Rainfall is unpredictable, but is concentrated in the rainy season from May to October. Average annual precipitation varies from 2,000 mm (79 in) in some areas to 1,270 mm (50 in) in the southwestern provinces of Nakhon Ratchasima, Buriram, Maha Sarakham, Khon Kaen andChaiyaphum. The rainy season begins with occasional short but heavy showers, eventually raining very heavily for longer periods almost every day, usually in the late afternoon or at night, until it ends abruptly at the onset of the cool season.