QPF Verification Information

This verification table shows statistics for either 12 or 24-hr Quantitative Precipitation Forecasts (QPF) valid at 1200 and/or 0000 UTC from various numerical weather prediction models and manually derived value added forecasts from three different National Weather Service forecast centers; California Nevada River Forecast Center (RFCQPF for 12z 24-hr only), the Weather Prediction Center of NCEP (WPC), and the Eureka and San Francisco Bay Area Forecast Offices (Official). These models and forecasts are defined in the table below. The statistics provided are derived from a contingency table showing 12 or 24-hr forecast (QPF) and observed 12 or 24-hr precipitation (QPE) divided into 7 categories. If the forecast and observed rainfall fall in the same category this is a hit. Otherwise this is a miss. The % correct, % to wet and % to dry are provided. The verification table shows the best guidance for the given duration of forecast selected, i.e. .5 (12-hr page) or 1 day (24-hr page) to 90 days, plus the next two best guidance forecasts and finally the worst guidance. Depending on the score chosen, color coding is used to show the quality of the particular model compared to the other forecasts. (green- top 25%, white- middle 50%, red- bottom 25% with brighter shading indicating the top or bottom 10%). If a duration longer than .5 (12-hr page) or 1 day (24-hr page) is chosen, a list of the wettest days over the watershed and the selected models performance is provided up to the ten wettest days. These statistics are for the first period only (12-hr lead time for 12-hr page or 0-hr lead time for 24-hr page). If a particular guidance forecast has no 12-hr or 0-hr forecast or is missing this will be noted with dashes. Three forecast products that fall into this category for the 0-hr lead time are Official, MOSGuide, and the NBM model. For these forecasts one should view the 12-hr verification page.

Threat Score (TS): A verification measure of categorical forecast performance equal to the total number of correct event forecasts (hits) divided by the total number of event forecasts plus the number of misses (hits + false alarms + misses). The TS is not affected by the number of no-event forecasts that are not observed (correct rejections). For this verification .5”/12hrs or greater precipitation is the threshold being verified.

Heidke Skill Score (HSS)¹: A skill corrected verification measure of categorical forecast performance similar to the Percentage correct (PC) but which takes into account the number of hits due to chance. Hits due to chance are given as the event relative frequency multiplied by the number of event forecasts.