Insider Blog: What’s a heat dome and why is it so hot this week?

We're just a couple days away from the official start of summer, but the sizzling heat has already settled in as above average temperatures are in the forecast for the entire week.

We typically see our first 90 degree day around May 31, but this year we didn't have a 90 degree day until this past Sunday, the 16th of June. We average around 40 days at 90 degrees or warmer each year in Columbia.

A stagnant pattern has set up for much of the country as upper high pressure is locked in across the East Coast/Mid-Atlantic. With a trough of low pressure to the west, this high likely won't start to flatten or retrograde back in our direction until late in the week.

High pressure signals quiet weather with lots of sunshine and little cloud cover, and allows sinking air. When air sinks, it warms up and causes temperatures to rise. The position of high pressure is also key in our case as winds rotate around the high clockwise. With the high to our east, winds pull in from the south/southeast, bringing in even more moisture directly from the Gulf of Mexico.

The trapped heat creates a "dome" effect as that air doesn't move around much, and the added humidity can make temperatures feel even hotter and become dangerous.

The heat index is what the temperature feels like to the human body when humidity is combined with the air temperature. The higher the relative humidity, the higher the heat index as temperatures approach 80+ degrees. It is harder for humans to cool off when perspiration can't evaporate and cool the skin.

When the heat index tops 100 degrees, it becomes unsafe for vulnerable populations to spend much time outdoors. This week, the heat index will be in the upper 90s through the start of the weekend.