Evaluation of Freezing Rain Local Storm Reports as a Verification Source

It is important to measure ice accumulation during freezing rain and drizzle events so that ice accumulation forecasts may be evaluated and improved. Ice accumulation can be measured by Automated Surface Observing System (ASOS) Goodrich ice detectors. However, the ASOS network is sparse with less than 1,000 stations across the United States. A second dataset recording ice accumulation is the National Weather Service Local Storm Report (LSR) archive. If LSRs are found to be reliable, they can be used to increase the coverage of ice accumulation observations across the United States.

To evaluate LSRs and their validity compared to ASOS ice accumulations, five winter seasons of LSR and ASOS data were collected. Multiple aspects of LSRs including source, report time, report location, and report magnitude were investigated and compared to ASOS observations. It was found that LSRs do not follow the diurnal trend of freezing rain/drizzle, but rather occur during hours of increased human activity, especially the morning commute. It was also found that LSRs tend to be estimated by human observers and binned into common fractional amounts of ice like 1/4 inch or 1/8 inch. Ice accumulation amounts in the LSR dataset had a moderate correlation to nearby ASOS Goodrich sensor observations. The focus of the presentation will be 1) to identify the characteristics of ice accumulation LSRs and, 2) to identify the extent that ice accumulation LSRs can be used for retrospective analyses or verification.