Orographic influence on frontally-produced flooding in northern Vermont

On 14-15 July 1997, a quasi-stationary backdoor cold front stalled over southeastern Québec, northern Vermont and New Hampshire. At least 50-150 mm of precipitation fell in less than 24 hours and was heaviest where orographic lift of the Green Mountains in northern Vermont aided the release of convective instability in the vertically stacked deep, moist layer. Like similar events in 1993, 1995 and 2002, the concentration of heavy rainfall and catastrophic flash flooding in northern Vermont only, led to the examination of the multiscale atmospheric and orographic factors most conducive to heavy precipitation and flooding. The 1997 flood discharges which were comparable to the 1927 floods of record along high-gradient streams in the Missisquoi River basin in northern Vermont, were also exacerbated by anthropogenic factors such as logging debris and land-cover changes. An empirical/hydrometeorological/GIS approach revealed that the orientation of synoptic atmospheric inputs over the mesoscale topography led to flow convergence, orographic lift, training and moisture stagnation. These dynamic mechanisms released the convective instability in a postfrontal moisture lobe which contained extremely high water contents. This resulted in convective training under steering northwesterly upper level flow producing heavy precipitation and catastrophic flooding along preferred corridors/major valleys in the Missisquoi River basin. This presentation will highlight lessons learned from this event and the contributions to the dynamics of flooding rains as influenced by topography.