The Earth's atmosphere is a complex system made up of centers of high and low pressure. These air masses interact to form sensible weather. In the upper atmosphere at mid to high latitudes these air masses interact in a complex process that produces what has come to be known as the jet stream.
The jet stream is defined as upper level winds that fluctuate in intensity and location, but always travel in the same direction. In the northern hemisphere, these winds travel from west to east, and go completely around the globe at mid-latitudes. The name of these winds is the circumpolar vortex, but they are refereed to as the jet stream, especially over North America. The location and intensity of these winds are determined by the presence of air masses and temperature gradients. It is a complicated interaction that is a result of ridges and troughs in the upper atmosphere created by differences in pressure systems. These circumpolar winds are most predominant in the winter when the temperature gradient is the greatest. Massive high pressure systems form over the cold continents of North America, Europe, and Siberia (the Siberian High is the largest and most dominant). Over the waters of the northern Atlantic and Pacific, relatively warmer air promotes the formation of massive lows (the Aleutian low and Icelandic low). This is a difficult concept to comprehend, especially without images. Picture looking at a map of the northern hemisphere with a continous bending wave drawn. Where the peak bends toward the poles there is warm air, while where it bends to the south there is cold air. This in effect is the jet stream, and it serves the purpose of a temperature boundary for air.
The winds involved with the jet stream are extremely high in velocity, reaching speeds of hundreds of miles per hour. This in turn pushes the weather in the same path of these winds, dictating what surface weather is like. Generally the strongest part of the jet stream is when the "wave" begins to bend upwards. Storms that form near the location of the jet stream will follow it's path. In the winter, the location of the jet stream is further south and the intensity is the greatest. For North America, the upward bend occurs along the eastern seaboard of America and heads towards Greenland and the Icelandic Low. This causes most storms to have a northeasterly path (Nor'easteres are commonly known storms that form off the coast and travel northward, producing heavy precipitation). During the winter months, there are a high volume of storms converging on the mid-Atlantic and New England states. In the summer when the jet stream is weaker and further north, there are less storm tracks across the Northeast because they are further into Canada. This will lead to relatively calm weather, with the exception of thunderstorms.
As for a relationship between the jet stream and storm severity, there is a definite correlation. More so, the frequency of storms is directly related to the location and intensity of the jet stream. Because the jet stream acts as a temperature boundary, there is a higher gradient when the jet steam is nearby. This increased gradient will lead to a higher probability of the formation of stronger storms.
The jet stream is a dominant factor in determining the type of weather a location will experience. The bends in the air flow act as always changing boundaries. An extreme shift in weather patterns can be attributed to a fluctuation in the jet stream, either north or south. As for storms, the jet stream plays a pivotal role in aiding in the formation and trajectory of many storms all over the world, not just in North America.