A rapidly worsening drought in the United States shows no sign of easing anytime soon.

A lack of rain and a long-lived heatwave are to blame for the explosive growth of the drought that has hit Montana, South Dakota and North Dakota, but is also growing into southern sections of Canada.

It is being called a "flash drought", which is defined by National Oceanic and Atmospheric Administration, NOAA, as "relatively short periods of warm surface temperature and anomalously low and rapid decreasing soil moisture".

The dire situation affecting the northern-central US border states and southern regions of Canada's Saskatchewan and Manitoba provinces only began in late May. The drought has now grown to extreme levels in the US and severe levels in Canada.

Lack of rain across much of the area was the initial cause of the crisis. But as the ongoing event moved into the summer months it was then the growing heatwave that expanded the severity of the drought situation.

In the state of Montana, Governor Steve Bullock declared a drought emergency in late June. For the northeastern part of the state, this is the worst drought they had seen since 1988. The town of Glasgow, Montana, shattered a 99-year-old record for the driest April through June period.

Fifteen North Dakota counties are eligible for emergency loans as they are now being designated agricultural disasters by the US Department of Agriculture.

The heatwave has been on the increase over the last several weeks with daytime high temperatures reaching into the mid-30s Celsius for the affected areas of the US and Canada.

Unfortunately, there is little relief in the near future as a dome of high pressure continues to remain over the northern Rockies and Plains states. Some forecast models suggest this excessive heat will persist and expand through late July.

Lack of moisture in the soil also means that less moisture is transferred back to the atmosphere, in turn leading to less cloud formation and less rain generated by clouds and thunderstorms.

Strong, large-scale summer wind patterns will also help to aggravate and intensify the growing drought situation across much of the area.