What's the cause of Southern California's heat wave?

It’s time to crank up those air conditioners, because the heat wave has arrived.

All across Southe Califoia, temperatures will crest into the triple digits and likely break records. By Monday, the inland valleys could see highs greater than 108 and the deserts could see highs greater than 120.

The high temperatures have prompted the National Weather Service to put out an excessive heat waing that will last through Monday for coastal areas, Tuesday for the inland valleys and Wednesday for the mountains and deserts.

But what is creating the heat, exactly?

National Weather Service meteorologists say an unusually strong high pressure system — one of the strongest to be seen in years — is compressing, and therefore heating, the air over a large swath of the southweste United States. Such systems happen during the summer, experts say, but usually not as early as June and not with such strength.

“These kinds of high pressure systems, we usually expect in mid- to late summer,” said Bill Patzert, a climate scientist at NASA’s Jet Propulsion Laboratory in La Cañada Flintridge. “The hottest months are usually August and September and in June we often expect ‘June Gloom.’ ... May and June tend to be our mildest months.”

Why this is happening is anyone’s guess, said National Weather Service meteorologist Brandt Maxwell.

“You can’t point to one thing and say, ‘Aha, that’s why the high is one of the strongest ever,’ ” he said, adding it could be the result of dry conditions or low-pressure systems over other areas of the United States.

Experts say the heat created by the system will be exacerbated for a couple of reasons.

The high pressure system is arriving on the cusp of the summer solstice, when the days are the longest, Patzert said, meaning more solar radiation and more time for things to stay hot.

Richard Miich, a professor of earth sciences at UC Riverside, says that sometimes North American monsoon conditions help keep summers cool. But, he says, that tends to happen in midsummer — meaning it’s too early in the season for that.

One thing that experts agree on is that it’s going to be exceptionally hot, so much so that people will want to avoid the outdoors during the middle portion of the day.

The National Weather Service is encouraging people who have to do strenuous activities to limit those activities to the moing or evening hours.

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“They can’t go out ruing in the middle of the day and go jogging,” said Miich. “They’ve got to do it in the moing or they’ve got to do it in the evening.”

The Weather Service also recommends that people know the signs and symptoms of heatstroke, wear loose-fitting clothing and drink plenty of water.

FORECASTED HIGHS

CHINO HILLS:

• 106 on Sunday

• 106 on Monday

• 96 on Tuesday

FULLERTON:

• 102 on Sunday

• 103 on Monday

• 91 on Tuesday

HUNTINGTON BEACH:

• 74 on Sunday

• 82 on Monday

• 74 on Tuesday

LOS ANGELES:

• 98 on Sunday

• 103 on Monday

• 87 on Tuesday

MONROVIA:

• 102 on Sunday

• 106 on Monday

• 94 on Tuesday

ONTARIO:

• 103 on Sunday

• 108 on Monday

• 99 on Tuesday

PALM SPRINGS:

• 116 on Sunday

• 121 on Monday

• 116 on Tuesday

RANCHO CUCAMONGA:

• 102 on Sunday

• 107 on Monday

• 97 on Tuesday

RIVERSIDE:

• 103 on Sunday

• 108 on Monday

• 98 on Tuesday

SAN BERNARDINO:

• 104 on Sunday

• 108 on Monday

• 100 on Tuesday

TEMECULA:

• 108 on Sunday

• 108 on Monday

• 97 on Tuesday

VAN NUYS:

• 104 on Sunday

• 107 on Monday

• 94 on Tuesday

YUCAIPA:

• 101 on Sunday

• 106 on Monday

• 98 on Tuesday

SIGNS AND SYMPTOMS OF HEAT STROKE

• Altered mental state

• One or more of the following symptoms: throbbing headache, confusion, nausea, dizziness, shallow breathing

• Body temperature above 103°F

• Hot, red, dry or moist skin

• Rapid and strong pulse

• Fainting or losing consciousness

Source: National Weather Service

SIGNS AND SYMPTOMS OF HEAT EXHAUSTION

• Heavy sweating

• Weakness

• Cool, pale, clammy skin

• Fast, weak pulse

• Possible muscle cramps

• Dizziness

• Nausea or vomiting

• Fainting

Source: National Weather Service