The series comes from the 'Current Population Survey (Household Survey)'
The source code is: LNS13008876
Source: US. Bureau of Labor Statistics
Release: Employment Situation
US. Bureau of Labor Statistics, Number of Civilians Unemployed for 15 to 26 Weeks [UEMP15T26], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/UEMP15T26/, September 2, 2015.
FRED BlogA long-term unemployment problem