On July 9 in northern France a high-pressure area formed which moved to the east of West Germany. On July 10 a low-pressure area formed over Portugal and Northwest Spain. This low-pressure area moved over Biscay, Spain on the morning of July 10 and it moved quickly towards the north-east. While it moved towards the east, on the eastern part of the low-pressure area, warm and moist subtropical air arrived from the Mediterranean Sea. This air spread to the south and middle of Germany. This caused high temperatures in the south of Germany (e.g. Karlsruhe 30.6 °C on July 10), but low temperatures in the north of Germany (North Sea area 15 °C on July 10). Meanwhile, on the north-western side of the low-pressure area cold and dry air from the northern polar region moved to the south. During the day the cold dry air moved westwards over France and Germany and moved into the warm and moist air, which had accumulated south-east of the low-pressure area. This caused strong thunderstorms, associated with hail, strong rain and wind gusts. One of this thunderstorm produced two tornadoes, while the storm front crossed from France into Germany on the evening of July 10. The low-pressure area arrived over northern Germany on July 11.
