An unprecedented heat wave is underway in France and the U.K. that is likely to topple all-time national high temperature records and potentially kill several thousand people. The scorching heat is moving north into a more vulnerable region after tormenting Spain and Portugal.

Why it matters: This is a climate change-related public health emergency. Already, hundreds are dead from heat-related causes in Portugal and Spain and the toll is likely to climb much further.

The big picture: The U.K. Met Office is forecasting the country's first-ever occurrence of 104°F (40°C) temperatures during this event, with a high of 97°F in London on Monday.

• The U.K.'s national temperature record stands at 101.66°F (38.7°C) set in 2019. That is likely to fall as early as Monday.
• According to Meteo France, even higher temperatures occurred there on Monday, with a preliminary peak of 108.68°F(42.6°C) in Biscarosse, a town in the country's southwest.
• The air mass responsible for this extreme event originated in northwest Africa, with a heat dome and an area of low pressure just west of Iberia acting as a heat funnel.

A reminder of the top ten hottest UK days on record#Heatwave2022 #heatwave pic.twitter.com/l97zKjlkR6

— Met Office (@metoffice) July 18, 2022

Context: Attribution studies of individual heat waves have shown that by increasing the global average surface temperature by about 1.2°C in the past century, human-induced climate change has dramatically boosted the odds of extreme heat events, along with their intensity and duration.

• Research has even shown certain heat extremes would have been virtually impossible without human-caused climate change.
• European heat waves in particular have increased their occurrence three to four times faster than in other parts of the Northern Hemisphere, according to a recent study.
• "The chances of seeing 40°C days in the U.K. could be as much as 10 times more likely in the current climate than under a natural climate unaffected by human influence," said Met Office climate scientist Nikos Christidis, in a statement issuing the country's first-ever Red warning for heat.

Threat level: Extreme heat can be deadly, particularly for vulnerable groups like the elderly, those with preexisting medical conditions and anyone without access to cooling. This is a particular concern for those in Great Britain, where only about 3% of homes have air conditioning.

• Heat waves are especially dangerous when overnight temperatures remain high, depriving people of relief. Numerous records for hot overnight temperatures have been set so far during this event.
• In Madrid, there have been at least five straight "torrid nights" when temperatures have not dropped below 78.4°F (25.8°C), Rubén del Campo, a spokesperson for the national meteorological agency, stated via Twitter.
• Massive wildfires associated with the heat are burning in southern France, parts of Spain and Portugal, with at least 28,000 evacuated in France alone. The worst fires in France are in the southwestern part of the country, in the region of Gironde.
• Extreme fire behavior, akin to fast-moving and destructive fires in the American West, has been observed with these blazes.

Zoom in: The heat wave is also proving disruptive to transportation networks. In the U.K., officials are warning against traveling by rail, since hot temperatures can warp railroad tracks.

• Rail service in the U.K. has been cut back for Monday and Tuesday, and museums in London are closing early Monday and closed Tuesday as the country braces for its hottest day on record.
• Even air travel may be affected, as aircraft tires sink into hot concrete and planes require longer runways to takeoff.

Only use public transport if your journey is absolutely necessary. Plan ahead, carry water and be prepared for delays, particularly on the London Underground which will be running with speed restrictions. Check @TfL for the latest travel advice.

— Mayor of London, Sadiq Khan (@MayorofLondon) July 18, 2022

By the numbers: Dozens of monthly and all-time temperature records have been set or tied in Western Europe.

• 116.6°F (47.0°C): Preliminary national July high-temperature record set Thursday in Pinhão, Portugal.
• 108.14°F (42.3°C): All-time high-temperature record set in Pamplona, Spain, on Sunday.
• 98.8°F (37.1°C): All-time temperature record set for Wales on Monday.
• 86°F (30°C): All-time highest temperature record set in Dublin, Ireland, on Monday.
• 100.58°F (38.1°C): High temperature Monday in Santon Downham, England, on Monday.

Go deeper: In photos: Wildfires raging across Europe force thousands to flee

Editor's note: This is a developing story. Please check back for updates.