Today is the 5-year anniversary of the historic 2021 PNW heat dome, could it happen again?

June 25-28, 2021. As the song went from Disney’s “The Little Mermaid,” it was Hot, Hot, Hot!

It has been five years this week since the historic “heat dome” parked over the Pacific Northwest during the last week of June, surging temperatures well past all-time record highs.

What is a heat dome?

A ‘heat dome’ is a strong high-pressure system aloft that gets cut off from the main westerly flow around the Northern Hemisphere, in this case across the Northern Pacific Ocean. The system’s sinking air motion and resulting low-level offshore flow toward the ocean produced the blistering temperatures. As an analogy, think of this upper-level high-pressure system with high-level air flow, as a large rock in a river, resting in one spot while the water flows around it.

Over time, descending air in a heat dome creates a progressively warmer air mass. From June 25 through the 28 in 2021, temperatures in the region warmed each day, peaking on the 28 and crushing previous all-time record highs. Bellingham soared to 99 degrees, Everett Paine Field hit 100, Arlington reached 103, SeaTac (SEA) Airport got to 108, Olympia and Forks peaked at 110, and Portland, Oregon suffered from a sizzling 116 degrees. The hottest temperature in the state was a scorching 120 degrees at Hanford near the Tri-Cities.

The previous all-time high temperature at SEA was 103 degrees, established on July 29, 2009. The 108-degree high on June 28, 2021 simply soared past the previous all-time record.

Heat wave impacts

During this historic heat wave, more than 250 people in the Pacific Northwest succumbed to the heat, and in Western Canada, more than 400 people perished. Extreme heat is the No. 1 weather-related hazard, causing fatalities worldwide. In fact, it kills more people than all other weather-related hazards like hurricanes, floods, tornadoes, and winter storms combined.

Yet with the ongoing warming of the planet, the global upper-level air pattern has been tending to produce more of these cut-off upper-level high-pressure systems that create extreme heat waves. For instance, the Southwestern U.S. had a heat dome that resulted in May temperatures occurring in March – well above average, with the thermometer topping 100 degrees across southern Arizona and California.

Last year, heat waves were felt not only in the U.S., but also in Europe, Japan, India, Australia, Argentina, and the Middle East. Temperatures in India and Pakistan soared to around 125 degrees. The hottest reading in the Northern Hemisphere was in Shabankareh, Iran, at 127 degrees. In the Southern Hemisphere, the temperature was 121 degrees at Geraldton Airport in Australia.

Can a heat dome happen again here?

The latest seasonal summer weather outlook for Western Washington reflects good odds of warmer-than-average temperatures into September and tips the odds toward below-normal precipitation during the driest time of the year. This summer weather outlook has been quite common so far this century.

With the ongoing warming of the planet, the intense heat event in the Pacific Northwest in June 2021 can no longer be entirely ruled out. The odds of another heat dome this summer or in the coming years are similar to rolling snake eyes three times in a row. Yet, it could happen again.

What to do if it gets hot?

It has already topped 90 degrees several times now in parts of Western Washington. More summer heat is quite likely ahead.

If sensitive to excessive heat, here are some precautions to help during a period of hot weather.

• Be sure to stay hydrated by drinking plenty of water – avoid alcohol.
• Spend time in an air-conditioned location for at least three hours each day to help relieve heat-related stress.
• If outdoors, seek shade and use sunscreen to avoid sunburn.
• During the heat of the day, avoid strenuous activities like running.
• If working outdoors, take breaks in the shade and drink plenty of water.
• Avoid leaving children and pets in cars, even for just a few minutes, with the windows open.
• Wear light, loose-fitting clothing to help reflect heat.
• If on area waterways, be sure to wear a properly fitted U.S. Coast Guard life jacket to avoid falling into the cold water and suffering cold water shock, potentially drowning as a result.
• For meals, eat easy-to-digest foods like salads and fruit.
• Be sure to check on elderly family members, friends, or neighbors to help ensure they are doing okay during hot weather, particularly if they take medications. Studies have revealed that common treatments such as heart medicines, antihistamines, and decongestants don’t mix well with heat. Continue to take the medications. Just stay cool and stay hydrated. Consult with a doctor for any concerns.

Always monitor the latest weather forecasts for potential heat waves. The National Weather Service has a publicly available heat-related resource called HeatRisk. This resource is a color-coded numeric index that shows a forecast of heat-related impacts.

HeatRisk takes into consideration how unusual the heat is for the time of year, the number of days with expected hot weather (including not only daytime temperatures but also overnight temperatures), and the elevated risk of heat-related health impacts. It is not just the heat of the day, but also the heat of the night.

Enjoy this summer; just do so safely, particularly during periods of hot weather.

Ted Buehner is the KIRO Newsradio meteorologist. Follow him on X and Bluesky. Read more of his stories here.