Ice Storm '98 in southcentral Canada and northeastern United States: A climatological perspective

Dubbed Ice Storm '98, an extreme weather event characterized by two synopti c systems in succession dropped about 70-100 mm (in terms of water equivale nt) of freezing precipitation over southeastern Ontario, southwestern Quebe c and northeastern New York during a 6-day period from January 5 to 10 in 1 998. Individually, the two synoptic systems were not dramatically more extr eme in freezing precipitation than other major freezing rain events (4 sinc e 1961) which occurred in the post over the affected area. Some regions in the target area, however, were impacted more by the second system. Based on an analysis of the 500 hPa vorticity field during the '98 event, we sugges t that the 1997/98 El Nino had a role in creating a flow environment conduc ive to the rapid formation of the second synoptic system. In contrast, othe r major freezing rain events in the last 30 years involved only one synopti c system per event lasting no more than 3 days, and producing 20-50 mm of p recipitation. We have also found that, 3 out of 4 past major freezing rain events since 1 958 were associated with the positive phase of the North Atlantic Oscillati on (NAO). Consistent with this usual past association between the NAO and a major freezing rain event, Ice Storm '98 also occurred when the phase of t he NAO was positive. Analysis of these 3 past and the '98 events also indic ates an apparent connection between the positive phase of the NAO and the n orthern Quebec high pressure system, which is an essential synoptic feature of a major freezing rain occurrence over the southcentral legion of Canada . As measured by their respective indices, the maximum positive NAO state l eads the maximum northern Quebec high by about 2 days (5 days in the '98 ev ent). There is some suggestive evidence to indicate that the persistence or the northern Quebec high pressure system is connected to the persistence o f the positive phase of the NAO.