A historic heatwave sweeping over the Southwest has triggered an urgent warning for millions to stay inside for the next four days.
The National Weather Service (NWS) has issued an extreme heat warning throughout Southern California beginning Tuesday at 10am PT (1pm ET) and lasting until 8pm on Friday as temperatures are expected to reach 104 degrees Fahrenheit in some areas.
In Los Angeles, millions are expected to bake under temperatures in the upper 90s, and the forecast may challenge the record for the earliest 100-degree day in the city since April 4, 1989.
Officials have warned the public to stay indoors during the hottest parts of the day this week, and also to take frequent breaks in the shade or in air conditioning if you have to be outside.
Similar warnings have been triggered in Nevada and Arizona, where this week's temperatures are projected to be 30 degrees above normal for mid-March and will likely break records for 100-degree days which have lasted for decades.
In both Las Vegas and Phoenix, extreme heat warnings have been issued for the next five days, spanning from Wednesday through Sunday.
The shocking forecast has been caused by a massive 'heat dome' forming over 14 states, including California, Nevada , Arizona , Utah, New Mexico, Colorado and Wyoming.
A heat dome works like a lid of high pressure. It builds overhead, trapping hot air at the surface level, suppressing clouds and intensifying sunlight.
The National Weather Service has issued an extreme heat warning throughout Southern California from Tuesday through Friday.
A heat dome is a high pressure system trapping heat and humidity near surface level.
In a statement, NWS officials warned: 'There is a high risk for dangerous heat illness for anyone, especially for the very young, the very old, those without air conditioning, and those active outdoors.'
NWS added that the millions living under the heat dome will need to watch for signs of heat exhaustion or heat stroke.
Heat exhaustion often starts with heavy sweating, fatigue, developing cool pale clammy skin and a fast weak pulse, muscle cramps, dizziness, nausea and headaches.
As for heat stroke, which is far more deadly and can develop within 15 minutes in extreme conditions, the first symptoms are often a throbbing headache, confusion, slurred speech, nausea, dizziness, and a fever above 103 degrees Fahrenheit.
Sufferers can also develop red dry or damp skin, a rapid pulse and can lose consciousness.
Move anyone affected by these conditions into a cool shaded spot right away and call 911.
Parts of Oregon, Idaho , Montana , Nebraska , Kansas , Oklahoma and Texas will also be affected, with meteorologists from AccuWeather warning that March temperatures, which are usually in the low 70s, could surpass 90 for several days.
A heat dome forms when the jet stream, the fast-flowing river of air high over the US, develops a bulge or ridge.
Temperatures throughout the Southwest are expected to be between 15 and 30 degrees above the normal averages for March.
The National Weather Service has issued extreme heat warnings in California, Arizona and Nevada which last through the weekend.
Weather officials in San Diego warned that the heat in the Coachella Valley, San Diego County Deserts and San Gorgonio Pass may reach between 106 and 114 degrees Fahrenheit this week.
'Do not leave young children and pets in unattended vehicles. Car interiors will reach lethal temperatures in a matter of minutes,' NWS San Diego said in their weather alert.
In Palm Springs, meteorologists with AccuWeather expect the temperature to surpass 105 degrees on Wednesday, Thursday and Friday. The historic average in this area for mid-March is just 81 degrees.
'Take extra precautions when outside. Wear lightweight and loose fitting clothing. Try to limit strenuous activities to early morning or evening. Take action when you see symptoms of heat exhaustion and heat stroke,' NWS added.
The agency has also urged anyone in the heat dome to drink plenty of fluids and check on relatives and neighbors regularly throughout the week.
Adults have been warned to never leave young children or pets alone in their cars, even for a short time, because car sealed interiors amplify the effects of the sun, turning vehicles into roasting ovens within minutes of turning off the air conditioning and potentially killing anyone inside.