A Unique Cold-Season Supercell Produces an EF1 'Snownado'

On November 23, 2013, the ‘forecast problem du jour’ in southern Ontario, Canada, was the onset of significant snowsqualls to the lee of the Great Lakes following the passage of an Arctic cold front. A few days later, however, it was discovered that a possible tornado had caused damage as the front moved through.

Subsequent analysis showed that a lone, low-topped supercell storm developed in the vicinity of the low pressure centre as it migrated southeastward, having rapidly intensified as it passed over the relatively warm Great Lakes. In addition, a damage survey found that the supercell did indeed generate a brief tornado with damage rated at EF1 on the Enhanced Fujita Scale.

What is unique about this event is that surface air temperatures in the region were below 0⁰C, and the tornado occurred in the presence of snow and graupel. In fact, dual-polarization data from the nearby TYX NEXRAD Doppler radar in Montague, New York, suggest that all precipitation associated with the supercell was frozen – mainly ice crystals with a graupel storm core. Despite the presence of ice crystals and graupel, however, no lightning was detected with this storm.

The goal of the case study is to extend our understanding of the spectrum of tornadic supercell characteristics and environments in order to improve severe weather forecasting and nowcasting, particularly in more northern climates.