[Update 5 pm AKST: this afternoon's sounding from Fairbanks showed an inversion of 31.7 °C or 57 °F as warming continued aloft. This has only been matched on 7 previous occasions since 1948.]
This morning's balloon sounding from Fairbanks airport revealed a temperature difference of more than 30 °C (55 °F) between the surface temperature and the temperature at 5300 feet above ground level. As shown in the chart below, deep warming has occurred in the past few days, and at 3 am this morning the temperature was only just below freezing at around 4000-5500 feet. Computer models suggest that the temperature is probably now above freezing at the same levels, despite valley surface temperatures below -20 °F.
An inversion of 55 °F or greater is quite unusual in Fairbanks; the last time it was observed was in January 2005. In the history of upper-air observations since 1948, the strongest inversion was 60 °F on December 4, 1956. The chart below shows the maximum inversion strength by winter since 1948-49, with the red line indicating the 10-year trailing average.
An even stronger inversion was observed in McGrath this morning (31.9 °C or 57 °F), but this is slightly less unusual there, as very warm air aloft can reach McGrath more easily. The strongest inversion observed in McGrath was 40.0 °C (72 °F) on January 13, 1966.