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It's time to take a meteorological term that we packed away for the winter out of storage: heat dome. It's a naturally occurring weather feature every summer in different parts of the world, and it just so happens to be preparing to set up shop over Minnesota. 

When temps surge into the 90s this coming Tuesday, it may be an early symptom of the larger pattern taking shape for the rest of June into much of July. Nothing is set in stone, but it's looking like a feverishly hot few weeks ahead. 

Tracking the heat dome

Our upcoming heat dome is developing in the Desert Southwest and will be breaking records in areas that are used to heat, like Las Vegas.

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The heat dome will then push east, giving Minnesota a brief taste as temps Tuesday soar into the 90s.

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The record high for Tuesday is 98, set back in the scorching 1987 summer. An upper-level low (cool air aloft) will touch off storms and give us a break from the 90s Wednesday into Friday, forcing the heat dome to compress south.

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Make sure to take a breath and come up for air because even hotter air potentially sets in next weekend, June 18-19, as the heat dome could essentially center itself over the region. 

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That’s the thing with heat domes: they can continue to expand and get hotter throughout June and July, and given the right conditions a heat dome can become the dominant weather feature/pattern for an area.

Could Minnesota have a rare 100-degree day in June?

Hitting 100 degrees in Minnesota doesn't happen all that often no matter the time of year, but it's happened in the Twin Cities just seven times in June in the record-keeping era. The last time it happened was June 7, 2011, when the temp rose to a blistering 103 in the Twin Cities. 

Regardless of month, the Twin Cities has hit 100 just five times in the last 30 years: 

  • May 28, 2018: 100 degrees
  • July 6, 2012: 102 degrees
  • July 4, 2012: 101 degrees
  • June 7, 2011: 103 degrees
  • July 31, 2006: 101 degrees
  • July 13, 1995: 101 degrees

Based on the latest models, there's about a 20% chance we reach 100 in the next 10 days, with June 18-19 seemingly the best chances for it to happen. Climatologically there's about a 10% chance, so the odds are in favor of NOT hitting 100 but the models suggest it's about twice as possible as a normal June. 

Long range heat?

The concern I have in the longer term over the next several weeks is that this heat dome will continue to dominate the central U.S. weather. This is not set in stone yet but the models are hinting at big-time heat. The European model is the most aggressive with the heat and dry weather.

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The above graphic shows temperature anomalies (difference from normal) forecasted from the European model for June 11 through July 25.

The Twin Cities area is forecast to have average temperatures of +3 degrees F above normal (again this is the European model, which is the warmest scenario at present). That may not sound like a lot, but our standard deviation in average temperatures for June and July is only 2.5 degrees, so that’s more than 1 whole standard deviation above normal. This is why we stress that just 1 or 2 or 3 degrees in climate change has big impacts. Let’s look at an example for us.

A standard deviation warmer than average for June and July would result in a doubling of the number of 90-degree days. That’s because when you increase an average for a pile of data (in this case highs and lows averaged over two months almost) you exponentially increase the extremes.

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Remember, a heat dome by definition is also dry due to the stability within it. This would coincide with below normal rainfall in areas that are subject to its influence. For this reason, the European model (as well as the others) are forecasting below normal precipitation in this same period.

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So we may go from a very wet spring with record high water levels into at least a very dry period, if not possible drought for some areas.

Climate change finger prints?

This is a good place to segue into the impact of climate change. We just talked about how an average of 2 to 3 degrees (F) doubles the number of 90s (extreme type temperatures for us). June average temperatures have warmed 2.1 degrees (F) in the last century for the Twin Cities. Again, that doesn’t sound like a lot, but it’s 0.8 standard deviations, which is a big impact. Such a shift makes something that would have had a probability of 2% a century ago now 13%, or more than a six-fold increase. 

By that math alone we can (in a simplistic scenario) state that a heat wave like this or any hot extreme in June/July has been made 6.5 times more likely due to climate change, all because of that 2.1 degrees F warming.

Pinpointing one specific weather event is complex and rapid attribution studies are now done to determine the climate change impacts on events. We know for example that weather disasters have increased by five times in the U.S. The Pacific Northwest heat wave last June was made 150 times more likely due to climate change and the recent heat wave in India has been determined to be 100 times more likely due to climate change.

So, while heat waves are a normal part of summer, they’re becoming longer and more intense and more frequent due to our warming of the planet. Those impacts spill into water resources and fire seasons in places like California and Australia. In addition to preventing further catastrophic, future warming we also need to adapt to this new reality, especially in cities and desert areas to prevent loss of life and disaster. 

What’s a heat dome?

What is a heat dome? It sounds more fancy than it is. Simply put, it’s an area of high pressure aloft that compresses air downward (heating it up and preventing clouds, precipitation from developing). This means that intense sunshine heats up and dries out the air and soil more and more over days or weeks. The heating migrates through the entire atmosphere creating a very stable airmass (i.e. little chance of developing storms).

Heat domes are seasonal. You need intense sun (May through July in the northern hemisphere, November through January in the southern hemisphere) and typically they begin over arid areas. In the United States this occurs over places like Texas or the Desert Southwest. In the southern hemisphere they develop over parts of interior Africa and Australia. 

It is no coincidence that the largest deserts occur near areas of 30 degrees north or south latitude. This is where dominant areas of high pressure routinely form as air descends after rising from thunderstorms closer to the equator. We call these ‘hadley cells’ in meteorology.

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These belts of circulation are important to understanding global circulation.

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Basically heat waves / heat domes develop seasonally with intense sun in areas that are already prone to high pressure. They then grow and expand until atmospheric circulation (like the jet stream) move them into other areas like the Midwest or the East, etc.

BMTN Note: Weather events in isolation can't always be pinned on climate change, but the broader trend of increasingly severe weather and record-breaking extremes seen in Minnesota and across the globe can be attributed directly to the rapidly warming climate caused by human activity. The IPCC has warned that Earth is "firmly on track toward an unlivable world," and says greenhouse gas emissions must be halved by 2030 in order to limit warming to 1.5C, which would prevent the most catastrophic effects on humankind. You can read more here.

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