Ice accumulation is the thickness of ice that builds up on exposed surfaces, mainly from freezing rain or freezing drizzle. It is measured and forecast in hundredths of an inch, and it is a main factor in how much an ice event damages trees and power lines.
Two ways to measure the same ice
Radial ice is the thickness of new ice around an exposed cylindrical object, such as a branch or a wire. Flat ice is the thickness of new ice on an elevated horizontal flat surface.
The National Weather Service (NWS) winter products directive, NWS Instruction 10-513, defines forecast ice in flat terms. NWS Service Change Notice 26-73, issued 08/13/26, announced that NWS ice forecasts and Local Storm Reports would switch to radial ice on or about 10/01/26, because radial amounts best describe impacts on branches and power lines.
How observers measure it
The NWS office in Jackson, Kentucky, suggests finding an ice-covered object out in the open, such as a small tree branch or a clothesline, and measuring the ice on it. Ice rarely coats an object evenly, because gravity pulls water to the underside before it freezes.
How the Weather Service uses it
Ice Storm Warnings are usually issued for around 1/4 inch of ice or more, but each office sets its criteria locally with its partners. As an example, the Baltimore/Washington office lists 1/4 inch or more of ice for an Ice Storm Warning.
Under the radial standard, ice is forecast in six-hour periods and distributed through the National Digital Forecast Database.
What the number does not tell you
Ice adds weight to branches and lines quickly, and heavy accumulations can bring down trees and topple utility poles and communication towers. A total by itself does not show how that load is spread across a storm.
Because radial and flat values differ for the same ice, older reports, criteria pages and new forecasts may not line up directly. Compare numbers only when you know which measure each one uses.