A heat wave is driving temperatures 5 to 15 degrees above normal across San Francisco and the greater Bay Area.

Highs could reach up to 100 degrees in parts of the East Bay, the mid to upper 90s along portions of the San Francisco Bay shoreline and in the Santa Clara Valley, and the mid to upper 80s in eastern San Francisco.

“The warmest part of the day for the city of San Francisco and adjacent shoreline will be late morning through mid-afternoon, when a return of weak onshore flow is expected to bring some relief,” the National Weather Service said.

A heat advisory is in effect through 7 p.m. Tuesday, Sept. 23. A pattern change will occur later tonight with the return of unsettled weather with possible rain and thunderstorms across the Central Coast into early Wednesday, Sept. 24.

What Is a Heat Advisory?

A heat advisory is an alert issued by the National Weather Service when dangerous heat is expected and could pose health risks.

• It’s typically issued when the heat index (a measure that combines air temperature and humidity) is forecast to reach a level that makes heat-related illness likely, especially for vulnerable groups like children, the elderly, or people working outdoors.
• A heat advisory is less severe than an excessive heat warning, which is issued when extreme heat is imminent or already occurring.

How Hot Is It Across California?

• Southern California: 85 degrees, partly sunny; peaking near 88 in the early afternoon before cooling in the evening.
• San Francisco (Bay Area): About 77, sunny to partly sunny; high around 81 to 82, cooling to the mid-60s to 70 at night.
• Sacramento (Central Valley): About 79, rising into the mid-90s with a possible high near 98 this afternoon.
• San Diego (Southern coast): About 74, mostly cloudy; warming into the upper 70s to low 80s and clearing later in the evening.
• Fresno (Central Valley): About 82, expected to climb into the upper 90s with a peak near 100 this afternoon.

What’s the Difference Between Temperature and Heat Index?

The temperature is the actual air reading — what a thermometer records in the shade. The heat index, on the other hand, represents what it feels like to the human body when both temperature and humidity are factored in, according to the National Weather Service.

When humidity is high, sweat doesn’t evaporate as easily, making it harder for your body to cool down. That’s why the heat index is often higher than the air temperature, and why it’s used to gauge the risk of heat-related illness.

What Are the Symptoms of Heat-Related Illness?

Extreme heat is the deadliest weather-related hazard in the United States, claiming more lives each year than hurricanes, floods or tornadoes.

Experts urge Californians to take precautions: stay hydrated, avoid outdoor activity during peak afternoon hours, wear light clothing and check on vulnerable neighbors, including older people and those without air conditioning, throughout these next couple of weeks and into the summer.

According to the Centers for Disease Control, there are multiple types of heat-related illnesses. Heat cramps and heat rash can be treated by staying indoors and getting hydrated. But some others are more serious.

Heat Exhaustion Symptoms

Heat exhaustion is a mild heat-related illness that occurs in hot temperatures, especially when you’re not drinking enough water or other nonalcoholic fluids. People most at risk for heat exhaustion are the elderly, those with high blood pressure, and individuals working or exercising in the heat.

Symptoms of heat exhaustion include:

• Excessive thirst
• Profuse sweating
• Headache
• Dizziness or confusion
• Nausea

If you or anyone you’re with experiences these symptoms, move to a shaded or air-conditioned area, use a fan, or immerse in cool water. Also, drink cool, nonalcoholic beverages like water or sports drinks with electrolytes.

Heat Stroke Symptoms

Heat exhaustion can escalate into heat stroke, a serious, life-threatening condition. Heat stroke occurs when the body’s temperature rises rapidly, and it can no longer cool itself down.

Early symptoms of heat stroke are similar to heat exhaustion but progress to more severe signs, including:

• Lack of sweating
• Body temperature rising to 106°F or higher within 10-15 minutes (even without a thermometer, the skin will feel very hot)
• Loss of consciousness

Heat stroke is a medical emergency. If you suspect someone is experiencing it, seek immediate medical attention. Without emergency treatment, heat stroke can cause death or permanent disability.

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Brandi Addison writes about weather across the country for the USA TODAY Network’s Weather Connect. Reach her at baddison@gannett.com.
This article originally appeared on Palm Springs Desert Sun: How hot will temperatures get across California today? See by region
Reporting by Brandi D. Addison, USA TODAY NETWORK / Palm Springs Desert Sun
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