National Weather Service United States Department of Commerce
What is the period of record? Since 1950 for tornadoes, and since 1996 for wind/hail.
Why the different periods of record? For various reasons, the severe weather climatology is flawed. Because of the highly localized nature of severe weather, it is extrememly rare for instruments to actually measure wind speeds associated with tornadoes, or even straight-line winds, for instance. Because of this, the vast majority of historical severe weather reports come from trained spotters, law enforcement, fire departments, etc, and even the public. While NWS personnel use expert judgment to determine the validity of severe weather reports, inaccuracies regarding the time, location, and magnitude of damaging wind speeds and large hail are inevitable. In addition, the historical record is far from complete. For instance, severe weather can occur in remote areas where people are not around to observe it, or severe weather can be observed, but not reported. While the official tornado record, which dates back to 1950, is generally considered to be fairly reliable, this is not the case for wind/hail, especially prior to the 1980s. Thus, while the entire 60+ year tornado record is included in the tornado maps, 1996 was chosen as the start of the wind/hail period of record. This coincides with the beginning of NWS Greenville-Spartanburg's assumption of warning responsibility for all 46 counties in its current County Warning Area.
Why "days" with severe weather rather than "number" of severe weather events? While large inaccuracies are assumed to exist in the period of record regarding the details of severe weather reports, two pieces of information are assumed to be fairly reliable: the day on which a severe weather event occurred, and the county in which it occurred, thus our focus on counties and "severe weather days." Also, while the meteorological literature is somewhat mixed regarding a "population bias" within the severe weather record (in which it is assumed that more reports of severe weather will come from areas that are heavily populated), it is generally assumed that such bias exists. Focusing on "days" with severe weather rather than "numbers" of severe weather reports helps eliminate this bias.  
Average # of days in a particular month (or season) with a severe weather event within 200 mi2 of any point? Really? Why so convoluted? The data have to be expressed in some way in terms of space and time, and believe it or not, this is the simplest! 200 mi2 was chosen as the areal qualifier because it roughly coincides with the smallest county in our forecast area. Because this is a relatively small area, a fairly large expanse of time was necessary (10 years in the case of wind/hail, 60+ years for tornadoes) so that meaningful numbers could be shown on the maps. (In other words, the maps look better this way!) For instance, if we used 1 year as our time range for wind/hail, the numbers on the maps would be less than 1, even for the busier months, and would be less than .01 for some of the less busy months. 
What dates encompass the seasons in some of the maps? We use "climatological" seasons. Thus, winter is December 1st-February 28th/29th, spring is March 1st-May 31st, summer is June 1st-August 31st, and autumn is September 1st-November 30th.
How often will these be updated? Being climatological maps, they should not change significantly from one year to the next, but each map will update once a year, two months after the last day in the "map month." (E.g., the maps for March 2015 will be updated on June 1st, 2015, the maps for June on September 1st, and the maps for autumn on February 1st). 
Who can I contact with other questions? Justin Lane