An Ice Storm Warning is a National Weather Service alert that freezing rain is expected to build up enough ice to meet the local office's warning criteria, usually around a quarter inch (about 6 mm) or more.
How the ice is measured
NWS Instruction 10-513, the national winter products directive in its November 2024 version, ties warning criteria to new ice on an elevated, flat, horizontal surface, recorded in hundredths of an inch. It describes significant ice accumulation as usually 0.25 inch or greater on that surface. Each office sets its own warning amount, and the Des Moines office, for example, uses 1/4 inch.
In August 2026 the NWS announced that, on or about 10/01/26, it would switch its gridded ice forecasts in the National Digital Forecast Database (NDFD) and its Local Storm Reports (LSRs) to radial ice. That is the thickness of new ice around an exposed cylindrical object, which the NWS says best describes impacts on tree branches and power lines.
Because flat and radial measurements describe ice differently, check which one an office's criteria and forecast text are using before comparing numbers.
Why that amount matters
The NWS notes that heavy accumulations of ice can bring down trees and topple utility poles and communication towers.
Travel can turn hazardous well before warning amounts. Even small accumulations are extremely dangerous to motorists and pedestrians, and bridges and overpasses freeze first.
Where it sits among winter alerts
Below warning amounts, freezing rain falls under the Winter Weather Advisory, which absorbed the former Freezing Rain Advisory in 2017. The Des Moines office, for example, lists freezing rain or drizzle under 1/4 inch at advisory level.
When ice comes mixed with snow or sleet, the event may instead be covered by a Winter Storm Warning, which includes ice and combinations of winter hazards. Ahead of the event, the possibility of an ice storm is carried in a Winter Storm Watch.
What the forecast hinges on
Freezing rain forms when snow melts completely in a warm layer aloft, then falls through a thin freezing layer near the ground without refreezing. If that cold layer is deeper, the drops refreeze into sleet instead.
Because the outcome depends on the depth of those layers, small errors in the forecast temperature profile can change how much ice actually accumulates.