|Source: The Atlantic|
This map shows 'How Much Snow It Typically Takes to Cancel School in the U.S.’ courtesy of Reddit user Alexandr Trubetskoy.
This clearly shows how the climate in the north of the U.S. is vastly different to the south. In the north, Chicago, IL, has an average January low of 18F. Whereas in the south, Austin, TX has an average low in January of 42F.
The trend of southern States requiring less snow - often just the warning of snow - to close schools is probably caused by how rare of an occurrence a snowstorm is. Due to this, some States do not budget for the infrastructure needed to clear snow and ice.
'"Can I make a comment that is unrelated to the economy very quickly?" Obama asked at a morning economic briefing. "It has to do with Washington. My children's school was canceled today. Because of what—some ice? As my children pointed out, in Chicago, school is never canceled."' President Obama discussing the school closures on this excerpt from the Washington City Paper highlights the seriousness of how being unprepared for snow can affect the southern economy. It also shows how American citizens opinions on the seriousness of snowy weather vary depending on location.
A problem with the map is that it assumes all school closures in the U.S. are caused by snow, whereas in reality many are closed by a below freezing windchill. It also was created using data "taken from hundreds of various points from user responses" which is a subjective method of data collection and therefore low in reliability.
In conclusion, there is a trend in the United States that where snowstorms are rarer, it becomes more likely that schools in the area will close in the event of one.