Large parts of Britain experienced their first heat wave of the year on Monday. Forecasters warned of the potential for record-breaking temperatures for May. Predictions showed that temperatures could reach up to 35 degrees Celsius (95 degrees Fahrenheit) in some areas of England.

If temperatures reach this high, it would break the previous May temperature record of 32.8 degrees Celsius (91 degrees Fahrenheit), set in 1944. The Met Office, Britain’s official weather service, described the current heat as “unprecedented for the time of year.” Normally, temperature records are broken by only small margins.

The unusually hot conditions built up over the week, peaking during the weekend. This was particularly the case across England and Wales. By Saturday, temperatures had climbed to 30.5 degrees Celsius (86.9 degrees Fahrenheit) in southeast England. In Britain, temperatures above 30 degrees Celsius in May are rare, with the last occurrence in 2012.

By Sunday, eight locations in southeast England met the country’s heat wave criteria, having recorded three consecutive days above their local temperature thresholds. Britain’s criteria for a heat wave differ from those in the United States. The National Weather Service in the U.S. defines a heat wave as “a period of abnormally hot weather generally lasting more than two days.” However, the Met Office in Britain declares an official heat wave when a location sees at least three consecutive days of temperatures meeting or exceeding the local heat wave threshold.

The forecast for Monday indicated exceptionally high temperatures for May. Data indicated conditions far above seasonal averages. Forecast models from the University of Maine Climate Change Institute and the National Centers for Environmental Prediction Global Forecast System supported these projections.