I have noticed multiple mentions of northward Northern Cardinal vagrancy in Oct-Nov 2020, and got to work looking into meteorological explanations. The results were as expected - there are several small weather events which could be tied to several isolated records, with one particularly notable event with a significant number of correlated records. In a nutshell: Northern Cardinals disperse every year in late Oct-early Nov, and every year, some move northwards on suitable warm conditions beyond their normal range. In 2020, there was an above-average number of "suitable" days for northward movement, leading to an above-average number of records, which was then significantly amplified by a prolonged anomalous heatwave - and suddenly Northern Cardinals are the talk of the (northern) town(s). Doing a quick check of the ebird database - there is a significant wave of records in the Nov 10-13th timeframe. A check of the weather reveals a corresponding record heat event from Nov 8-11... Here's some maps (red arrows added by me, indicating flow of warm air):
Nov 8, 2020 - warm front pushing through the middle of the continent. Ridge (high pressure) in the SE rotating gentle S winds into the Eastern Great Lakes / S. Quebec.
Nov 9, 2020 - SE Ridge pushing the warm front N over/above the high pressure - stiff warm winds pushing well into the boreal forest.
Nov 10, 2020 - ridge moving more offshore but still putting the squeeze on the frontal boundary... Weak low pressure near the Great Lakes along a boundary which runs from the Gulf of Mexico to the Labrador Sea.
Nov 11, 2020 - Exit, stage right. Initial warm air mass has moved off the map, but the SE ridge continues to hold back the frontal boundary enough to continue warm conditions & southerly flow in the NE CONUS and into Quebec & the Canadian Maritimes.
The pattern of warm weather Nov 8-11 with records concentrated Nov 10-13 is something I would expect, as the birds are not all found immediately after the arrive and the subsequent passage of colder weather could encourage them to move to feeders etc.
Beyond that, there may well be additional external factors that promote Northern Cardinal movements beyond the weather. Was 2020 an atypically successful breeding year in the northern parts of the species range? Is the 2020 "irruption" year of finches and other passerines promoting an increase in dispersal distance (due to lack of suitable food supplies for the winter?). Generally speaking, Northern Cardinal is one of those species which is considered non-migratory, but can show significant spring or fall migratory behaviour if you're looking closely. I would assume that multiple factors are at play in making 2020 an excellent year for Northern Cardinal dispersal; however, the species would NOT be fighting cold weather/north winds/snowstorms to move well north of their typical range (in these numbers) - and I believe it is the abundance of suitable warm weather which is allowing them to move into regions where their dispersal and numbers are impossible to ignore.
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