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A heat wave is not unusual during the summer months. More and more, you may be hearing meteorologists use the term "heat dome" to describe the weather pattern responsible for these prolonged stretches of heat.
A heat dome is an exceptionally warm air mass associated with a region of high pressure in the upper levels of the atmosphere, according to the American Meteorological Society. In simple terms, very hot weather on Earth's surface sits under high pressure a few miles overhead.
The name comes from the way the atmosphere responds to heat. As the atmosphere warms, it expands upward. At the same time, high pressure aloft causes air to sink, dry out and suppress cloud formation. With fewer clouds blocking the sun, temperatures continue to climb, reinforcing the dome.
When a heat dome is centered directly overhead, the risk of prolonged hot and dry weather greatly increases. But there are other risks too.
The jet stream, which steers weather systems across North America, tends to ride along the edge of heat domes rather than through them. This means that regions along the edge of heat domes are at a much higher risk of fast-moving thunderstorms, often with the threat of strong winds.
If a heat dome remains in place for several days, repeated rounds of thunderstorms along its edge can bring further risks of flooding to already inundated regions.
The United States has experienced several heat domes since early June.
Heat domes have generally been centered over the central United States and the Ohio River Valley, bringing persistent heat to these areas, occasionally allowing very warm air to spread into upstate New York as the pattern shifted east at times.
Because the jet stream has tracked around the heat domes, the strongest thunderstorms have generally remained north of the Capital Region. But that has produced an unusual side effect.
Thunderstorms riding the northern edge of a heat dome over southern Canada have sparked wildfires responsible for much of the recent smoke and poor air quality across the Northeast.
While thunderstorms in southern Canada are common, the hotter and drier conditions associated with the northward extension of the heat domes have dried forests that are typically much wetter. That has made it easier for wildfires to ignite and spread.
Over the coming week or two, heat domes are expected to shift west toward the Rocky Mountains and Pacific coast. That should provide the central and eastern United States with a break from the recent stretches of hotter weather.
The trade-off is that rain chances will increase as weather systems again move along the edge of heat domes across the central and eastern United States.
There are indications that heat domes will remain mainly over the western United States in the final days of July and early August. Confidence remains low, however, for forecasts that extend more than 10 days into the future.
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