Large-Scale Antecedent Conditions Associated with 2014–2015 Winter Onset over North America and their Impact on Predictability

Winter 2014–2015 began with a vengeance in mid-November 2014 over much of the CONUS east of the Rockies. The coldest November weather in 35 years was ushered in by a sequence of arctic air masses that progressively advanced eastward and southward across the CONUS. An epic lake-effect snowstorm that buried parts of western New York on 18–19 November was followed by a series of snow and ice storms that disrupted Thanksgiving holiday travel. This presentation will show how the evolution of the antecedent Northern Hemisphere (NH) circulation governed the initial onset of abnormally cold and stormy conditions over the CONUS east of the Rockies in November 2014.

We will show how the recurvature and extratropical transition (ET) of Supertyphoon Nuri in the western Pacific in early November 2014, and its subsequent explosive reintensification as an extratropical cyclone (EC), disrupted the North Pacific jet stream and downstream NH circulation. The Nuri ET/EC produced high-latitude ridging and the formation of an omega block over western North America, resulting in downstream baroclinic development, downstream Rossby wave dispersion, and the development of a deep trough over eastern North America. This sequence of Nuri-induced NH circulation changes culminated in the aforementioned epic lake-effect snowstorm in western New York on 18-19 November 2014 and the subsequent storms that disrupted Thanksgiving travel. Atmospheric predictability on the 5–10 day time scale across the North Pacific and North America was poor centered on the time that Nuri explosively intensified as an EC. However, once the aforementioned omega block was well established over northwestern North America atmospheric predictability on the 5–10 day time scale improved significantly. The implications of this predictability improvement will be discussed.