Synoptic and Mesoscale Structure of a Severe Freezing Rain Event: The St. Valentine's Day Ice Storm

Abstract

A severe freezing rainstorm produced as much as 4.5 cm of freezing rain during an 18-h period at Champaign, Illinois, on 14?15 February 1990, resulting in over $12 million in damage, week-long power outages, and a federal disaster declaration. The ice storm occurred during the University of Illinois Winter Precipitation Program based in Champaign. The early mesoscale evolution of this storm was documented for several hours with a 10-cm Doppler radar and Cross-chain Loran Atmospheric Sounding System soundings launched every 3 h. The freezing rain event occurred when convective bands developed over a slow-moving warm front during a period of strong overrunning. The strongest convection developed in a period of about 1 h, with a narrow elongated band northwest of the radar producing very heavy sleet and a band just south of the radar producing heavy freezing rain, along with in-cloud lightning. An analysis of conditional symmetric instability yielded no evidence that centrifugal accelerations were important to the development of convection in this storm. Frontogenetic forcing was strongest several hours before the development of the bands but apparently was also insufficient to trigger convection until the local atmosphere became marginally unstable to upright convection. The transition from a conditionally stable to an unstable atmosphere in the vicinity of the bands was directly associated with locally strong warm advection above the warm frontal surface. Forecast guidance, including the nested grid model (NGM) thickness, precipitation, and 850-mb temperature forecasts, and model output statistics of both the limited fine mesh (LFM) model and the NGM all predicted that the warm front would progress northward and that freezing rain would convert to rain before significant glaze accumulations occurred. Forecasts of midtropospheric parameters such as 1000?500-mb thickness and 850-mb temperature indeed verified; however, surface temperature forecasts were significantly in error, with errors ranging from 5° to 10°C during the period of heaviest glaze accumulation. The observed surface temperature never rose above 0°C during the period of ice accumulation or throughout the following day. The isothermal conditions observed during and after the storm appeared to be the result of sublimation and melting of ice that had accumulated on surface objects. The available evidence suggested that ice sublimation and melting, in addition to cooling the boundary layer, maintained a small wedge of cold air at the surface over which warmer air rose as it advected northward. The result of ice sublimation and melting was to retard the movement of the surface warm front, although warm air aloft was free to move over the narrow wedge of cooled surface air. By maintaining the surface temperature near 0°C, diabatic processes extended the duration of time that heavy glaze accumulations remained on trees and wires, allowing more damage to occur.