Tropical Moisture, Arctic Anticyclones, and Upper-Level Jets: An Ice-Storm Recipe Par-Excellence

During 21 – 23 December 2013 a widespread destructive ice storm, arguably the most significant event of this type in 2013, impacted parts of lower Michigan, northern New York, northern New England, and adjacent Ontario, Quebec, and New Brunswick. More than 1,000,000 customers lost power in the United States and Canada. Ice accumulated to more than 3 cm on the ground, on trees, and on power lines in several places with the Toronto metropolitan area being especially hard hit. The ice storm was concentrated in the equatorward entrance region of an exceptionally strong jet stream that was characterized by a radiosonde-measured, record-breaking, peak 250-hPa wind speed of 248 kt at Goose Bay, Labrador, at 1200 UTC 22 December. The jet core wind speed more than doubled in the 48 h ending at 1200 UTC 22 December.

The meridional 1000–500-hPa thickness gradient across the aforementioned jet was > 84 dam per 1000 km (equivalent to a layer-mean temperature difference of 42 C) and was manifest by 1000 – 500-hPa thickness values that ranged from < 480 dam over Labrador to > 564 dam over southern New York and New England. This strong meridional thickness gradient was established by a strongly confluent flow pattern that enabled the juxtaposition of arctic and tropical air masses over eastern North America. Anticyclogenesis over eastern Canada associated with an arctic potential vorticity anomaly and an arctic air mass strengthened the lower-tropospheric meridional thickness gradient and contributed to strong lower-tropospheric frontogenesis. Heavy rainfall associated with a tropical air mass that was characterized by +4 to +5 standardized precipitable water anomalies contributed to diabatically enhanced upper-tropospheric ridging over the Great Lakes and the Northeast and to the establishment of deep tropospheric baroclinicity.

Shallow arctic air, driven southward by surface anticyclogenesis in the poleward entrance region of the strong jet, was able to seep even farther southwestward and southward in regions of terrain flow channeling in the St. Lawrence, Champlain, Hudson, and Connecticut River Valleys, as well as along the western side of the Adirondacks. As this shallow arctic air penetrated southward, lower-tropospheric baroclinicity was enhanced in the equatorward entrance region of this strong jet. Warm-air advection and deep ascent in the tropical air mass above this shallow arctic air in the equatorward entrance region of the aforementioned jet set the stage for a widespread disruptive ice storm from the Great Lakes eastward across southern Canada, northern New York, and northern New England. A synoptic and climatological analysis will be used to show important meteorological details of jet-related ice storm. The results will also be interpreted in the context of the potential for adverse societal impacts.