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In Brazil the adoption of semi-confinement leads to special adapted management methods in dairy production, and milk production can be improved by the use of technology that assures better herd management. Index variables such as air temperature, relative humidity and air speed were considered. The values of rain index, solar radiation and pasture soil temperature were also considered as stress agents that could potentially influence milk production. The objective of this research was to identify the variables that potentially would interfere in the milk yield in Jersey cows, and to develop an index for predicting milk production for high productivity milking cows lodged in pasture under hot weather conditions. The experiment took place in Ribeiro Preto, in Southeastern Brazil, latitude 210 11 S, longitude 470 48 W, and altitude 621 m, using two treatments with 14 Jersey cows each: Treatment A cows waited for 30 minutes prior to milking in a room with a shower and a fan, and Treatment B the cows did not have access to this room (control). At other times the cows had access to a pasture. Differences in the effect of the treatment in the average milk yield were not statistically significant for the treatments. The analysis for studying the effect of designing the model led to a probabilistic model relating the variables milk production and rain index coeficient: Milk Production (kg) = 10.67 (kg) + 0.017 (kg/mm).