The meteorological situation of 3 and 4 December 1970 produced heavy rains and floods in the Caribbean side of the mountains of Costa Roca. No significant synoptic systems is observed in the lower troposphere. In the higher troposphere from 500 mb upward, however, an upper trough in the middle latitude westerlies deepens and intrudes in the tropics reaching the latitude of Costa Rica. This upper trough takes an orientation NE-SW in the tropics and results in the formation of a shear line or a cut low. The orographic system of costa Rica is the agent of the heavy orographic rains when an anomalous flow from the NE in the upper troposphere extends upward the normal NE winds in the lower troposphere, and the ascending motion associated with the upper low, increases the thickness of the layer with hydrostatic potential instability. The mountains produce the blocking of the windward low level flow due to the increased stability of the surface layer during the day, associated with the persistent overcast and precipitation. As a result, the Froude number mantains its low value during the day. The blocking appears as a return current from the west in the first few hundred meters. The maximum precipitation is of the order of 350 mm in one day, 700 mm in two days, and 800 mm in three days. These amounts of rainfall are of the same magnitudes as those produced by hurricanes in one day in Central America, and larger for two and three days. The rain fall is characteristically of the continuous and moderate type, but not convective. A mechanism for the gradual liberation of the potential instability due to the initial conditional instability is proposed. The spatial distribution of precipitation shows two regions of maximum: one at the intermediate highs of the slopes exposed directly to the maritime flow, and the other in the plains to the east of the mountains and close to the sea shore. The first is interpreted as a direct orographic effect, and the second as a result of the convergence to the east of the mountains between the return current of the blocking and the synoptic flow from the NE. Others characteristics of the spatial distribution of precipitation reflect the strong influence of topography: regions with orographic barriers in the direction of the flow have considerable less precipitation, regions with low passes between mountains and exposed directly to the maritime flow have more rain on the lee slopes.Se analiza la situación meteorológica del 3 y 4 de diciembre de 1970, que produjo grandes lluvias e inundaciones en la Vertiente del Caribe de Costa rica. Las cartas de la troposfera inferior no muestran ningún sistema sinóptico significativo, pero se destaca la presencia de un disturbio de importancia desde el nivel de 500 mb para arriba. Se trata de una vaguada en los Oestes de latitudes medias que se profundiza y se extiende en las latitudes tropicales hasta Costa Rica. En los trópicos la vaguada de altura toma una orientación NE-SW y da lugar a una línea de cortante o una baja segregada. El sistema orográfico de Costa Rica actúa como generador de lluvias abundantes al producirse un flujo anómalo del NE en la troposfera superior y al aumentar el espesor de la capa potencialmente inestable por encima de sus valores normales. Por otra parte, las montañas bloquean el flujo en la capa superficial a barlovento, danto lugar a una corriente de retorno del Oeste, contrario al flujo sinóptico del NE. Las precipitaciones máximas – del orden de 350 mm en un día, de 700 mm en dos días y de 800 mm en tres días – son del tipo contínuo moderado y no convectivo. Se propone un mecanismo de liberación gradual de la inestabilidad potencial por el ascenso orográfico. Se destaca que precipitaciones de esta magnitud son mayores que las producidas por cualquier otro disturbio sinóptico, incluyendo los huracanes, en América Central.