South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 100 PM EDT Thu Mar 13 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ SOUTH AMERICAN FORECAST BULLETIN 13 March 2025 AT 13:00 UTC ...Climate/Teleconnections Conditions... MJO:The MJO is currently in phase 2. The enhanced convective phase is currently crossing Africa and entering the Indian Ocean Basin. This phase also shows enhanced precipitation over the eastern part of Brasil. The large scale upper-level convergence over the Americas will be increasing limiting the coverage of deep convection. ENSO conditions:La Nina conditions are present, a transition to ENSO neutral is expected from April to May. ...Synoptic conditions over tropical South America... In Ecuador and the far southwest of Colombia, elevated sea surface temperatures (SSTs) over the east Pacific, the Pacific Intertropical Convergence Zone (ITCZ), and weak onshore winds at lower altitudes are likely to enhance the development of sea breezes. This process will facilitate the movement of moisture inland during Thursday and Friday, which will encourage isolated diurnal convection. The increased availability of moisture will further bolster convective activity in the region, contributing to ongoing atmospheric instability and cloud formation. Additionally, upper-level divergence will promote the upward movement of air, creating ideal conditions for the formation of convective clouds. However, a slight reduction in moisture influx and rainfall is expected by Saturday. The following maxima are expected: Coast of Eacudor: a maxima of 40 - 80mm on Thursday, and 20 - 45mm on Friday and 20 - 35mm on Saturday. Southwest Coast of Colombia: a maxima of 25 - 50mm on Thursday. Southern Colombia and Venezuela: a maxima of 30 - 60 mm on Friday and Saturday. The transport of moisture by trade winds around the Intertropical Convergence Zone (ITCZ) into central tropical South America, along with upper-level divergence, will continue favoring precipitation along the northern extension of the NET/ITCZ. Trade winds carry warm, humid air from the ocean to the ITCZ, where it converges and favors convective clouds development and favors precipitation maxima of the region. In addition, upper-level divergence over the region enhances this process by promoting upward motion. Therefore, this interaction will sustain precipitation along the NET/ITCZ, affecting northwestern Brasil and southern Venezuela and Colombia over the next three days. ...Synoptic conditions for Brasil... Northeastern Brasil: Upper-level divergence, in conjunction with the trade winds approaching the northeast coast of South America from Guyana to northeastern Brasil, will promote convergence along the coast. This instability is expected to have the most significant impact on northeastern Brasil on Thursday. On Friday these conditions will continue to impact northeastern Brasil, but to a lesser extent. On Saturday, the region will continue to experience a steady influx of moisture at low levels associated with weak tropical perturbations. However, upper-level conditions will not provide sufficient support for deep convection. As a result, expect to see mainly stratiform clouds with a decrease in precipitation maxima throughout the day. Southern Brasil: A persistent and expansive high-pressure system in the low to mid troposphere centered between Pantanal and southeast Brasil will promote moist easterly to northeasterly winds in southwest Amazon and Bolivia, that will result in some showers around the periphery of the high pressure during the next three days. In the Serra do Mar, local effects such as sea breezes, combined with an approaching frontal boundary, will increase the likelihood of showers and thunderstorms from Thursday (today) through Friday. Expect periods of rain, particularly in the afternoons when convection is most likely to occur. The peak will be on Saturday with maxima of 20 - 35mm. ...Synoptic conditions for Bolivia, Paraguay, Uruguay and Argentina and Southern Brasil... From Thursday through Saturday, a zonal upper-level jet will be positioned over northern Argentina and southeastern Brasil. This jet stream will contribute to enhanced weather activity in the region. From Thursday through Saturday, a mid-level short wave will be moving over northern Argentina and Paraguay. This system will bring limited potential for showers and localized thunderstomrs across Paraguay and the ParanĂ Basin in Brasil from today (Thursday) into early Saturday. As the strong surface high pressure system advances from Thursday evening into Friday, dry and stable conditions will prevail over Argentina, Uruguay, and partly southeastern Paraguay. Overall, drier air at mid levels will be dominanting Argentina and central Paraguay, therefore conditions will remain mostly stable with occasional clouds and isolated showers possible in the affected areas. By Saturday, a weak upper-level trough and a short wave at mid-levels will move across Argentina, interacting with a surface low pressure system. This combination will create a line of convergence over central Argentina with a maxima of 15 - 20mm, primarily affecting the southern Cuyo and northern Patagonia. The sea breeze will enhance moisture convergence over the northern Patagonia coast, promoting increased convection and higher precipitation rates. Expect scattered showers and thunderstorms, particularly in the afternoon and evening hours, with the potential for localized heavy rainfall rates and slight risk of severe weather. In Bolivia and Peru, easterly winds at both the low and upper levels will enhance moisture influx from the interior of the continent. Further the Bolivian high will be promoting upward motion over the northern Bolivia and eastern Peru. In the Altiplano and southern Peru diurnal convetion will be enhance due to an upper trough and jet dynamic approaching from the southwest, favoring precipiation maxima over the next three days. Overall, conditions will be warmer and more humid as moisture continues to flow into the area for Friday and Saturday. ...Synoptic conditions for Chile... As the surface high-pressure system (south Pacific anticyclone) advances toward Chile, stable conditions are anticipated over central and southern Chile over the next three days. This stability is linked to subsidence caused by the high-pressure system, which suppresses vertical motion and restricts cloud formation. In the southern and austral regions, conditions will also remain predominantly stable, with the presence of stratiform clouds. By Friday, an upper-level jet is expected to move into the Zona Austral, aiding in the development of stratiform rain. However, low-level moisture advection is anticipated to be limited, which may restrict the intensity of precipitation. A maxima of 15 - 20mm on Thursday and maxima of 15mm on Friday and Saturday. Overall, expect mostly dry and stable weather conditions across these regions, with some light rain possible in the south by the end of the week. For more QPF graphical information: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Ledesma...(WPC) Galvez...(WPC)