Other facts from our historical weather data: Which is the warmest month in Río Grande? January has an average maximum temperature of 14°C (57°F) and is the warmest month of the year. Which is the coldest month in Río Grande? The coldest month is July with an average maximum temperature of 6°C (43°F). Which is the wettest month in Río Grande? January tops the wettest month list with 56mm (2.2 in) of rainfall. Which is the driest month in Río Grande? October is the driest month with 29mm (1.1 in) of precipitation. No idea where to travel to this year? We have a tool that recommends destinations based on your ideal conditions. Find out where to go with our weather planner.