When prevailing credit has been earned, negative conditional scores will reduce it, down to a minimum total score of zero. On the other hand, when prevailing score is zero, positive conditional scores give partial compensation. Positive conditional scores do not increase positive prevailing scores, and negative conditional scores do not decrease zero prevailing scores. In other words, conditional scores act as insurance for the prevailing forecast. Conditional forecasts are discouraged unless the prevailing forecast needs protection against a possible zero prevailing score.
Conditional scores can also affect each other.
If it is negative, the Total Raw Score is zero. If positive, the score is reduced by all negative scores further to the right. This results in Total Raw Scores that always range between 0 and 7. Difficulty factors allow TAF verifications to be compared for different weather situations or different sites, i.e., they help normalize the scores. Results Figure 2 shows hour by hour deterioration of total ceiling raw scores as the TAF progresses.