The “current icing potential” (CIP) algorithm combines satellite, radar, surface, lightning, and pilot-report observations with model output to create a detailed three-dimensional hourly diagnosis of the potential for the existence of icing and supercooled large droplets. It uses a physically based situational approach that is derived from basic and applied cloud physics, combined with forecaster and onboard flight experience from field programs. Both fuzzy logic and decision-tree logic are applied in this context. CIP determines the locations of clouds and precipitation and then estimates the potential for the presence of supercooled liquid water and supercooled large droplets within a given airspace. First developed in the winter of 1997/98, CIP became an operational National Weather Service and Federal Aviation Administration product in 2002, providing real-time diagnoses that allow users to make route-specific decisions to avoid potentially hazardous icing. The CIP algorithm, its individual components, and the logic behind them are described.

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An ATR72 commuter aircraft crashed near Roselawn, Indiana, on 31 October 1994 killing all 68 people on board. Available weather data, including those from a Next Generation Radar, a radar wind profiler, a Geostationary Operational Environmental Satellite, and pilot reports of icing have been examined in combination with analysis fields from the Rapid Update Cycle model and forecast fields from the Pennsylvania State University/National Center for Atmospheric Research MM5 numerical model. Synthesis of this information provides a relatively complete and consistent picture of the ambient meteorological conditions in the region of the ATR72 holding pattern at-3.1 km above mean sea level. Of particular interest is the evidence that these conditions favored the development of supercooled drizzle drops within a strong frontal zone, as indicated by cloud-top temperatures of-10° to-15°C, weak radar reflectivity, and strong, vertical wind shear within the cloud and warm front.