It’s been nearly a decade since we’ve seen such a moisture-laden storm this close to the end of the year.
A low-pressure system rolling toward the Great Lakes will drag a tremendous plume of Gulf moisture north of the border, bringing near-record levels of moisture to Ontario by Sunday.
GET THE LATEST: Significant rains, warmth push into southern Ontario Sunday
We measure moisture in the atmosphere using precipitable water (PWAT). Measured in inches or millimetres, it’s how much rain would fall if you condensed all the moisture in one small column of the atmosphere. Higher PWAT values indicate more moisture, which can fuel heavier rainfall rates.
PWAT values on Sunday are forecast to exceed 30 mm, which is incredibly rare for this time of year. That’s more in line with what you’d expect to see during a summertime setup over this part of the world.
Typically, the atmosphere over southern Ontario at the end of December is much drier, averaging a PWAT value of just 7 mm—meaning this event will be about 500 percent of the normal value.
Back on Dec. 27, 2015, a weather balloon launched from Buffalo measured 33 mm of moisture in the atmosphere. The subsequent storm brought 20-30 mm of rain to the Greater Toronto Area (GTA), with locally more across southwestern Ontario.
In general, we can expect similar rainfall values to the 2015 late-December rain event, with even locally up to 40 mm of rainfall with this anomalous setup.
It’s unlikely that stations around southern Ontario will achieve their wettest December day on record, but several stations are likely going to see a top-five wettest December day.
Here are the stats for the wettest December day on record for select cities across the region:
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)
Given the weak sun angle, there is very limited surface instability available for thunderstorms to tap into around this time of year. We have to rely on the internal dynamics within an approaching low-pressure system to create the setup needed for lightning and thunder.
While some of those dynamics will be in place on Sunday, it’s unlikely most places will see thunderstorms. A few lightning strikes are possible given elevated instability in spots—so don’t be surprised to hear a rumble or two as you go about your day.
Stay tuned to The Weather Network for more forecast updates across Ontario.
