http://mashable.com/2014/07/10/polar-vortex-redux-redux-record-cold-july-weather-on-the-way/
The strange weather pattern has its roots near Hudson Bay, Canada, where so much of last winter's cold originated. The cold air will be spinning around underneath an area of low pressure at upper levels of the atmosphere, which the jet stream, which is the river of air at about 30,000 feet, is going to steer south, into the U.S., over the weekend.
The dip in the jet stream, known as a "trough," is connected via a long chain of events to once-
Super Typhoon Neoguri, which struck Japan on Wednesday as a weakened tropical storm, according to
Jeff Masters of Weather Underground.The large and powerful nature of this storm has set in motion a chain-reaction set of events that will dramatically alter the path of the jet stream and affect weather patterns across the entire Northern Hemisphere next week. Neoguri will cause an acceleration of the North Pacific jet stream, causing a large amount of warm, moist tropical air to push over the North Pacific. This will amplify a trough low pressure over Alaska, causing a ripple effect in the jet stream over western North America, where a strong ridge of high pressure will develop, and over the Midwestern U.S., where a strong trough of low pressure will form. This jet stream pattern is similar to the nasty "Polar Vortex" pattern that set up during the winter of 2014 over North America, and will cause an unusually cool third week of July over the portions of the Midwest and Ohio Valley, with temperatures 10 - 20°F below average.