Task: Question Generation
Input (English): Passage: Almost 250 different fires started in Yellowstone and the surrounding National Forests between June and August. Seven of them were responsible for 95% of the total burned area. At the end of July, the National Park Service and other agencies had fully mobilized available personnel, and yet the fires continued to expand. Smaller fires burned into each other, propelled by dry storms which brought howling winds and dry lightning strikes but no rain. On August 20, the single worst day of the fires and later dubbed "Black Saturday", more than 150,000 acres (610 km2) were consumed during one of many intense fires. Ash from the fires throughout the park drifted as far away as Billings, Montana, 60 miles (97 km) to the northeast. The wind driven flames jumped roads and firelines, and burning embers started new fires a mile (1.6 km) or more ahead of the main fires. Ground fires raced the fuel ladder to the forest canopy and became crown fires with flames over 200 feet (61 m) high. On that single day, more Yellowstone land burned than in all other fires combined since the establishment of the park. Throughout the summer, fires made huge advances of 5 to 10 miles (8.0 to 16.1 km) a day, and there were even occasions when more than 2 miles (3.2 km) in one hour were recorded.One large group of fires was known as the Snake River Complex. These fires were in the southern section of the park, in the headwaters region of the Yellowstone and Snake Rivers. The largest fire in the group was the Shoshone fire which was started by lightning on June 23. The prescribed natural burn policy was still in effect, and at first no efforts were made to suppress this fire. It smoldered with little movement for several weeks, then rapidly started expanding towards the northeast on July 20.The Red fire started near Lewis Lake on July 1, and like the Shoshone fire, advanced little for several weeks. The fire then moved northeast on July 19, and combined with the Shoshone fire in August. As these two fires advanced towards the Grant Village area, evacuations were ordered so fire fighting crews could concentrate on structure protection. In the midst of a large lodgepole pine forest, the Grant Village complex was the first major tourist area impacted that season. A number of small structures and some of the campground complex were destroyed. After the Red and Shoshone fires combined, they were referred to as the Shoshone fire, since it was much larger.
Output (English): What rapidly started expanding towards the northeast on July 20?