The heat dome that killed 29 people in New Jersey is moving southwest. [1] Phoenix, Las Vegas, and Denver now sit in its projected path, with temperatures expected to reach 110 degrees Fahrenheit in the Desert Southwest by mid-week. [2] What makes this forecast different from an ordinary summer heat advisory is what it says about nights.

This paper's July 5 edition framed the 600,000-plus customer outage as a live shelter test, with the Northeast emergency still in progress. The 29 deaths in New Jersey that followed — all confirmed indoors — underscore what the shelter test actually measured: the capacity to survive heat that does not relent. That is precisely what a heat dome does to desert cities. It removes the one variable that makes desert summer survivable.

Phoenix already records overnight lows above 90 degrees Fahrenheit in peak summer. The urban heat island effect — absorbed and re-radiated heat from concrete, asphalt, and rooftop equipment — keeps city temperatures 10 to 15 degrees above surrounding desert at night. A dome amplifies this. It suppresses the atmospheric mixing that would normally allow overnight temperatures to drop, trapping heat close to the ground and keeping nighttime readings at or above the threshold where the human body cannot fully dissipate accumulated daytime heat load. [1]

This is the documented mechanism for compounded mortality. A single day above 100 degrees is survivable for most people in reasonable health. Two consecutive days, with a night between them that offers no recovery, begin to accumulate physiological debt. Three or more such days — each night warmer than the last because the dome holds — produce exponential mortality risk for the elderly, the chronically ill, the unhoused, and anyone in an unair-conditioned building. [3] The New Jersey deaths unfolded over five days. Phoenix, Las Vegas, and Denver will face the same dome duration, with starting temperatures already 10 to 15 degrees higher than New Jersey's baseline. [2]

Desert infrastructure is built for heat. Phoenix has cooling centers, hydration stations, and public transit systems that serve as de facto cooling refuges. Las Vegas has 24-hour casinos that remain air-conditioned at all hours. Denver, at elevation, has historically escaped the worst of Southwest desert heat. But infrastructure built for ordinary desert summer — manageable days, recoverable nights — is not the same as infrastructure designed for a multi-day no-recovery-window dome. The distinction is not academic: New Jersey had cooling centers too. The people who used them lived. [1]

AccuWeather forecasts 110 degrees Fahrenheit for Phoenix and Palm Springs, with highs across Nevada reaching into triple digits. [2] Those are the headline numbers. The number that determines mortality is the overnight low — whether Phoenix's nights during the dome stay at 92 or above, whether Las Vegas sees sustained overnight temperatures above 85, whether Denver loses its elevation advantage if the dome sits long enough. [3]

The National Weather Service is the source for heat watches, heat advisories, and excessive heat warnings at the city level; residents in Phoenix, Las Vegas, and Denver should monitor NWS Phoenix and NWS Las Vegas advisories rather than AccuWeather's general forecasts. [2] The advisory language distinguishes between "heat advisory" (dangerous heat index values) and "excessive heat warning" (extreme and potentially life-threatening conditions) — the difference matters for whether emergency services are pre-positioned.

The preparation window that the Northeast did not have — the advance notice before the dome's full force arrived — is open for western cities as of July 6. That window is measured in days, not weeks. Wellness checks on elderly neighbors and people living alone, identification of residents who cannot afford or access air conditioning, and direct outreach to households with powered medical equipment are the actions the 29 New Jersey deaths demonstrate should happen before the dome peaks. [3]

What the dome removes is time. Phoenix already knows heat. It does not know heat that never relents at night.

-- DARA OSEI, London