Towards a Meteorological Impact Scale of Extratropical Cyclones for Western Canada

Abstract:

Most potentially damaging geophysical phenomena have been categorized into scales. This is true for tornados (Enhanced Fujita), hurricanes (Saffir-Simpson), earthquakes (Richter, Mercalli), wind (Beaufort), tsunamis (Papadopoulus-Imamura) and recently atmospheric rivers (Ralph et al). The principal reason for those scales is to allow any end users (public and regulators alike) to understand the power of such events and allow preparation for those events that are predictable. Such scales are particularly valuable for emergency managers as they allow them to understand the severity and likely impacts of such events. No such system yet exists for extratropical cyclones at higher (> 35 degree) latitudes. Emergency managers currently rely on weather forecasts which provide them with total rainfall or snowfall, rainfall intensities and wind velocities. It is then up to the regional emergency managers to decide how to prepare for the predicted storm. We develop a new region-specific categorization that includes storm characteristics such as atmospheric moisture flux, total precipitation, precipitation intensity, and their respective return periods, the size of likely affected region and total storm runoff. As a frame of reference, it also describes typical storm consequences and classifies them in terms of the size of the affected population, undesirable ecological consequences, the percent of gross domestic product to repair the damages, potential fatalities and recovery time. One key limitation of the system is that it cannot capture individual convective storms that emerge locally due to unstable air masses, but this can identify when these smaller storms may organize as part of a larger mesoscale or synoptic scale system. We hope that this system, once calibrated for specific regions, helps regulators, emergency managers and the public prepare themselves appropriately for such storm impacts.