Tropical and subtropical influences on Montreal’s record-breaking rainfall event of 8-9 November 1996

On 8-9 November 1996, 134.5 mm of precipitation was recorded at Montreal’s Trudeau International Airport (CYUL), setting the all-time record for a single rainfall event at the station. Previous work has shown that precipitable water values at Montreal just prior to and during this event were over 30 mm, or 400% of the climatological value. In addition, radar imagery shows substantial convection in the St. Lawrence valley associated with a frontal boundary that stalls over the region, along which a frontal wave forms in the latter part of the event. Preliminary indications also suggest a tropical and/or subtropical origin of the air mass that is at least in part responsible for this extreme event. There is a plume of high low-level equivalent potential temperatures and low coupling index (CI) values extending northeastward from Texas and Mexico, towards Montreal, suggesting the presence of weak static stability during this event. Moreover, 5-day backward air parcel trajectories show that the air parcels in the lower- and mid-troposphere during the entire time of precipitation (~30 hours) in the vicinity of Montreal, originate almost exclusively from the Gulf of Mexico (during the first part of the event) and Atlantic Ocean (during the latter part of the event).

Finally, the importance of the St. Lawrence River valley as a mechanism for focusing heavy precipitation will be highlighted, as it has been previously shown to do so in cases of extratropical transitions (ET). In particular, results will focus on the reasons why Montreal’s Mirabel Airport (CYMX), located 40 km northwest of CYUL and on the extreme northern edge of the St. Lawrence valley, received over 50 mm less precipitation during the event.