Near-Record Rainfall Heading to Ontario in late 2024

A rare and near-record amount of moisture is heading towards Ontario as we approach the end of 2024. It’s been almost ten years since we’ve seen such a moisture-laden storm at this time of year. A low-pressure system coming towards the Great Lakes will bring a significant amount of Gulf moisture north of the border, resulting in near-record levels of moisture in Ontario by Sunday.

When it comes to measuring moisture in the atmosphere, we use precipitable water (PWAT). This is the amount of rain that would fall if all the moisture in a small atmospheric column were condensed. Higher PWAT values indicate more moisture, which can lead to heavier rainfall. On Sunday, PWAT values are expected to exceed 30 mm, which is extremely rare for this time of year and more characteristic of a summertime setup in this area. Typically, southern Ontario has a much drier atmosphere at the end of December with an average PWAT value of 7 mm, making this event about 500 percent above normal.

Looking back to December 27, 2015, a weather balloon launched from Buffalo measured 33 mm of moisture in the atmosphere, which resulted in 20-30 mm of rain in the Greater Toronto Area (GTA) and even more in southwestern Ontario. We can expect similar rainfall values to that late-December storm event in 2015, with the potential for locally up to 40 mm of rainfall during this unusual setup.

While it’s unlikely that stations across southern Ontario will experience their wettest December day on record, several stations are likely to see a top-five wettest December day. Some statistics for the wettest December day on record in select cities across the region include:

– Toronto (YYZ): 40.9 mm (Dec. 6, 1962)
– Hamilton (YHM): 50.6 mm (Dec. 29, 1990)
– London (YXU): 47.5 mm (Dec. 17, 1921)
– Wiarton: 45.5 mm (Dec. 6, 1962)
– Windsor (YQG): 72.6 mm (Dec. 21, 1967)

As for the risk of thunderstorms, the weak sun angle at this time of year limits the surface instability available for thunderstorms to develop. However, the internal dynamics within the approaching low-pressure system may create conditions necessary for lightning and thunder. While it’s unlikely that most places will experience thunderstorms, a few lightning strikes are possible due to elevated instability in some areas. So, don’t be surprised if you hear a rumble or two as you go about your day.

Stay tuned to The Weather Network for more forecast updates across Ontario as this moisture-laden system approaches.