The middle of July is typically one of the hottest weeks of the year in the Midwest and Central United States.

Well, not this year. Instead, the region that was locked in the deep freeze for the entire winter and much of the spring — Lake Superior's ice cover lasted until June, setting a new record — is about to shiver again. OK, maybe not shiver. But it's going to be 20 to 30 degrees cooler than average for this time of year.

Six-to-10 day temperature outlook from the National Weather Service, showing the huge area of below average temperatures across the eastern half of the country. Image: NOAA

High temperatures in northern Minnesota on Sunday may barely crack 70 degrees Fahrenheit, with a high of just 60 degrees on Monday, and overnight lows that could dip into the 40s. This is fall jacket weather, when it should be air conditioner, ice cream truck, and lemonade weather.

Is this the polar vortex all over again, but in the summer?

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.

On jet stream charts, this looks like a textbook January weather pattern (it bears "a haunting resemblance," according to the Capital Weather Gang blog), not July, so it's possible that Mother Nature just mixed up her "J" months.

Temperature departures from average forecast for Sunday, July 13, 2014. The cold air, in blue and green, will be poised to enter the Midwest. Image: WeatherBell Analytics

This cannot strictly be viewed as an actual polar vortex event, but there are some similarities to the weather pattern in place during the winter of 2013-14. The real polar vortex, which to meteorologists means something different than it has meant colloquially, concerns a pattern of winds in the upper atmosphere — at or above the height that most jet aircraft fly — which typically pens in extremely cold air across Canada and the Arctic, and grows particularly strong during the winter.

Polar vortexes exist in both hemispheres. They are not a new phenomenon, having been in weather textbooks for decades.

In January, the vortex weakened and wobbled a little bit to the south of its typical position. This and a few other factors helped bring some of the extremely cold air southward. That's when the term “polar vortex” took on a life of its own via social media, and now is used colloquially by many people to refer to unusual cold.

Temperature departures from average forecast for Tuesday, July 15, 2014. The colder than average temperatures will have spilled almost to the Gulf Coast, as warm weather builds in the West and Northwest. Image: WeatherBell Analytics

Here's how the National Weather Service's Weather Prediction Center in Maryland describes the upcoming event (ALL CAPS is their style, because apparently, government weather forecasters like to shout):

A HIGHLY ANOMALOUS AMPLIFICATION OF THE FLOW ACROSS NORTH AMERICA IS INDICATED BY THE GLOBAL NUMERICAL MODELS AT THE MEDIUM RANGE. A DEEP UPPER LOW... NOT THE POLAR VORTEX AS ITS ORIGINS ARE FROM THE NE PACIFIC... WILL SWING THROUGH THE GREAT LAKES EARLY NEXT WEEK WITH AN IMPRESSIVE COLD SHOT OF AIR INTO THE CENTRAL AND THEN SOUTHERN PLAINS AND THE MIDWEST.

According to the National Weather Service forecast office in Chicago, the coldest that temperatures have been since 1979 at the 850 millibar pressure level, which is at about 5,000 feet above the surface, have been around 45 degrees Fahrenheit. Computer model forecasts for next week show such temperatures bottoming out at 41 degrees Fahrenheit, indicating the unusual nature of this cold snap. Forecasters look at 850 millibar temperatures to get an idea of what surface temperatures may be.

Forecast temperature departures from average for the entire globe, on Tuesday, July 15, 2014. Image: The Climate Reanalyzer

The upcoming cool conditions will be offset by record warmth in the West, where 100% of California is experiencing drought conditions, and record heat may occur in Seattle and Portland early next week as well. The heat in the West, combined with the dryness, poses a wildfire risk.

The cool air mass will affect much of the Midwest, Central and Plains states as well as the Ohio Valley before spilling into the Mid-Atlantic and Northeast. In many areas, including Chicago, Washington, D.C., and New York, the cool weather will be a welcome respite from the hot, humid and stormy conditions that have been the theme this week.

In fact, the cold front may make it all the way to the Gulf Coast, which is extremely unusual for this time of year.

It's worth noting, though, that while the U.S. experiences unusual cold, much of the world will be remarkably warm for this time of year. May was the world's hottest such month on record, and it's likely that June and July will rank in the top five as well.

Correction, July 11: We updated the captions on the maps to show the correct dates.