

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 PM EDT Thu Jul 20 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.


A tropical wave over the far eastern Atlc has its axis extending from near 18N27W to 12N28W to 04N28W, moving westward at 20 kt. This wave is along the leading edge of a deep pool of moisture associate with a well pronounced monsoon trough off the coast of Africa. The GFS guidance continues to show well-defined 700 mb over the vicinity of the wave. The METEOSAT-9 imagery along with the Total Precipitable Water (TPW) imagery animation both show an extensive area of Saharan dry air and dust northwest through northeast of the wave.

A tropical wave with a large amplitude is over clearly identified over the central Atlantic along a position from near 24N48W to weak low pressure near 14N49W 1013 mb and to near 08N50W. This system is moving west-northwestward around 15 kt. The Total Precipitable Water (TPW) imagery animation depicts very deep moisture south of 20N between 48W-54W and from 20N-24N between 44W-51W. Latest satellite imagery shows an increasing convection of the scattered moderate to strong intensity within 240 nm of the low in its northwest quadrant. Scattered moderate convection is within 30 NM of 18N50W, and within 30 nm of a line from 08N50W to 07N54W. The METEOSAT-9 and GOES-E satellite imagery show Saharan dry air and dust north of 19N, but appears to be thinning out with time. A portion of this wave is forecast to move across the eastern Caribbean during the upcoming weekend bringing increasing moisture and probabilities for scattered showers and thunderstorms with gusty winds.

A tropical wave crossed the Lesser Antilles this morning, and produced wind gusts up to 31 kt at Martinique at 1230Z. The wave is analyzed along 61W south of 20N, moving westward around 20 kt. The TPW imagery only shows a small area of moisture within 120 nm east of the wave from 13N-17N. The 700 mb streamline model field from the GFS clearly indicates troughing just to the east of this wave. Scattered showers and possible isolated thunderstorms are within 210 nm west and 180 nm east of the wave. This activity is capable of producing strong gusty winds as it moves across the eastern Caribbean through late tonight and into Friday morning, and across the central Caribbean on Saturday and Saturday night.

A tropical wave is over the central Caribbean Sea, the remnants of former Tropical Storm Don. The wave axis extends from near 18N74W to inland NW Colombia at 11N74W. The wave is under the southern portion of an upper level trough. A small upper level low forming along this portion of the upper trough is helping to generate scattered showers and thunderstorms west of the wave from 10N-15N between 75W-79W. This activity is preceded by outflow boundaries racing westward with strong gusty winds. In addition, diffluence aloft on the southern periphery of the upper low is helping to support this activity. The wave will quickly move across the rest of the central Caribbean through this evening, and across the western Caribbean on Friday. Strong gusty winds will continue to be possible with the associated shower and thunderstorm activity.

A western Caribbean tropical wave has its axis extending from near 22N82W to inland northeastern Honduras and to over eastern Nicaragua. It is moving westward at 15-20 kt. The TPW imagery animation shows low/mid-level moisture content within 240 nm east of the wave south of 19N. In the upper levels dry air and Saharan dust are evident in the latest satellite imagery. Scattered moderate isolated strong convection is west of the wave across Honduras from 14N-16N. Scattered showers and isolated thunderstorms are east of the wave to 79W and south of 15N. The remainder of the wave will move inland central America on Friday.


The monsoon trough axis extends from 14N17W to 11N27W to 10N32W. The ITCZ then extends from 10N32W to 07N38W to 09N45W. The ITCZ resumes W of a tropical wave near 10N50W and extends to the coast of South America near 09N61W. Aside from the convection associated with the tropical waves, scattered moderate isolated convection exists within 180 nm north of the ITCZ axis between 33W-42W.



In the upper levels, an upper level low is located over central Mexico near 24N100W. This feature continues to enhance scattered showers and thunderstorms along the coast of Mexico as of early this afternoon. A small upper level low is centered just northeast of the Yucatan Peninsula moving westward. A larger upper level low is moving southwestward along the southeastern coast of Georgia. This low will shift southwest to the NE Gulf Friday night, and retrograde westward to the western Gulf by Saturday while weakening to a trough. Instability associated with this feature should help set-off scattered showers and thunderstorms over some areas of the eastern and central Gulf in addition to the isolated showers and thunderstorms that are presently occurring over much of the central, eastern and SW Gulf. In addition, expect scattered showers and thunderstorms to remain active over the Bay of Campeche through Saturday. Otherwise, high pressure of 1019-1020 mb will remain over the central to eastern Gulf, with its associated gradient maintaining generally gentle to moderate anticyclonic flow throughout.


The main features in the basin are the now three tropical waves moving through the basin. See above for discussions on these features which mentions any associated shower/thunderstorm activity. Scattered showers and isolated thunderstorms are seen over the far SW Caribbean within 120 nm of the coasts of Costa Rica and Panama. Elsewhere, mostly fair weather conditions are observed, with areas of Saharan dust spreading westward across much of the waters north of about 15N.


A fast moving tropical wave recently passed just to the south of the island, and is along 74W. In its wake, dry air and dust aloft have moved in over the Caribbean north of about 15N including the area of Hispaniola. Latest satellite imagery is showing sea breeze low clouds developing just inland the southwest side of Haiti, and over some inland sections of the northern and eastern portions of the island. Isolated showers may be developing with some of these clouds. With dry air in place over the island and nearby regional waters, shower and thunderstorm activity is expected to be limited to some extent this afternoon. A tropical wave over the far eastern Caribbean just west of the Lesser Antilles early this afternoon will pass just to the south of the island late Friday afternoon and evening. The wave should be just to the southwest of the island on Saturday. Moisture associated with this wave may help erode some of the dry air in place as it advects towards the island Friday into early on Saturday leading to increases chances for scattered showers and isolated thunderstorms to affect some portions of the island during those times.


The northern portions of three tropical wave are moving through the far southern waters. See Tropical Waves section above for discussions on these features. In the upper levels, a large upper level low is identified on water vapor imagery to be located along the southeastern coast of Georgia moving southwestward. Another upper level low is to its southeast near 26N72W, with a trough extending southwestward to eastern Cuba. Well to the east and northeast of these features, another upper level low moving to the east-northeast is near 30N49W, with a trough extending southwest to near 23N51W, where it becomes a cyclonic shear axis to 23N60W and to northern Hispaniola. At the surface, a nearly stationary 1026 mb high is centered over the central Atlantic near 31N56W, with a ridge stretching southwestward to just east of east-central Florida. A surface trough extends from near 32N77W to just north of the NW Bahamas. The combination of the upper level low along the southeastern coast of Georgia along with the surface trough acting on plenty of deep moisture and instability in place over the far western and northwestern portions of the basin is resulting in scattered to numerous showers and thunderstorms north of 23N and west of about 75W. Scattered showers and isolated thunderstorms are noted within 150 nm of the low in southeast, south and southwest quadrants. An area of numerous showers with embedded scattered thunderstorms is noted within 180 nm of the upper level low near 30N49W in the southeast quadrant. Isolated showers and isolated thunderstorms are north of 27N between 49W- 55W. Little change is expected with the ongoing convective activity through at least Friday night.

Weather conditions are very stable elsewhere as an extensive area of Saharan dust continues to migrate westward over the basin. It appears, based on latest satellite data, that the leading edge of the dust has reached as far west as the Bahamas.

For additional information please visit http://www.hurricanes.gov/marine

$$ Aguirre