258 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) $$  137 FXUS01 KWBC 181933 PMDSPD Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 332 PM EDT Mon May 18 2026 Valid 00Z Tue May 19 2026 - 00Z Thu May 21 2026 ...Severe weather and flash flooding threats continue across the Central Plains and Midwest into tonight before shifting south and eastward through midweek... ...Extreme fire weather concerns across the southern High Plains through this evening... ...Heavy wet snow continues across the Central Rockies before tapering off tonight... ...An early-season heatwave will challenge temperature records across the eastern U.S. through Wednesday... A strengthening frontal system will be the focus for another round of severe weather and flash flooding across the Central Plains and Midwest into tonight. Rich moisture streaming northward from the Gulf will interact with a wavy, slow-moving cold front draped from the Upper Midwest to the southern Plains, triggering numerous showers and thunderstorms this afternoon and evening. This activity will be bolstered by an extremely unstable air mass, setting the stage for thunderstorms to become strong to severe. The greatest threat for severe storms will be across the Central Plains and the mid/southern Missouri Valley where the Storm Prediction Center has maintained a Moderate Risk (level 4/5) of severe weather. The most intense storms will be capable of producing damaging winds, very large hail, and a few potentially strong tornadoes. In addition to the severe threat, there is also a heightened risk of heavy rain and flash flooding, especially across parts of Missouri, Iowa, Nebraska, and Kansas. Slow-moving clusters of storms will produce numerous downpours with rainfall rates of 1-3"/hr. These drenching storms will likely repeatedly track over the same areas, leading to an increased risk of flash flooding. As a result, the Weather Prediction Center continues to highlight a Moderate Risk (level 3/4) of excessive rainfall through tonight. The frontal system begins to pick up speed as it pushes eastward, shifting the severe weather and heavy rainfall threat to the Southern Plains, Ohio Valley, and Mid-Atlantic Tuesday into Wednesday. Meanwhile, a late season winter storm continues to bring heavy mountain snow to the Central Rockies this afternoon. Snow will continue through this evening before gradually tapering off tonight. Several inches of heavy wet snow accumulation will be measured once snow comes to an end. Unseasonably cold air will follow in the wake of the storm, leading to frost and freeze concerns from the Intermountain West to the northern High Plains tonight into early Wednesday morning. On the hotter and drier flank of this dynamic and multifaceted storm system, the concern shifts to extremely critical fire weather conditions across portions of the southern High Plains this afternoon and evening. Strong gusty winds combined with susceptible fuels and relative humidity values in the single digits and teens will support the ignition and extremely rapid spread of new and ongoing wildfires. These critical conditions could become exacerbated when a cold front sweeps southward and rapidly shifts surface winds from westerly to northerly later this afternoon and evening. With snow, severe, and fire weather in full swing across the Rockies into the Plains, the Eastern U.S. is entering an early-season heatwave. A Bermuda high pressure system will remain anchored over the western Atlantic, steering a long fetch of warm, southerly flow across the eastern half of the country. High temperatures will soar well into the 80s and 90s the next few days, challenging high temperature records across parts of the Ohio Valley, Mid-Atlantic, and Northeast. Widespread moderate to major HeatRisk is forecast, highlighting the season's first extended period of potentially hazardous heat for millions across the Eastern U.S. Miller Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php $$