When rain freezes on contact with the surface, latent heat of fusion is released which can warm the surrounding air to 0°C, potentially changing freezing rain to plain rain. As a result, for freezing rain to persist for many hours, either (a) cold surface temperatures at freezing rain onset or (b) cold-air advection at the surface during freezing rain, are necessary. These features are often aided by terrain. For example, northeasterly winds channeled down the in the St. Lawrence River Valley (SLRV) have been show to play a role in cooling surface air during freezing rain events at Montreal1, 2. This is a key factor that places Montreal among the locations observing the most freezing rain in North America (40-50 hours/year).
Ice storms occur when freezing rain is able to persist through a combination of these mesoscale terrain features and favorable synoptic patterns. Montreal has been affected by severe ice storms on several occasions, most notably in January 1998, when over 60 hours of freezing rain occurred over a period of several days and paralyzed Southern Quebec. We are currently analyzing the mesoscale, synoptic, and planetary-scale patterns of ice storms over North America to help forecasters better recognize conditions leading to these destructive weather events. By identifying long-duration (6+ hour) freezing rain events at surface stations and then grouping together events at other stations concurrent in time, we generate maps of freezing rain events based on where the freezing rain was observed. The map for the January 1998 ice storm is shown in figure 1a. Notice that freezing rain fell in a swath all the way back to Oklahoma, with maximum amounts occurring in the SLRV.
This pattern of long-duration freezing rain events spanning from the South Central US to the Northeastern US/Eastern Canada has occurred on several occasions during our period of study (1979-2015). A composite of 14 of these most severe (greatest total freezing rain hours) events from the NCEP Climate Forecast System Reanalysis (CFSR) is shown in figure 1b. Freezing rain falls ahead of a quasi-stationary front within a deformation zone/area of frontogenesis spanning from Oklahoma to the Canadian Maritimes. North of this front, arctic air flowing southward associated with the anticyclone over the Canadian Prairies maintains below-freezing surface temperatures, while southerly flow to the west of the Atlantic anticyclone transports warm, moist air from the Gulf of Mexico over the surface cold air, forming the above-freezing inversion layer necessary for freezing rain formation. We are currently examining this and other patterns that lead to long-duration freezing rain in North America. For more details, see my recent presentations at http://www.meteo.mcgill.ca/~cmccray/presentations
1 Razy, A. et al. 2012. Synoptic-Scale Environments Conducive to Orographic Impacts on Cold-Season Surface Wind Regimes at Montreal, Quebec
2 Ressler, G. et al. 2012. Synoptic-Scale Analysis of Freezing Rain Events in Montreal, Quebec, Canada