<Tr> <Td_colspan="14"> Source: NOAA (relative humidity 1962--1990, sun 1961--1990) </Td> </Tr> <P> Winter in North Dakota is characterized by cold (below freezing) temperatures and snowfall. Snow is the main form of winter precipitation, but freezing rain, ice, sleet, and sometimes even rain are all possible during the winter months . Common storm systems include Alberta clippers or Panhandle hooks, some of which evolve into blizzards . Annual snowfall averages from 26 inches (66 cm) in the central part of the state to 38 inches (96.5 cm) in the northeast and southwest . Temperatures as low as − 60 ° F (− 51 ° C) have occurred during North Dakota winters . </P> <P> Spring is a time of major transition in North Dakota . Early spring commonly sees snowstorms, but by late spring as temperatures begin to moderate the state can experience tornado outbreaks, a risk which diminishes but does not cease through the summer and into the fall as North Dakota lies at the northern edge of Tornado Alley . Springtime flooding is a relatively common event in the Red River Valley, due to the river flowing north into Canada . The spring melt and the eventual runoff typically begins earlier in the southern part of the valley than in the northern part . The most destructive flooding in eastern North Dakota occurred in 1997, which caused extensive damage to Grand Forks . </P> <P> Summer sees heat and humidity predominate in the east, while hotter and less humid conditions are generally present in the west . These humid conditions help kick off thunderstorm activity 22--34 days a year . Summer high temperatures in North Dakota average in the mid 80s (30 ° C) in the west to the upper 70s (25 ° C) in the east, with temperatures as hot as 121 ° F (49 ° C) possible . The growing season in North Dakota usually begins in April, and harvest begins in September and October . Tornadoes are possible in North Dakota from April through October, but the peak tornado month is July, followed by June and August . The state averages 13 tornadoes per year . Depending on location, average annual precipitation ranges from 14 in (35.6 cm) to 22 in (55.9 cm). </P>

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