| Summary: | including sleet, freezing rain, and hail. Extreme weather events such as tornadoes, dust storms, and blizzards are also common at various times of the year. Most snow storms originate as a low pressure system in the province of Alberta, Canada. Known as "Alberta Clippers," these storms move quickly southeastward across the state providing low amounts of snow and brisk northerly winds afterwards. Occurring less frequently, the heaviest snowfall occurs when storms originate in southeastern Colorado, known as Colorado Lows, and move northeastward across South Dakota and into Minnesota. This low-pressure system draws greater moisture from the Gulf of Mexico, which is then lifted over colder air across North Dakota. These storms are capable of producing snowfalls sometimes in excess of 20 inches in a single storm. No matter the season, there is a potential for snow in North Dakota as it has snowed in the state during every month except July. Annual snowfall varies from near 30 inches to just over 50 inches across the state. The southeastern portion of the state generally has higher snowfall amounts than the northwestern region. The coldest months are December, January, and February, with daily average temperatures in the single digits Fahrenheit in early January across the state. The polar jet stream, a band of high velocity air located in the upper half of the troposphere that defines the storm track for precipitation-producing conditions, often becomes favorably positioned across North Dakota during the spring and summer, helping to convert the northward flux of Gulf of Mexico atmospheric moisture to precipitation. As summer progresses, the jet stream shifts northward into southern Canada giving North Dakota a drier and warmer climate. The hottest weather is typically found in the months of June, July, and August, but the hot conditions can sometimes begin as early as April and extend into early October. Owing to the large contrast between summer and winter temperatures. North Dakota has the third greatest temperature variation in the United States and the greatest for all non-mountainous states. Summer conditions result in greater humidity in the east, while hotter and less humid conditions generally occur in the west. The return in spring and summer of warm and humid conditions results in the formation of thunderstorms, which provide the majority of the total annual precipitation across the state. There are on average slightly more than thirty tliunderstorm producing days across North Dakota each year. Thunderstorms primarily occur during the months of May, June, and July, but have occurred in even' month of the year. The frequent strong intensity of these storms results in approximately $340 million in crop damages each year. At times the stronger thunderstorms will also produce tornadoes. The state averages thirty-two tornadoes per year with July the peak tornado month, followed by June and August. Summer high temperatures in North Dakota average in the mid-80s Fahrenheit in the west to the upper 70s Fahrenheit in the east. While temperatures in excess of 120 degrees Fahrenheit are possible, temperatures above 100 degrees Fahrenheit are infrequent and occur predominantly in the western half of the state. During winter, deep polar and arctic air masses frequently spread southward from Canada across North Dakota. This results in some of the coldest conditions in the 48 contiguous states during the months of December through February. Often the coldest weather comes as the polar jet stream dives southeastward from eastern 518 2013-2015 North Dakota Blue Book
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