"Relentless" heat wave spreads across southern U.S.

• The National Weather Service forecasts the heat to expand this week, as triple-digit highs continue in the desert Southwest and southwestern Texas, while oppressively humid temperatures in the mid- to upper-90s affect the Mid-South and Gulf Coast.
• Overnight minimum temperatures are set to break records. "This means little in the way of relief for areas desperately needing to cool off from the sizzling daytime highs," the NWS stated in a forecast discussion Monday.
• High temperatures into the 110s in Phoenix, Las Vegas and California's highly populated Central Valley region continue to pose a serious threat to people's health.

Threat level: This heat wave is a deadly threat, and extreme heat is the top annual weather-related killer in the U.S.

• The elderly, young and people with chronic illnesses are at the highest risk for heat-related illness, but even healthy individuals can quickly succumb to such high temperatures.
• "Record-breaking heat is expected in the Four Corners states, Texas to the Lower Mississippi Valley, and South Florida each day" this week, the NWS said in a forecast discussion on Tuesday morning.
• The NWS office in Phoenix said Monday it expects the near-record temperatures to continue through at least Friday with daily highs between 110°F to 115°F.

Of note: Temperatures are smashing records in cities that already have a reputation for extreme heat.

Zoom in: In Phoenix, the high temperature was forecast to reach 118°F on Monday. The local NWS office confirmed at 12:15pm local time that the city had hit its 18th consecutive day of high temperatures at or above 110°F.

• This ties Phoenix's all-time record for such a stretch. "We are very likely to break the record tomorrow," the NWS office in Phoenix said.
• In jeopardy too is the city's record for the hottest seven days. Phoenix has already beaten its record for the longest streak of days with overnight low temperatures at or above 90°F, with eight so far. The overnight minimum temperature Monday morning was a record-breaking 95°F.
• Death Valley, Calif., hit 128°F Sunday — close to its highest reliably recorded temperature in modern record-keeping of 130°F.

Between the lines: This extreme event features a blazingly dry heat in the Southwest as well as sultry air in the South and Southeast. There, the air is rich with moisture from a record-warm Gulf of Mexico.

• The steamy waters surrounding South Florida are playing a role in Miami's dangerous heat.

• The weather pattern responsible for the extreme heat consists of a broad and unusually potent area of high pressure aloft, known as a heat dome, parked over the Southwest.

Context: Human-caused climate change is worsening heat waves by making them more intense, longer-lasting and more likely to occur.

• An initial analysis from research and communications nonprofit Climate Central shows that in some areas affected by this heat wave, climate change has likely made record-breaking temperatures at least five times more likely than in a world without added amounts of heat-trapping greenhouse gases.
• Recent research has shown some deadly extreme heat events were virtually "impossible" without climate change.

Our thought bubble: The ongoing heat wave is just one of several extraordinary climate events playing out worldwide, each tied in part to climate change.

• Record heat is hitting Europe and Asia, while global temperatures continue to hit unprecedented milestones.

Meanwhile, in Canada, the country's worst wildfire season on record sent smoke into the U.S. for a second consecutive day Monday — leaving millions of people under air quality alerts from the U.S. West to the Northeast.

More from Axios:

• In photos: Extreme heat strikes multiple continents
• What the world's increasing heat does to our bodies
• What this summer's weather reveals about climate change

Editor's note: This article has been updated with new details throughout.