Key takeaways
- Ice dams need three things: snow on the roof, a warm upper roof surface, and a cold roof edge.
- A season that swings across 32 degrees repeatedly produces more of that pattern than a season locked in deep cold.
- Strong El Nino winters in the Great Lakes and Ohio Valley tend to run milder with more mixed precipitation.
The forecast says milder. That is not the relief most homeowners assume it is. Ice dams are built by thaw and refreeze, and El Nino winters specialize in both.
Published September 28, 2026 · Roof Gutter Now LLC
Every year we get the same call in February from someone who says the winter was mild, so the water coming through the bedroom ceiling makes no sense. It makes perfect sense. Ice dams are not a cold problem. They are a temperature difference problem.
The mechanism, in plain terms
University of Minnesota Extension describes ice dams as a product of nonuniform roof surface temperatures. The part of the roof over your heated living space climbs above 32 degrees and melts the bottom of the snowpack. That water runs down and reaches the overhang, which hangs out past the heated walls and stays below freezing. It refreezes there. The ridge of ice grows, the next round of meltwater pools behind it, and water works its way backwards under the shingles.
Building scientist John Straube laid the same process out in Building Science Digest 135, pointing at air leakage and insufficient insulation as the heat sources that warm the deck from below.
Why mild and swinging is worse than cold and steady
Consider two Januaries. In the first, temperatures stay between 5 and 15 degrees for three weeks. Snow sits on the roof, but the roof surface rarely gets above freezing, so little melts and little refreezes. In the second, you get four days near 40, then a night at 18, then another warm stretch, then a snow-to-rain storm.
The second January is the one that fills our schedule. Each thaw pushes water toward the eave and each refreeze locks it there. Add a rain event on top of an existing snowpack and you get liquid water with nowhere to go but backwards.
Worth knowing: a roof does not need much snow to dam. Two or three inches of wet snow with a warm attic underneath can build enough ice at the eave to back water up under the first course of shingles.
The El Nino connection
NOAA describes a typical El Nino winter as warmer across the northern tier with a drier Ohio and Tennessee valley, as the storm track shifts south. Regional climate centers make the same point for the Midwest: milder winters with less total snow, but still the potential for ice storms and big individual events.
Less total snow. More time near freezing. More mixed precipitation. That is a thaw-and-refreeze winter, and it is the kind that produces ceiling stains in homes that went ten years without a problem.
What actually breaks the cycle
- Fix the heat source. Air sealing and insulation stop the melting at the top of the roof. This is the permanent fix.
- Give water a path. Heat cable at the eave, through the gutter and down the downspout keeps a channel open through every freeze-thaw cycle.
- Clear the snow before it matters. Roof snow removal takes away the raw material.
If you already have a stain on a ceiling from a past winter, read what to do when an ice dam has formed but has not leaked yet.
Weather outlooks describe probabilities, not guarantees. Seasonal forecasts from NOAA and other agencies are updated monthly and can change. Nothing here is a prediction of what will happen at any specific address. Heat cable products are supplied by Radiant Solutions Company and installed by Roof Gutter Now LLC.