Ice Impact Index – an experimental product from NWS-Memphis

A new experimental ice index will be used at 10 NWS offices in Missouri, Oklahoma, Arkansas, Texas and Tennessee this winter. The Scaled Predictive Ice Storm Aftermath Index (SPIA Index, or just “Ice Impact Index”), which will assign a number from 1-5 to a forecast ice event, will gauge potential damage due to ice accumulations on exposed electric utility systems. The index takes into account forecasted ice accumulations and wind speeds – the higher the index, the greater the damage potential from icing.  The target audience in emergency management officials, utility companies, and the public.

The Ice Impact Index will be at its highest when ice accumulations are large and winds gusts are high. These conditions place a huge stress on utility systems and can result in widespread, prolonged power outages. This index will assist in preparation for dangerous ice storms, but hopefully we will not have to use it much this winter!  Below is the index and expected impacts from each level:

Index Utility Damage and Impact Description
1 Some local utility interruptions possible…typically lasting a few hours.
2 Scattered utility interruptions possible…typically lasting less than 12 hours.
3 Numerous utility interruptions possible…lasting up to 5 days. Damage to some main feeder lines possible.
4 Prolonged and widespread utility interruptions. Damage to many main feeder lines possible. Utility outages lasting up to 10 days possible.
5 Catastrophic damage to exposed utility systems possible. Outages lasting several weeks possible in some areas.

Here is a graphic showing what ice accumulation and wind speed forecasts are necessary to produce each Ice Index Impact level:

The Ice Impact Index will be issued separately from any Ice Storm Warnings, Freezing Rain Advisories, and other NWS “watch, warning, advisory” products.

For more details on the Ice Impact Index, visit this page on the NWS-Memphis website.  The NWS’ official Product Description Document can be found here.  Participating offices include: Tulsa, OK; Little Rock; Memphis; Nashville; Norman (Oklahoma City); Jackson, MS; Springfield, MO; Pleasant Hill, MO; St. Louis, MO; and Paducah, KY.

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