The previous record for longest observed stretch of above-freezing conditions aloft in January through March was 72 hours or 3 days, during January 24-27, 1983. However, it should be noted that the vertical resolution of the sounding measurements has improved over time, so it has become less likely over time that above-freezing temperatures aloft could be missed by the balloon measurements; this might make it "easier" to maintain a continuous stretch of above-freezing observations. Looking only at the 850 mb measurements, which have been reported consistently since 1948, the longest stretch of above-freezing temperatures in December through March was 48 hours (in 1953, 1958, and 2007), compared to 72 hours in the event of recent days.
It is also worth noting that the previous January-February record highest temperature at any height, +7.8 °C, was exceeded for three consecutive measurements in this event. The new record is +9.2 °C.