Unusually deep wintertime cirrus clouds observed over the Alaskan subarctic

Unusually deep wintertime cirrus clouds at altitudes exceeding 13.0 km above mean sea level (MSL) were observed at Fairbanks, Alaska (64.86°N, 147.85°W, 0.300 km MSL), over a 12-h period beginning near 1200 UTC 1 January 2017. Such elevated cirrus cloud heights are far more typical of warmer latitudes and, in many instances, associated with convective outflow, as opposed to early winter over the subarctic on a day featuring barely 4 h of local sunlight. In any other context, they could have been confused for polar stratospheric clouds, which are a more common regional/seasonal occurrence approaching such elevated heights. The mechanics of this unique event are documented, including the thermodynamic and synoptic environments that nurtured and sustained cloud formation. The impact of an unusually deep and broad anticyclone over the wintertime Alaskan subarctic is described. Comparisons with climatological datasets illustrate how unusual these events are regionally and seasonally. The event proves a relatively uncharacteristic confluence of circulatory and dynamic features over the wintertime Alaskan subarctic. Our goal is to document the occurrence of this event within the context of a growing understanding for how cirrus cloud incidence and their physical characteristics vary globally.