AXNT20 KNHC 241057

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
657 AM EDT Mon Jul 24 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 1030 UTC.


An eastern Atlantic tropical wave has its axis extending from 17N26W to 05N25.5W, moving westward at 15 kt. Both CIRA LPW and SSMI TPW imagery confirms the extensive presence of dry air mainly in the northern wave's environment. Convection associated with this wave remains at a minimum as dry air to the north of the wave has filtered some into its environment. Only scattered moderate convection is seen within 120 nm either side of the wave along and near the monsoon trough from 05N-08N.

A central Atlantic tropical wave has its axis extending from 21N40W to 15N39W to 09N39W, moving westward at 15 kt. This wave has a broad low-level cloud field associated with it, and is depicted in a somewhat inverted-V shape pattern as seen in the first visible satellite images. No deep convection is noted with this wave as the Saharan Air Layer (SAL) and associated dust has also mixed in with this wave's environment. Only isolated showers and weak isolated thunderstorms are noted east of the wave within 30 nm of 17N39W.

Another tropical wave has its axis extending from 17N52W to 06N53W, moving westward at 15-20 kt. Moisture has increased during the overnight hours mainly west of the wave as upper level dynamics provided by an upper trough that extends east to west across the wave is providing additional instability and lift to the environment around the wave south of 14N. Latest satellite imagery shows increasing scattered moderate convection within 45 nm either side of line from 12N52W to 12N56W to 12N58W. Similar activity is east of the wave within 30 nm either side of line from 13N50W to 12N53W. A swath of Saharan dry air and dust is present over and around this wave north of 14N.

The northern portion of an eastern Pacific tropical wave has moved inland central America to over Guatemala and the far south-central part of the Yucatan Peninsula. It is moving westward at 10-15 kt. The Total Precipitable Water (TPW) imagery animation continues to display abundant deep atmospheric moisture within the environment of this wave. This moisture is seen in infra-red satellite imagery as mainly broken to overcast mid to high level debris cloudiness from previous deep convective activity. Scattered strong convection has recently developed over far southeastern Belize and far northwestern Honduras. Scattered moderate convection is just offshore the northeast part of the Yucatan Peninsula within 30 nm of a line from 20N85W to 21N87W. Isolated showers and thunderstorms are over some areas of the Yucatan Peninsula. This activity is likely to become scattered to locally numerous this afternoon and into this evening.


The monsoon trough axis extends from 13N17W to 08N29W to 07N36W, where latest scatterometer winds then indicate the ITCZ begins and continues to 06N43W to near 06N51W. It resumes at 07N53W to near the coast of South America at 07N57W. Aside from convection with the tropical waves, scattered moderate convection is within 60 nm south of the trough axis between 28W-34W, and within 60 nm south of the ITCZ between 46W-51W. Similar convection is within 120 nm north of the ITCZ between 45W- 51W, and within 60 nm south of the ITCZ between 42W-45W.



Weak surface ridging dominates across the Gulf waters anchored by western Atlantic high pressure. With this, light to gentle anticyclonic winds prevail across the basin. A small upper level low is located over the central Gulf near 26N91W. A swath of dry air is evident within about 420 nm either side of the low from 24N-27N, while moisture is seen north of 27N and south of 24N. Isolated showers and thunderstorms are within the moisture areas. Scattered showers and isolated thunderstorms are occurring south of 24N between 85W-91W, and south of 22N west of 91W. Similar activity is quickly developing over the NE Gulf north of 27N and east of 86W, and also north of 28N between 86W-89W. With little change expected in the current synoptic pattern through the next 24-48 hours, expect scattered showers and thunderstorms to pretty much develop over mainly the southern and northern waters, with isolated showers and thunderstorms possible elsewhere. A thermal trough will move westward from the Yucatan Peninsula into the Bay of Campeche with convection every evening. This trough will be enhanced some by the northern part of a tropical wave that is currently along the coast of the southern Yucatan Peninsula.


A tropical wave has moved further inland central America this morning. Refer to the section above for details. A very moist and unstable atmosphere over the western portion of the Caribbean is leading to clusters of scattered moderate convection there, and over the waters between eastern Cuba and Jamaica. In addition, the eastern Pacific monsoon trough extends along 10N to NW Colombia. This continues to enhance scattered shower and thunderstorm activity over the far SW Caribbean south of 13N and west of 80W to near the coasts of Costa Rica and Panama. All of this activity is expected to remain active over the next 48 hours or so. Elsewhere, a broad high pressure over the Atlantic stretches across the basin supporting fair weather and maintaining a pressure gradient to support fresh to near gale-force winds in the south-central Caribbean through early this morning and again late tonight.


With dry sinking air aloft advecting westward across the island, only isolated showers and thunderstorms are possible just offshore the island. Conditions should become generally fair during the rest of the morning and into much of the the afternoon. Scattered showers and thunderstorms are expected to redevelop with daytime heating combining with local island effects today and again on Tuesday. This activity may be limited in coverage, and it is possible some of it may linger into the evening hours before it dissipates.


Three tropical waves are moving across the basin. Refer to the section above for details. An upper-level low lifting northeastward is north of the area at 33N74W, with a trough extending south-southwestward to the SE Bahamas and to southeastern Cuba continues to support scattered showers and thunderstorms over the northwest portion of the basin. This activity is shifting eastward with time. A large upper-level low moving westward is near 24N45W. A resultant surface trough is along a position from 31N50W to 23N52W. Scattered showers and isolated thunderstorms are within 120 nm west of the trough from 25N-28N, and also to the southwest of the trough from 20N-23N between between 55W-60W. The remainder of the basin is under the influence of a broad surface ridge anchored by a 1022 mb high centered near 29N61W. Very stable and dry conditions are observed north of 13N east of 50W where METEOSAT-9 imagery depicts an extensive area of Saharan dust that is pressing westward. No major changes expected through next couple of days.

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

$$ Aguirre