McAllen, like much of South Texas, has a subtropical climate , featuring long, very hot and humid summers, and brief, warm winters. The city has two distinct seasons; a wet season from May to October and a dry season from November to April. The normal monthly mean temperature ranges from 60.8 °F (16.0 °C) in January to 87.4 °F (30.8 °C) in August. The warm season is extremely long, as average high temperatures from May through September are above 90 °F (32 °C) and average low temperatures are above 70 °F (21 °C), with relatively high dew point values, resulting in higher relative humidity values and heat index values. Heat indices consistently reach over 110 °F (43 °C) during these months. Winter temperatures in McAllen and surrounding Rio Grande Valley are some of the warmest in the contiguous United States outside of South Florida, and comparable to the Coachella and Imperial valleys, but with warmer nighttime lows and higher dew points.
Average annual precipitation is 22.20 inches (564 millimeters). Most precipitation occurs in the warm season, with the least precipitation distinctly occurring in the cooler winter. As September is the peak of the north Atlantic hurricane season and tropical storms and hurricanes occasionally drop copious amounts of rainfall on the region, this month tends by far to be the wettest, averaging 4.47 in (114 mm) of rain. The driest month is November, with only 0.89 in (23 mm) of precipitation. Since 1941, it has snowed twice, once when the city received 1.7 in (43 mm) on December 25, 2004.
Temperatures consistently rise above 100 °F (38 °C) from June through August, with exceptionally high humidity. The highest temperature ever recorded in McAllen was 111 °F (44 °C), set on June 22, 2017. The lowest temperature ever recorded in McAllen was 13 °F (−11 °C), on January 12, 1962.
While usually a dry, hot climate, McAllen, Texas has received unusual weather phenomenons. In 2013, McAllen, Texas received a large hail storm that destroyed many vehicles and buildings. The storm came suddenly and many people were unprepared. In December 2017, McAllen received almost 3 inches of snow, nearly 13 years since the last snowfall on Christmas Day 2004. In February 2021, McAllen reached record low temperatures between 10 °F (−12 °C) and 20 °F (−7 °C) and lost power, heat, and water for a week due to Winter Storm Uri. Regardless of these freak weather events, McAllen is prone to tropical storms, hurricanes such as Hurricane Dolly (2008) and Hurricane Hannah (2020), and heatwaves.