During the winter of 1996-97, two forecasters from NCAR formulated experimental probabilistic forecasts of icing conditions in the regions around six large cities in the midwestern United States. The purposes of this forecasting exercise were (1) to determine the capabilities of the forecasters to make probabilistic forecasts of this weather phenomenon and (2) to investigate the characteristics of a forecast verification method based on icing observations in the regions around large cities. In the middle of the experiment, feedback was provided to the forecasters feedback regarding the quality of their forecasts. The experiment was extended through winter 1997-98 for one city (Cleveland).
Pilot reports (PIREPs) of icing are used to verify the forecasts. To compensate for known problems with using PIREPs for verification, a "cities-based" approach is used to evaluate the probabilistic forecasts. This approach allows computation of all standard verification measures, including the false alarm ratio (FAR), for Yes/No forecasts, and makes it possible to evaluate the probabilistic forecasts. The cities-based verification approach assumes that if icing conditions exist in the regions around larger cities/airports (i.e., where there is a lot of air traffic), the conditions will be reported.
The forecasts included 12-hr outlooks and 2-to-4-hr short-term forecasts. Three categories of icing conditions were forecast: general icing (i.e., all icing severities and types), moderate-or-greater (MOG) icing severity, and so-called "worst" conditions (mixed or clear icing type, with MOG severity). The probabilistic forecasts for the different categories were evaluated both individually and as a vector. In addition, consensus and "average" forecasts were evaluated, as well as the forecasts by the individual forecasters.
Results of the forecast evaluation indicate that the forecasters were able to create reliable and reasonably accurate probabilistic icing forecasts for all categories of forecasts, with the best results for short-term forecasts of general icing conditions. However, the forecasts are most reliable when the region around the cities is extended to a larger area than was originally specified in the experimental plan. The quality of the forecasts improved in the second half of the experiment, after the forecasters were offered feedback on the performance of their forecasts; apparently the forecasters were able to incorporate information from the feedback to improve their forecasts. The results (1) suggest that it is possible to formulate skillful probability forecasts of icing conditions; and (2) confirm the usefulness of the cities-based icing verification approach.
The 8th Conference on Aviation, Range, and Aerospace Meteorology