An Evaluation of Freezing Rain Local Storm Reports and Their Uses

It is important to measure ice accretion and its impacts during freezing rain and drizzle events so that forecasts and impact messaging may be evaluated and improved. Ice accretion can be measured by Automated Surface Observing System (ASOS) Goodrich ice sensors. However, the ASOS network is sparse with less than 700 ice-detecting stations across the continental United States. Additionally, ASOS stations are unable to report the impacts of freezing rain and drizzle. A second source of ice accretion observations are National Weather Service Local Storm Reports (LSRs). LSRs record both the magnitude of ice accretion and any reported impacts. If LSRs are found to be reliable, they can be used to increase the coverage of ice accretion observations. Additionally, LSRs could used for the development of impact-based freezing rain and drizzle forecasts and messaging.

To evaluate the utility of freezing rain and drizzle LSRs, they are compared to ASOS ice observations over a thirteen-year period. It was found that LSRs do not follow the diurnal trend of freezing rain and drizzle, but rather trend with periods of increased human activity, especially the morning commute. It was also found that ice accretions tend to be estimated by human observers into common fractional amounts like 1/4 inch or 1/8 inch. Most LSRs did not record the start time or end time of ice accretion. LSRs had a moderate correlation with nearby ASOS observations, despite well-known ice accretion variability within short distances. Lastly, relationships are drawn between ice accretion and reported impacts. Overall, uncertainty in ice accretion magnitude and duration limits the ability to use LSRs in quantitative ice estimation. However, freezing rain and drizzle LSRs do supplement the ASOS network with increased coverage and offer useful impact information.