A stalled low pressure system off the east coast of the Canadian Maritimes is being blamed for redirecting the jet stream, producing a steady cold polar flow of air out of the northwest across the Great Lakes region, resulting in steady accumulations of snow in the SE lee of the lakes. The wintry blast, which started at the tail end of last week, is not expected to abate before late Wednesday or early Thursday. Its not yet been determined whether records have been set yet, but portions of southwestern Ontario have received as much as 114cm of snow since the squall period began.

Lake effect snows occur when cold winds blow across the warm fall and early winter waters of the Great Lakes, picking up moisture. Once these winds come onshore, the air mass rises and cools, depositing the moisture as snow in the lee of the lakes. This is a surface effect. Most of these LES squalls are less than 12,000 feet in depth, and as a result, disappear from radar about 100/160km miles out from the radar site as the beam overshoots the snow deck. The Great Lakes region is prone to these squalls up until about February, when the lakes begin to freeze over much of their surface, depriving the wind of moisture. This transition can be seen as January skies change from cloudy and flurry, to clear and cold in early February.

The following two animations show the composite reflectivity pattern from KBUF in Buffalo, NY, showing the band alignment with the NW–>SE wind direction for december 5, 2010 and December 6, 2010, respectively. The darker the blue, the more intense the snow. Moist winds off Lake Huron and Georgian Bay in the upper left materialize as snow bands further enhanced by lakes Erie and Ontario as they cross into Ohio and New York State. The snow bands are largely continuous back to the upper lakes, but, are not seen because the radar beam has passed over top of the snow bands at the Lake Huron and Georgian Bay shorelines. (Click on animation image if animation doesn’t self start.)

The second animation for December 6, 2010 (cities layer is suppressed) shows the orographic (cooling as the moist air rises over landform) condensation into snow, in the second half of the animation as a crescent shaped band forming southeast of Georgian Bay, near the centre of the animation. The wind rises up the gentle slope of the Oak Ridges moraine, just above the north shore of Lake Ontario.

Environment Canada discussion:

Weather summary for all of Southern Ontario and the National Capital
Region issued by Environment Canada Toronto at 6:15 AM EST Tuesday
7 December 2010.

Significant lake effect snowfall reports.
==weather event discussion==

The traditional snow belts of Southern Ontario are in the midst of a
major multi-day snow squall event. Brisk northwest winds from Lake
Huron and Georgian Bay have produced impressive snowfall
accumulations over the past couple of days. Heavy snow squalls will
continue today and Wednesday likely giving remarkable snow totals
well over one metre in and around the London area as well as some
locales to the southeast of Georgian Bay.

Snowfall reports as of 6 PM Monday and fallen since Saturday night,
except for lucan which has received a staggering 114 cm as of
6 AM.
Location snowfall amounts (cm snow)

London 45-55
London aiport 53
Southeast of London 65-70
Lucan (northwest of London) 114
Dorchester (east of London) 42
Goderich 15-20
Mount Forest 10
Lakelet (near Clifford) 18
Paisley (ne of Kincardine) 5-10
Niagara 14
Toronto downtown 2-4
Toronto north 12
Newmarket 30
Maple 15-20
King City 20
Schomberg 33 as of 8 AM Monday
Beeton (south of Alliston) 81
Thornbury 40
Barrie 10-15

Please note that this summary contains the observations at the time
of broadcast and does not constitute an official and final report of
the weather events or the high impact events attributed to the
weather events.