Forecasts of "normal". (10th February 2021)

• Record Type: Journal Article
• Authors: Mason, Simon J.; Ferro, Christopher A. T.; Landman, Willem A.
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 147:Number 735(2021)
• Page Start: 1225
• Page End: 1236
• Publication Date: 2021-02-10
• Subjects: bivariate normal -- deterministic forecasts -- normal -- probabilistic forecasts -- seasonal climate forecasting; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.3968
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract: The difficulty of forecasting "normal" climate conditions is demonstrated in the context of bivariate normally distributed forecasts and observations. Deterministic and probabilistic skill scores for the normal category are less than for the outer category for all‐but‐perfect models. There are two important mathematical properties of the normal category in a three‐category climatologically equiprobable forecast system that affect the scores for this category. First, the normal category can achieve the highest probability less frequently than the outer categories, and far less frequently in contexts of weak to moderate skill. Second, there are upper limits to the probability the normal category can reach. These mathematical constraints suggest that summary measures of skill may underestimate the predictability and forecast‐skill of extreme events, and that subjective inputs to probabilistic forecasts may need to take greater account of limitations to the predictability of normal conditions. Abstract : Maximum possible forecast probability for the normal category as a function of the correlation skill for bivariate normally distributed predictions and observations given different sample sizes, n is shown in the figure above. The effects of sample size on the maximum forecast probability for the normal category are indicated in the figure is small (reducing the maximum probability by less than 1% for sample sizes larger than 20), so the primary control on the maximum probability is the correlation skill.