Summary (tl;dr)

"Extreme heat watch" is trending as extensive and unseasonably intense heat events are gripping regions across the United States and Europe in late September 2026, driven by ongoing climate change and the intensifying "Super El Niño" phenomenon.

Essential Background

An "Extreme Heat Watch" is issued by weather services when conditions are favorable for dangerously high temperatures and heat index values within the next 24 to 72 hours, signaling a significant risk of heat-related illness. This alert serves as a precursor to an "Extreme Heat Warning," which indicates that life-threatening heat conditions are imminent or already occurring. The heat index, which combines air temperature and humidity, is a critical factor for these alerts; in many parts of the United States, a warning is typically issued when the heat index is projected to reach or exceed 105°F for at least two consecutive days with insufficient nighttime cooling. Globally, temperatures have risen by 2 degrees Fahrenheit (1 degree Celsius) since pre-industrial times, making heatwaves more frequent, longer, and more intense. The summers of 2026 and August 2026 were recorded as the hottest on record in the United States, a trend linked to global warming and a developing "Super El Niño" in the tropical Pacific, which is expected to intensify through late 2026.

The Full Story

As September 2026 draws to a close, significant and unseasonal heat is causing "extreme heat watch" to trend. Southern California is currently under an Extreme Heat Watch, with inland valley temperatures anticipated to reach up to 108°F. Simultaneously, a substantial "Heat Dome" has returned to Europe, ushering in an unseasonably warm spell across Western, Central, and Northern Europe that is projected to extend into early October. Last week, parts of Spain and Portugal in Western Europe experienced temperatures as high as 40°C (104°F).

Earlier in September, large parts of the United States also experienced extreme heat. From August 31 to September 5, 2026, a prolonged heatwave brought heat index readings near or above 105°F for five consecutive days across central and southern U.S. regions. On September 8, Extreme Heat Warnings and Advisories were in effect across parts of the central and southern U.S., the Southwest, and California, with heat index values reaching 112-113°F in areas of Arkansas, Oklahoma, and southern Texas, and temperatures soaring to 116°F in Death Valley and the Colorado River Valley. By September 15, a heat dome settled over the south-central United States, pushing temperatures towards 104°F in states including Texas, Oklahoma, Arkansas, Missouri, and Tennessee, leading to over 41 million people being under a National Weather Service heat alert. Notably, Nashville, Tennessee, broke a 99-year-old daily temperature record on September 15, reaching 100°F.

Why It Matters

The trending "extreme heat watch" highlights a critical public health and safety concern. Extreme heat is a significant cause of mortality, with 2,325 heat-related deaths in the United States in 2023, and a 68% increase in deaths among individuals over 65 in the past two decades. Prolonged exposure to high temperatures can lead to severe health issues such as heatstroke, heat exhaustion, and dehydration, and can worsen existing conditions like heart, respiratory, and kidney diseases. Vulnerable populations, including the elderly, young children, pregnant women, outdoor workers, and those without air conditioning, face particularly elevated risks. Beyond health, extreme heat stresses infrastructure, notably the power grid, contributing to outages and disruptions. These persistent and record-breaking heat events are increasingly understood as direct consequences of human-induced climate change and the current El Niño conditions, pointing to a future of more intense and frequent heatwaves globally.

Geographic Location

• Southern California, United States (under Extreme Heat Watch)
• Inland Valleys, Southern California, United States (temperatures up to 108°F forecast)
• Orange County, California, United States (coastal and inland areas affected by heat)
• San Bernardino County, California, United States (valleys affected by heat)
• Riverside County, California, United States (valleys affected by heat)
• San Diego County, California, United States (coastal areas and valleys affected by heat)
• Western Europe, Europe (unusually warm September, heat dome)
• Spain, Europe (temperatures up to 40°C in late September 2026)
• Portugal, Europe (temperatures up to 40°C in late September 2026)
• France, Europe (unseasonably warm temperatures due to heat dome)
• Central Plains, United States (dangerous heat in early September 2026)
• Middle Mississippi Valley, United States (dangerous heat in early September 2026)
• Arkansas, United States (heat index up to 112°F on September 8, 2026)
• Oklahoma, United States (heat index up to 112°F on September 8, 2026)
• Southern Texas, United States (heat index up to 113°F on September 8, 2026)
• Nashville, Davidson County, Tennessee, United States (record-breaking heat on September 15, 2026)