The makings of a Heat Wave
We need to take a term out of storage from last summer you may have heard: ‘Heat Dome.’ A heat dome is a naturally occurring weather feature every summer in different parts of the world.
The upcoming heat may be a symptom of the larger pattern taking shape for the rest of June into much of July. Nothing is set in stone yet but if you are not a fan of the heat- it’s not looking good.
What’s a heat dome?
What is a heat dome? It sends 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 mass and soils more and more over days or weeks. The heating migrates through the entire atmosphere creating a very stable air mass (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 worlds 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.
(Credit: NOAA)
These belts of circulation are important to understanding global circulation.
(Credit: NOAA)
Basically heat waves / heat domes develop seasonally with intense sun in areas that are already prone to high pressure. They then grow and expand more until atmospheric circulation (like the jet stream) move them into other areas like the Midwest or the east, etc.
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.
The heat dome will then push east. This is when we get our first taste, somewhat briefly Tuesday with highs soaring into the 90s.
The record high is 98 on Tuesday 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. The heat dome will then compress south.
Make sure to take a breath and come up for air because then even hotter air potentially sets in for next weekend (June 18th-19th).
That’s the thing with heat domes. They can really continue to expand and get hotter throughout June and July given the right conditions and become the dominant weather feature/pattern in an area.
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 this. The European model is the most aggressive with the heat and dry weather.
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 deg 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 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.
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.
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 Segway into the impact of climate change. We just talked about how an average of 2 to 3 deg (F) doubles the number of 90s (extreme type temperatures for us). June average temperatures have warmed 2.1 deg (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 6 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, due to that 2.1 deg F warming.
Pin pointing one specific weather event is complex and rapid attribution studies are now done to determine the ‘forensics’ of climate change on events. We know for example that weather disasters have increased by 5 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.
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