Is it unusual to see the coldest air (aloft) of the season in March? Not especially; the chart below shows the month in which the coldest air was observed at 850 mb for each winter since 1948-1949. It's nearly as common to see the coldest air in March as in December. The last time the coldest air was observed in March was in 2003, which followed the very warm winter of 2002-2003; obviously the odds of this happening are heightened after persistent warmth earlier in the season. Over the history since 1948, the earliest that the coldest air was observed was November 16, 1956, and the latest was April 18, 1959.
At the surface, Fairbanks airport observed 6 consecutive days with a daily minimum of -30 F or lower, culminating in -37 yesterday, March 15. This is fairly impressive for so late in the season; it's the most consecutive days reaching -30 this late in the winter since 1966. However, it can't compare to the 13 straight days reaching -30 F from March 9-21, 1959.
The -37 F yesterday morning is the coldest this late in the winter since 1995 (-37 F on March 24). The record for coldest this late or later is -46 F on March 15, 1964.