Experts say an erratic jet stream has helped stall a lot of hot air over Mexico and Texas.

Sweltering high temperatures are setting records in Texas, prompting heat advisories from forecasters and putting nearly unprecedented strain on the state’s power grid. The National Weather Service says a heat dome is to blame, and that it’s expected to expand in the next few days, keeping Texas under its severe heat while reaching north and east to cover more of the U.S.

The main driver of the heat is an area of high pressure over the southern U.S. and Mexico. Forecasters on Monday called it a “stagnant upper-level ridge,” also known as a heat dome.

Areas of high air pressure push air down toward the ground. As it sinks, the air heats up, and the resulting dome keeps that hot air from moving elsewhere.

Sinking air can also lower humidity and cloud cover, which can allow more sunlight through to contribute to hot conditions on the ground.

SEE MORE: Father, stepson die at Texas national park as temperature reached 119

But what keeps this high pressure in place?

Experts say the behavior of the jet stream is unusual this year. Typically, heat and moisture from the tropics help push the jet stream north during the summer months in the U.S.

But this year, a more erratic jet stream has helped cause lingering areas of high and low pressure. One such high-pressure spot helped dry out parts of Canada, which contributed to the wildfires that turned parts of the northeastern U.S. smoky earlier in June. Now it’s contributing to the stubborn high-pressure ridge over the southern U.S.

National Weather Service forecasters say that heat is expected to spread into the Plains and Midwest regions as the week goes on, because the high pressure ridge responsible will move northeastward. They warn that Arizona, New Mexico and Texas will continue to see triple-digit temperatures, and that temperatures will creep up toward 100 in the central Plains, middle/lower Missouri Valley and the lower Mississippi Valley.

SEE MORE: Texas removes mandate for construction worker water breaks amid heat

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