Creation and Evaluation of a CONUS-Wide Gridded Analysis-of-Record for Ice Accumulation in Preparation for NWS Operations

Freezing rain is a dangerous form of precipitation that can cause extremely hazardous travel and power outages. While the National Weather Service (NWS) generates forecasts of ice accumulations, there is limited validation data available to evaluate the accuracy of these forecasts. Moreover, there is no real-time analysis of ice accumulation to aid in nowcasting. This talk focuses on the development and evaluation of such an analysis: the Freezing Rain Accumulation National Analysis (FRANA).

FRANA is an hourly gridded analysis of ice accumulations derived from the Freezing Rain Accumulation Model (FRAM), Multi-Radar Multi-Sensor (MRMS) QPE, and the surface hydrometeor phase (ptype). FRAM requires wetbulb temperature, wind, and a precipitation rate to calculate an ice accumulation. The HRRR wind/wetbulb is used along with the MRMS QPE to calculate measurable ice accumulations. MRMS base reflectivity and the surface ptype are also used to determine where freezing rain is falling and to identify trace ice. A retrospective analysis of FRANA during 3 winter seasons was conducted and compared to ASOS goodrich ice accumulations. Overall, FRANA produces a reasonable ice accumulation footprint for events that have a defined precipitation field. However, freezing rain events that are weakly forced with very light precipitation are challenging to detect on radar and thus result in poor FRANA performance. FRANA is running experimentally on MRMS for the 2023-2024 winter season and is being evaluated by NWS forecasters during the Winter Weather Experiment (WWE). Potential impacts from weak events will be investigated from an impacts-based decision-support perspective during the testbed. An overview of FRANA, verification scores, and preliminary feedback from forecasters/testbed activities will be presented.