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The AR scale is described by Ralph at all 2019 in the bulletin of the American Meteorological Society.

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It combines AR intensity and duration.

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The intensity is defined as an event maximum integrated vapor transport or IVT magnitude.

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The duration is the hours with IVT magnitude greater than 250 kilograms per meter per second.

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This is commonly known as AR conditions.

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The scale is derived for a location using observed or forecast data.

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For example, step one, pick a location and identify an event with IVT greater than 250 in the past or in the forecast period.

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The AR intensity, again, is the maximum IVT magnitude during the event.

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This sets the preliminary rank.

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The AR duration is the continuous period with IVT greater than 250.

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The final value of the AR scale is the preliminary rank plus one for durations greater than 48 hours

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and the preliminary rank minus one for durations less than 24 hours.

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For example, an AR intensity of 875 kilograms per meter per second would yield an AR-3.

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If the duration of that event was greater than 48 hours, it would be promoted to an AR-4.

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If the duration was less than 24 hours, it would be demoted to an AR-2.

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The AR scale can also be illustrated on a map.

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The map on this image demonstrates an AR-5 making landfall on the US-West Coast between 7 and 10 February in 2017.