McLean experiences a humid subtropical climate (Köppen climate classification *Cfa*), featuring four distinct seasons. Summers are characteristically hot and humid, with average July highs in the upper 80s to low 90s (°F) and frequent evening thunderstorms. Winters are generally cool to cold, with January lows averaging in the upper 20s.
Rainfall is well-distributed throughout the year, averaging about 43 inches annually. Snowfall is highly variable but averages around 15 to 20 inches per season, occasionally delivered by potent coastal storms known as nor'easters. Spring and autumn are widely considered the best times to visit, offering mild temperatures, lower humidity, and vibrant seasonal foliage. Natural disasters are rare, but the area can occasionally experience remnants of tropical storms or localized flooding in low-lying river areas.