FXCA20 KWBC 181914 PMDCA Tropical Discussion - International Desks NWS Weather Prediction Center College Park MD 313 PM EDT Mon May 18 2026 Forecast Bulletin 18 May 2026 at 1915 UTC: Across Mexico... A dynamic mid-to-upper level atmospheric pattern will persist across northeast Mexico over the next three days. On Monday, an upper-level jet streak will drive the development of a mid-level shortwave trough in the afternoon. At the same time, southerly low level winds will be strong and it will support the transport of moist tropical air into northeast Mexico and the Central Plains of the United States. Expect enhanced moisture convergence in Tamaulipas and northern Veracruz with this pattern, where moderate rainfall accumulations are anticipated. Thereafter, upper-level divergence will continue to dominate across the region, accompanied by a series of developing mid-level shortwave troughs. However, precipitable water values will decrease across the Gulf states on Tuesday and Wednesday. Moisture convergence will peak across Tamaulipas and Nuevo Leon on Wednesday, driven by a brief resurgence of easterly and southeasterly low level winds. Thus, expect moderate rainfall accumulations for Wednesday in this region. In central Mexico, diurnal showers and thunderstorms will prevail over the next three days. The most intense precipitation accumulations are expected on Tuesday and Wednesday, when a mid-to-upper level trough will increase upper-level divergence and vertical lift. While there will be light accumulations due to orographic effects on Monday, moderate rainfall is likely across the broader region on Tuesday and Wednesday. Across the Bahamas, Greater Antilles, and Lesser Antilles... A potent, negatively tilted mid-to-upper level trough currently has an axis across the southeast Bahamas and the Greater Antilles. Upper level wind speeds will begin to increase in the exit region starting on Tuesday, causing the trough to transition to a more neutral tilt thereafter before losing amplitude on Wednesday. This dynamic pattern will support upper divergence and instability across the region. Meanwhile, lowered surface pressures driven by the upper trough will maintain a deeply moist environment. At the same time, low level troughs will be sustained, leading to the increase in moisture convergence and cyclonic rotation. Thus, expect daily moderate total precipitation maxima particularly in the Bahamas and Cuba. Elsewhere in the Caribbean, weaker moisture plumes and low level troughs traversing the islands will yield light daily precipitation accumulations. Across Central America and Tropical South America... As the aforementioned negatively tilted upper trough begins to lose its amplitude, its base will retrograde into the south Caribbean Sea. This will enhance upper divergence across Colombia and Venezuela while increasing subsidence across Central America on Tuesday and Wednesday. Along the Caribbean coast of Central America, the low level conditions will drive the precipitation patterns. Moisture pooling combined with orographic effects will further enhance precipitation chances, yielding a moderate total precipitation maxima across Costa Rica and Panama over the next three days. On Wednesday, a low level trough wave will move west of the Andes, significantly increasing low level moisture convergence. The arrival of the trough is expected to intensify the Panamanian trough, strengthening low level cyclonic rotation at the low levels. Elevated total precipitation maxima is likely with these conditions along the Pacific coast of Colombia. Meanwhile, a series of tropical waves will be increasing precipitation chances across the Amazon Delta, Guianas, and Venezuela for the next three days, yielding moderate total precipitation maxima. Another region of interest is the western Amazon Basin. On Monday, an upper level trough will cross the Andes from west to east, supporting upper divergence across the jungle. In the low-to-mid levels, dominant easterly winds will assist the transport of moisture into the region, yielding elevated rainfall accumulations on Monday. Meanwhile, in the central Amazon, expect the propagation of a series of low level trough on Tuesday and Wednesday. These features will support the development of thunderstorms and increase moisture convergence leading to moderate rainfall accumulations. Elsewhere in tropical South America, the presence of the Intertropical Convergence Zone (ITCZ) near the Amazon Delta will continue to support daily thunderstorms and moderate rainfall accumulations. Otherwise, daily diurnal showers and thunderstorms will be likely across the rest of the region. Positions of Tropical/Easterly Waves Initialized at 12 UTC Type SOF INIT 19/00 19/12 20/00 20/12 21/00 21/12 22/00 22/12 TW 10N 24W 27W 31W 35W 39W 43W 46W 49W 53W TW 07N 53W 56W 59W 61W 64W 67W 69W 72W 74W TW 10N 79W 80W 81W 83W 85W 87W 90W 92W 94W TW 12N 94W 97W 100W 103W 107W 110W 113W 116W --- Tinoco-Morales...(WPC) $$