Orographic Influence on Frontally-Produced Flooding in Northern Vermont—The July 14-15, 1997, Event

On July 11-14, 1997, a quasistationary backdoor cold front stalled over southeastern Québec, northern Vermont, and New Hampshire, producing at least 50-150 mm of precipitation in 24 hours or less. In northern Vermont, convective training under steering northwesterly upper level flow produced heavy precipitation and catastrophic flooding along preferred corridors/major valleys in the Missisquoi River basin. This study identifies the multiscale atmospheric and orographic factors most conducive to heavy precipitation and flooding. An empirical/hydrometeorological approach revealed that the orientation of synoptic inputs over the mesoscale topography led to flow convergence, orographic lift, training, and moisture stagnation. These dynamic mechanisms released the convective instability in the postfrontal moisture lobe which contained extremely high water contents. Flash flooding was produced along high gradient input tributaries, with single event flooding along parts of the main stem. The impacts of flooding were exacerbated by anthropogenic influences, including increases in impervious area and the juxtaposition of roads and rivers.