Following on the heels of the hottest June in the history of the lower 48 states, an extended, intense and widespread heat wave is likely to develop next week.

The heat wave will start in the Plains states and parts of the intermountain West, eventually spreading to the West Coast, South and Midwest by Thursday, July 21. The heat wave is also likely to seep northward into southern Alberta and Ontario.

Cities like Dallas, Denver, Oklahoma City, Kansas City, Chicago, Minneapolis, Fargo and eventually New York City and Washington, D.C. may experience sizzling heat and stifling humidity by the end of the week.

During the height of the heat wave, a majority of the contiguous U.S. is likely to see high temperatures well above 90 degrees Fahrenheit, or 32 degrees Celsius. There will also be areas that will see temperatures climb into the 100s Fahrenheit, or above 38 degrees Celsius.

Another hazard may also be found along the northern fringe of the heat dome, from southern Canada across the northern Great Lakes states and into New England. Atmospheric disturbances tend to ripple across the outer edges of such weather patterns, triggering large complexes of severe thunderstorms.

June temperature departures from average. Image: Noaa

While this is an extended range forecast with details that are subject to change, the warning signs for a major heat wave are present, and have been for several days, in several of the main computer models used to help predict the weather.

Both the European, or ECMWF, and the GFS models are showing the extreme heat developing early in the week and growing more intense and widespread as the week goes on.

One parameter meteorologists look at to judge the severity of a hot or cold streak is the height of a given pressure surface in the atmosphere.

With hot air masses, a particular pressure surface will be found higher in the atmosphere, since hot air rises and causes a bulge to develop that pushes pressure surfaces above where they would normally be located.

On the other hand, cold air masses feature below average heights.

Computer model projection for height anomalies on Friday July 22, 2016. Image: weatherbell analytics

Both the European and GFS models, among others, are depicting the height of the 500 millibar pressure surface, which is normally located around 5,000 meters, or 18,000 feet, to be at or above 6,000 meters, or 19,685 feet.

This is a rare event that is an indication that this event may be unusually severe.

Heat waves such as this one, which will also involve high humidity, due to a flow of air that will pump Gulf of Mexico moisture northward into the Plains, have the potential to cause heat-related injuries and fatalities.

One of the most confident conclusions in climate science is that heat waves are becoming more intense and more common as the world warms in response to increasing amounts of greenhouse gases in the air.

Skewed records

Another trend that is happening as the world warms is that record highs are outpacing record lows by an ever-growing margin.

Computer model projection for the odds that the 500 millibar pressure surface will be above 5970 meters on July 21, 2016. Image: weatherbell analytics

For example, during June, there were more than seven times as many warm temperature records than there were cold temperature records in the U.S.

+/- one sigma of ensemble mean in Topeka (EPS 00z) includes 6000 m threshold. pic.twitter.com/AEBglIaQvJ — Ryan Maue (@RyanMaue) July 13, 2016

More specifically, there were 5,768 record high temperatures set or tied during the month, compared to just 411 record lows during the same period.

The July heat wave will ensure that this records gap either merely continues, or, more likely, grows much wider. Unlike the June heat, though, this event is not likely to peak across the Southwest, but rather across the Central states and Midwest.

Over longer periods of time, the record highs to lows ratio is becoming more skewed in favor of warm temperature records, reflecting the increase in average temperatures that raises the odds of warm records.

If global average surface temperatures were not going up, the ratio between record highs to lows would be expected to be closer to 1-to-1.