Freezing rain is a significant winter hazard that occurs when raindrops freeze upon contact with cold surfaces, forming a layer of ice. This phenomenon can lead to treacherous travel conditions, damage to trees and power lines, and widespread power outages.
To assess the potential impact of ice storms, meteorologists and emergency managers often refer to the Sperry–Piltz Ice Accumulation Index (SPIA Index). Developed in 2007 by Sidney Sperry and Steven Piltz, the SPIA Index predicts the severity of ice storms by considering factors such as expected ice accumulation and wind speed. The index ranges from 0 to 5, with higher values indicating more severe conditions:
Level 0: Minimal risk of damage to exposed utility systems; few outages expected.
Level 1: Some isolated utility interruptions possible; roads and bridges may become slick and hazardous.
Level 2: Scattered utility interruptions expected, typically lasting 12 to 24 hours; travel conditions may be extremely hazardous due to ice accumulation.
Level 3: Numerous utility interruptions with some damage to main feeder lines and equipment expected; tree limb damage is excessive; outages lasting 1 to 5 days.
Level 4: Prolonged and widespread utility interruptions with extensive damage to main distribution feeder lines and some high-voltage transmission lines/structures; outages lasting 5 to 10 days.
Level 5: Catastrophic damage to entire exposed utility systems, including both distribution and transmission networks; outages could last several weeks in some areas; shelters needed.
The SPIA Index emphasizes that even moderate ice accumulations can cause significant damage, especially when accompanied by strong winds. For instance, a half-inch accumulation of ice with wind speeds exceeding 25 mph can elevate the event to a Level 4, indicating a high risk of prolonged power outages and infrastructure damage.
For Clinton County, Ohio, a Winter Weather Advisory is in effect from 5 PM this afternoon until 2 AM Thursday. Forecasts indicate the possibility of freezing rain beginning as early as Wednesday afternoon, persisting into Wednesday night. The greatest potential for ice accretion is expected near and north of the I-70 corridor. Antecedent* road and ground temperatures are relatively warm, which may limit impacts through the evening. However, untreated surfaces could become hazardous past midnight as temperatures approach the freezing mark. By that time, air temperatures are expected to rise, and precipitation should taper off between 2 to 4 AM. New ice accumulation of less than 0.1 inches is possible. Given these conditions, isolated slick spots are possible, and minor ice accumulation on trees and power lines could lead to scattered power outages.
Residents are advised to monitor local weather updates, exercise caution while traveling, and prepare for potential power outages by ensuring they have necessary supplies on hand.
*Antecedent means something that comes before or precedes something else. In the context of weather, antecedent road/ground temperatures refer to the temperatures of roads and the ground before the upcoming weather event occurs.
For example, if the ground temperature is still above freezing before freezing rain starts, it may take longer for ice to accumulate on roads, reducing the immediate impact. However, if temperatures drop quickly or are already near freezing, ice could form more rapidly, making travel hazardous.