There’s an unusual twist beneath all this high pressure.
While barometres got an unusual workout above 1050 hPa across eastern Ontario, the strong high paradoxically triggered an unusual feature across Lake Ontario. Most of Ontario benefitted with sunny skies on Saturday, but a localized thermal low developed over Lake Ontario: snow occurred in a region dominated by record-breaking high pressure.
First, let’s define a thermal low. We’re looking at a localized low-pressure feature that develops with surface heating, initiating rising air and triggering a pressure drop.
You need a strong temperature contrast, so arctic air ‘chilling’ over Lake Ontario provides the perfect backdrop. When sufficient lift and instability are achieved, you can squeeze snowfall out of the bone-dry environment offered by the record-breaking high.
As air converged across the axis of Lake Ontario on Saturday, a thermal low was born. You can spot the classic swirl as we play out a video of satellite imagery on Saturday afternoon.
The broad easterly flow associated with the high will steered the thermal low west, peppering the shores of Lake Ontario with light flurries.
We can even zoom in and take it a step further. Cold air from the surrounding shoreline sinks in to replace the air rising over the lake.
It’s created a small-scale rotation that’s visible on satellite. You can see a quasi-warm front propagate from the centre of the low, as the relatively warmer air from the lake spirals outwards, while a mini-cold front forms on the southern end of the low as colder air from the land wraps behind the low.
This unusual event highlights the complex influences of temperature, pressure, and moisture that can create localized weather phenomena, under what can seem to be stable conditions.
Check out The Weather Network for more information on weather conditions across Ontario.
