The Washington PostDemocracy Dies in Darkness

Heat warnings for southern U.S.; severe thunderstorm risk in the north

More than 50 million Americans are expected to see highs in the triple digits, while heavy thunderstorms will be likely across the northern U.S.

High temperatures on Friday as predicted by the National Weather Service. (Pivotal Weather) (FTWP)

The “ring of fire” isn’t just a line from a Johnny Cash song — it’s a thing on weather maps, too. This week’s overarching weather pattern across the Lower 48 will feature another stubborn ring-of-fire setup, with anomalously hot temperatures for some and the risk of severe thunderstorms for others.

The premise of a ring-of-fire pattern is that a heat dome, or ridge of high pressure bringing exceptional heat, is flanked by thunderstorms that ride along its periphery. They feed off the temperature contrast at the edge of the high pressure and tap into momentum from the jet stream aloft. Heat domes shunt the jet stream poleward.

Heat advisories and excessive-heat warnings blanket much of the South, where heat indexes could push past 105 degrees. As it stands, more than 50 million Americans are expected to see highs in the triple digits.

To the north, waves of severe thunderstorms will roll across the northern Rockies, Plains and Upper Midwest, presenting the risk of damaging straight-line winds and hail. Tornadoes will become a possibility by this weekend, too.

Beneath the heat dome

Temperatures are already rising beneath the heat dome.

Kansas City, Mo., Springfield, Mo., Paducah, Ky., Memphis and Nashville were included in the excessive-heat warnings. The National Weather Service in Jackson, Miss., wrote that “the combination of persistent heat and humidity will reach extremely dangerous levels through Saturday,” as heat indexes approach 115 degrees.

What extreme heat does to the human body

Actual air temperatures will fall shy of records, generally hovering in the mid- to upper 90s. But after dew points in the lower to mid-70s are factored in, heat indexes will flirt with dangerous levels. Because of the moisture in the air, sweating won’t work as intended — the sweat can’t evaporate and remove heat from the body. That’s the reason heat stress increases as the humidity spikes.

Highs across the South each day through Saturday should range about 5 to 8 degrees above average — mainly in the upper 90s across Alabama, Georgia and Tennessee, with a few lower 100s in western Mississippi, northern Louisiana, southern Arkansas and adjacent East Texas.

Oklahoma City could hit 104 on Friday, with 105 looking likely in Dallas and a high of 107 predicted for Wichita Falls, Tex. A cold front will bring some relief in the form of thunderstorms by late Saturday.

A number of locations will also set records for elevated overnight minimum temperatures, with some places failing to dip below the upper 70s. On Saturday morning, most of northern Mississippi won’t see a low below the stifling upper 70s. Little Rock is likely to see its low bottom out at a sultry 80 degrees.

Elevated overnight temperatures significantly exacerbate heat risk, often preventing the body from cooling down. That can prove deadly for vulnerable populations — including the elderly and the homeless — without access to adequate cooling shelters.

The heat dome will shift west into the weekend and set over the Four Corners region, baking the Desert Southwest.

Severe thunderstorm risk

Across the northern fringe of the heat dome, daily disturbances embedded in swift jet stream flow will touch off rounds of severe thunderstorms.

On Thursday, two risk areas were present: one over the Midwest and parts of the Mid-Atlantic, covering primarily the Carolinas west through parts of the Tennessee Valley toward Illinois and Missouri, and the other across Montana and the northern Intermountain West.

In the eastern risk area, only pulse-type thunderstorms with localized gusty downdraft winds and heavy rain are expected, though a low-end tornado threat may be present in Missouri north of Interstate 44.

In Montana, there’ll be a higher risk of a few rotating thunderstorms with hail, perhaps to the size of ping-pong balls, though thunderstorms may eventually merge into windy line segments. A very isolated tornado can’t be ruled out.

Friday will feature similar risk areas, though a lesser, more isolated risk over the eastern half of the country. That’s because instability, or “juice” to fuel thunderstorms, will be rather limited. Some wind or hail is possible over Montana again, with isolated severe thunderstorms bubbling up over the Rockies and High Plains to the south.

Over the weekend, it’s looking increasingly likely that a shortwave — or lobe of high altitude cold air, low pressure and spin — will saunter along the international border. That will probably touch off rotating thunderstorms in eastern Montana and most of South Dakota on Saturday, and across eastern parts of the Dakotas and Minnesota on Sunday. Storms will probably be supercells capable of all severe hazards, including a few tornadoes.