Why the Snowiest Towns Are Not the Worst for Ice Dams
The number people check first
Ask whether a town gets ice dams and most people quote its snowfall. Across the 1,697 cities in this dataset where a typical house gets at least one dam day, snowfall and dam days do rise together (a rank correlation of 0.82), but the days with snow lying on the ground track dam days almost perfectly (0.99). A dam needs snow sitting on a roof in freezing air. Snow that falls and melts off in two days does very little.
Snowy places that come out lower than expected
Of the 192 cities that average 60 inches of snow or more, 43 are not high or severe on a typical house. The lowest are Cheyenne, WY (66 in of snow, 10.3 dam days), West Jordan, UT (82 in of snow, 10.7 dam days), Millcreek, UT (82 in of snow, 10.7 dam days). Their snow comes in storms separated by thaws, or arrives late in the season when the sun clears roofs quickly.
Less snow, more dams
The reverse also happens. Among cities with under 50 inches a year, the highest are Bemidji, MN (49 in of snow, 51.3 dam days), Fergus Falls, MN (50 in of snow, 47.3 dam days), Detroit Lakes, MN (48 in of snow, 46.7 dam days). Their winters are cold enough that a moderate snowpack stays on the roof for weeks, and every one of those weeks is a chance for attic heat to melt it from below.
What to look at instead
Each city page shows days with an inch or more of snow on the ground and how many of them stay below freezing all day. Those two numbers are the climate side of the risk; the attic is the other side. See air sealing or heat cable for what the attic changes, and which month dams form for when in the winter those days fall.