Question: The 1947 Glazier–Higgins–Woodward tornadoes were compared to the tornado that took place on what date?
Context: The tornado outbreak of April 9–11, 2011 was one of several tornado outbreaks in the U.S. to take place during the record month of April 2011.  Producing 43 tornadoes across five states, widespread damage took place; however, no fatalities resulted from the event due to timely warnings.  In Wisconsin, 16 tornadoes touched down, ranking this outbreak as the state's largest April event on record as well as one of the largest single-day events during the course of any year.  The strongest of these storms was an EF4 tornado that touched down south of Pocahontas, Iowa on April 9.  Between 0256 and 0257 UTC (9:56 and 9:57 pm CDT) that day, five tornadoes were on the ground simultaneously in Pocahontas County, Iowa, all of which were from one supercell thunderstorm. The late-May 1957 tornado outbreak occurred from eastern New Mexico to Oklahoma, western Arkansas, southern Kansas, eastern Colorado, and southeastern Wyoming on May 24–25, 1957.  37 tornadoes touched down over the area, most of which took place across northern and western Texas, in addition to southern Oklahoma.  The strongest tornado was rated at F4 status south of Lawton.  Unusually, some tornadoes touched down during the early morning hours, whereas most Plains tornadic systems are nocturnal.  Four deaths were attributed to the tornadic activity.  The tornado outbreak was related to the arrival of a strong shortwave trough. The Tornado outbreak of May 1–2, 2008 was a tornado outbreak that took place across the Southern and Central United States on May 1 and May 2, 2008.  The outbreak was responsible for at least seven fatalities (six from tornadoes) and 23 injuries in Arkansas.  There were at least 29 tornado reports from Iowa to Oklahoma on May 1 and 67 more in Arkansas, Missouri, Mississippi, Tennessee, Louisiana and Texas on May 2.  At least 63 tornadoes were confirmed by weather authorities. The 1947 Glazier–Higgins–Woodward tornadoes were a series of related tornadoes spawned by a single supercell that swept through Texas, Oklahoma, and Kansas on April 9, 1947.  Most of the damage and all the deaths are still blamed on one large F5 tornado, known as the Glazier-Higgins-Woodward Tornado, that traveled nearly 125 miles from Texas to Oklahoma.  This event was often compared to the Tri-State Tornado, because it was originally thought to have left a 219-mile path, but it is now believed to have a been part of a family of 8 or 9 tornadoes. The Late-March 2007 tornado outbreak was a tornado outbreak that took place across the central United States.  It developed in the High Plains from South Dakota to central Texas on March 28, 2007, which produced most of the tornadoes.  Several more tornadoes were reported the next three days before the system weakened on March 31.  It affected western Nebraska, western Kansas, extreme eastern Colorado, and much of Oklahoma, and Texas.  It was the second major outbreak of 2007, four weeks after an outbreak farther east.  The outbreak produced 80 confirmed tornadoes, with five deaths and extensive damage being reported.  In addition to the tornadoes, widespread hail as large as softballs and destructive straight-line winds as strong as 90 mph were reported. The tornado outbreak sequence of May 21–26, 2011 was a deadly series of tornado activity across the Midwestern and Southern regions of the United States.  It was one of the largest tornado outbreaks on record.  Most of the tornadoes developed in a corridor from Lake Superior southwest to central Texas; isolated tornadoes occurred in other areas.  An especially destructive tornado destroyed one-third of Joplin, Missouri, resulting in 162 deaths and 1000 injuries.  The Joplin tornado is the deadliest in the US since April 9, 1947, when an intense tornado killed 181 in the Woodward, Oklahoma area.  Tornado-related deaths also occurred in Arkansas, Kansas, Minnesota, and Oklahoma.  Overall, the tornado outbreak resulted in 184 deaths, 6 of those non-tornadic, making it second only to the 2011 Super Outbreak as the deadliest since 1974, and the second costliest tornado outbreak in US history behind that same April 2011 outbreak, with insured damage estimated at $4–7 billion. The Tri-State Tornado of Wednesday, March 18, 1925 was the deadliest tornado in U.S. history.  It was also the most exceptional tornado during a major outbreak of at least twelve known significant tornadoes, spanning a large portion of the Midwestern and Southern U.S.  The tornado alone inflicted 695 fatalities, more than twice as many as the second deadliest, the Great Natchez Tornado of 1840.  The 151 to track left by the tornado was the longest ever recorded in the world as it crossed from southeastern Missouri, through southern Illinois, then into southwestern Indiana.  Although not officially rated by NOAA, it is recognized by most experts (such as Tom Grazulis<ref name="F5/F6"> </ref> and Ted Fujita) as an F5 tornado, the maximum damage rating issued on the Fujita scale. The Mid-October 2007 tornado outbreak was a widespread tornado outbreak that took place across much of the eastern half of North America starting on October 18, 2007 and continuing into the early hours of October 19.  The outbreak was also responsible for five deaths; three in Michigan and two in Missouri, plus many injuries (including some from non-tornadic events).  At least 64 tornadoes were confirmed including 16 on October 17 across six states including Texas, Oklahoma, Arkansas, Louisiana, Mississippi and Missouri with wind damage reported in Oklahoma, Kansas, Illinois, Iowa, Arkansas and Mississippi.  On October 18, at least 48 tornadoes were confirmed across eight states including Florida, Alabama, Mississippi, Tennessee, Kentucky, Illinois, Indiana and Michigan, plus widespread straight line wind damage.  Until 2010, this event held the record for largest tornado outbreak ever recorded in the month of October according to NOAA. The June 2010 Northern Plains tornado outbreak was one of the most prolific summer tornado outbreaks in the Northern Great Plains of the United States on record.  The outbreak began on June 16 with a several tornadoes in South Dakota and Montana.  The most intense storms took place the following day across much of eastern North Dakota and much of Minnesota and North Dakota.  The system produced 93 tornadoes reported across four states while killing three people in Minnesota.  Four of the tornadoes were rated as EF4 on the Enhanced Fujita scale, the most violent tornadoes in a 24-hour period since there were five within 15 hours on February 5–6, 2008.  This was the region's first major tornado outbreak of the year and one of the largest on record in the region, comparable to a similar outbreak in June 1992.  The 48 tornadoes that touched down in Minnesota on June 17 marked the most active single day in the state's history.  June 17 was the second largest tornado day on record in the meteorological summer, behind the most prolific day of the 2003 South Dakota tornado outbreak on June 24, 2003. From November 15 to 16, 2006, a major tornado outbreak occurred across the Southern United States and into the Mid-Atlantic States.  It took place along a sharp cold front that tracked across the entire region from west to east.  Ten people were killed by the many tornadoes that damaged many communities.  The bulk of the tornadoes took place on November 15, but the deadliest tornado took place on the morning of the November 16 in southern North Carolina.
Answer: March 18, 1925