Precipitation modulation by the Saint Lawrence River Valley in association with transitioning tropical cyclones

The St. Lawrence River Valley (SLRV) is an important orographic feature in Eastern Canada that has been shown previously to contribute to pressure-driven wind channeling and locally enhanced precipitation, predominantly in the cold season. We qualitatively assess the impact of the SLRV on named tropical cyclones that have undergone or are undergoing extratropical transition and track near the SLRV. Such cases can result in heavy precipitation during the warm season that result in risks to life and property. Katrina (2005) is one example of a storm that produced 55% of Montreal, Quebec's total August rainfall in only one day.

Using National Hurricane Center best track data, we find 39 cases of named tropical cyclones to have tracked within 500 km of the SLRV from 1979-2010. Utilizing the NCEP North American Regional Reanalysis (NARR), we find that 28 (Group A) of the 39 cases had large differences between 30-m total frontogenesis and 1000-hPa geostrophic frontogenesis (frontogenesis difference), and that areas of frontogenesis difference were oriented parallel to the SLRV. For a subset of 10 Group A cases from 2004-2008, a qualitative comparison is performed using high-resolution (15 km) gridded Canadian precipitation data. Comparisons with NARR frontogenesis differences show that enhanced areas of precipitation were also oriented parallel to the SLRV and were concomitant with the largest frontogenesis differences.

The suggested physical pathway to enhanced ascent and precipitation within the SLRV during Group A cases is rooted in 1) near-surface ageostrophic frontogenesis, and 2) weak atmospheric stability in the SLRV. The valley-enhanced near-surface frontogenesis is due to an along-valley pressure gradient established by the approaching named tropical cyclone, which induces pressure-driven wind channeling within the valley. Regions of weak stability are often co-located with the valley-enhanced frontogenesis; this scenario allows the shallow frontogenesis to result in deeptropospheric ascent and enhanced precipitation along the SLRV.