How Phoenix and Los Angeles handle heat and water advisories

What to take away

Two cities, two alert desks

Phoenix and Los Angeles sit in the same desert-adjacent climate, but their warning systems are wired differently. In Phoenix, the Maricopa County Department of Public Health owns the heat message. The City of Phoenix Water Services Department owns the water message. Those two rarely arrive together.

Los Angeles splits the work three ways. The Los Angeles County Department of Public Health issues heat guidance, the National Weather Service office in Oxnard — NWS Los Angeles/Oxnard — issues the heat alerts themselves, and the Los Angeles Department of Water and Power issues power and water advisories. The alert products carry the same names you see in Phoenix: Heat Advisory, Excessive Heat Watch and Excessive Heat Warning.

That split is the whole story for a reporter. A single hot afternoon in Los Angeles can produce a county health notice, a weather service alert and a utility conservation request, each with its own language and its own boundary.

The forecast underneath all of them comes from the National Weather Service, which publishes the definitions behind each alert level. Learning to read a weather alert first saves you from repeating a term you have not checked.

How Maricopa County sets its heat alert levels

The levels Maricopa County uses — Heat Advisory and Excessive Heat Warning — are the National Weather Service's heat products for the Phoenix area, relayed by the county health department alongside its own response. The escalation turns on forecast risk, not one temperature: the criteria weigh daytime highs, overnight lows, humidity and how long the hot stretch runs.

Overnight lows carry the most weight in practice. A body that never cools overnight does not recover, so a 105 degree day with 90 degree nights can trigger a stronger alert than a 110 degree day that drops into the 80s.

When an Excessive Heat Warning goes up, the county typically opens cooling centers and sends outreach teams to unsheltered residents, and it lists both on its heat relief network map. Confirm which centers are open and their hours before you publish; the county revises that list during the event.

The National Weather Service publishes the heat safety guidance behind those thresholds, including how heat exhaustion differs from heat stroke. Cite the county's alert level, not the phrase "extreme heat." The level tells a reader how serious the day is.

Phoenix water notices and who signs them

Phoenix water notices come from the city's Water Services Department, and they cover maintenance, pressure changes and water quality. A pressure problem and a quality problem are not the same event, and residents routinely confuse them.

The city draws on Colorado River water, Salt River Project water and groundwater. When shortage conditions shift on the Colorado, the department publishes what that means for residents. During heat waves it may ask households to shift watering to early morning, which signals system stress rather than an emergency.

The city water page is the primary source, with conservation tips, rate information and emergency contacts. Link to it instead of paraphrasing it.

If a quality advisory is issued, the department must follow the federal Public Notification Rule under the Safe Drinking Water Act, which sets how fast a utility has to tell customers about a problem with their water. Arizona's primacy agency, the Arizona Department of Environmental Quality, enforces it in Phoenix; California's Division of Drinking Water at the State Water Resources Control Board does the same job for LADWP. A Tier 1 notice — one that carries an immediate health risk — must reach customers within 24 hours. Our drinking water advisory explainer walks through what a utility owes the public and on what clock. A short boil-water notice still affects every household in the service area, so treat it with the same care as a heat warning.

LADWP advisories: power first, then water

The Los Angeles Department of Water and Power issues advisories about both electricity and water. During heat waves it often asks customers to raise thermostats, delay major appliances and hold off on electric vehicle charging until peak hours pass.

Those requests are voluntary until the grid tightens. The utility also posts planned outages for repairs and unplanned outages from equipment failure, usually with maps of the affected area.

On the water side, a main break or a pressure drop can trigger a boil-water notice for a specific neighborhood. That notice stands until tests confirm the water is safe.

LADWP advisories are separate from county health alerts, and both can land the same day. Say which agency issued which message. Restoration times are estimates, so label them as estimates rather than promises.

CDC guidance both cities adapt

The Centers for Disease Control and Prevention publishes national heat guidance that Phoenix and Los Angeles both fold into their own messaging. The core is to stay cool, stay hydrated and stay informed, and to check on people at higher risk.

That group includes older adults, infants, people with chronic conditions and outdoor workers. Both cities repeat the list in their alerts.

The CDC also describes the signs of heat illness. The National Weather Service adds guidance for during a heat wave that pairs with the CDC material. Neither agency issues a local alert, and neither replaces the county notice.

For any question about medication and heat, name the authority and send readers to the current CDC page rather than an old summary.

OSHA and the California heat standard

The Occupational Safety and Health Administration frames worker heat protection around water, rest and shade, with time to acclimatize for new workers. Its heat guidance for workers covers training, break scheduling and emergency response, and the agency runs a heat illness prevention campaign alongside it.

The federal guidance is not the only rule in play. California operates its own Heat Illness Prevention standard — Cal/OSHA's Title 8 of the California Code of Regulations, section 3395 — which reaches further than the federal guidance in some respects, with its own requirements for water, shade, rest and training. Arizona has no state heat standard, so federal OSHA guidance carries the weight there.

That difference changes what an employer owes a crew. Phoenix outdoor workers in construction, agriculture and landscaping face some of the highest temperatures in the country. In Los Angeles, the same exposure hits construction crews, delivery drivers and farmworkers in nearby areas, and LADWP schedules some of its own field work for cooler hours.

Ask employers what they do. Water, rest, shade, training and a response plan are the five things to check.

Comparing the two cities

Run our public safety alert reporting checklist before you publish that story.

The National Weather Service safety hub is the place to verify terms, including how a heat advisory differs from an excessive heat warning. Our breaking news updates guide covers the timestamp and update log a live heat story needs.

A worked example

Maricopa County issues an Excessive Heat Warning for Phoenix with overnight lows above 90. The same afternoon, LADWP asks Los Angeles customers to conserve power. A story covering both should say who issued each message and what residents should do.

That is three paragraphs. The first covers the county warning and cooling centers. The second covers the LADWP request. The third covers the shared CDC and OSHA guidance.

If a Phoenix water notice is also active, add a fourth and say whether the water is safe to drink. Never bury that under heat advice. In Los Angeles, a boil-water notice leads the story; water safety outranks a conservation request.

Then watch the clock. Heat alerts get extended, downgraded or widened through the day, so a note telling readers when the story was last updated is standard. Our daily news explainer guide covers that structure.

Common questions

What separates a heat advisory from an excessive heat warning in Maricopa County?

A heat advisory signals moderate risk; an excessive heat warning signals more severe and prolonged risk. The county weighs temperature, humidity, overnight lows and duration rather than one number.

Who issues Phoenix water supply notices?

The City of Phoenix Water Services Department. The notices cover maintenance, pressure, quality and drought conditions, and they are separate from any heat alert.

What do LADWP advisories cover?

Both water and electricity. During heat waves LADWP may ask customers to conserve power, and after a main break or pressure loss it may issue a boil-water notice for a specific area.

Does the CDC issue local heat alerts?

No. The CDC publishes national guidance that local agencies adapt, and Phoenix and Los Angeles both cite it inside their own alerts. The local alert level always comes from the county health department or the weather service, never from the CDC.