Monthly & Seasonal outlooks? What do they mean?
Forecasts are expressed as the probabilities of the observation of mean temperature (total precipitation) falling into the most likely of three classes - above - near - or below normal (median). Classes are defined by limits that divide the 1981-2010 climatological distribution into thirds. Thus each class has a climatological chance of occurrence of 33.3%.
A forecast probability of either above or below normal in the three-Class system implies a corresponding reduction in the probability of the opposite class and a fixed probability (at 33.3%) of the near normal class for Probability anomalies up to 30%. For probability anomalies greater than 30% of Above or Below normal the probability of the opposite class is fixed at 3.3% (a -30% anomaly) and the probability of the near normal class is reduced by the excess forecast probability anomaly over 30%. Note that this is only a crude approximation of the true probability of the non-specified classes and is generally less accurate for extreme shifts (20% or more) in the probability anomaly of the most likely class.
Examples: forecast probability anomalies of 20%, 30% and 40% for above normal imply probabilities for all three classes (above - near - below) of 53.3% - 33.3% - 13.3% --- 63.3% - 33.3% - 3.3% and 73.3% - 23.3% - 3.3% respectively.
Occasionally the forecast calls for an increased chance of the observation falling in the middle class. When this occurs - half of the increased probability of the middle class is subtracted from each of the extremes.
For users who prefer a 2-class system to the current 3-class system - conversion to a 2-class system can be done very simply by altering 50-50 climatological probabilities for the below versus above normal two class categories by the probability anomaly seen on our maps. For example -- a 20% anomaly toward above Normal would convert to an 70% chance for above and a 30% chance for below in a 2-class system - a 30% anomaly to 80 and 20% - and a 40% to 85 and 15%.
Future revisions of this message will be issued whenever significant changes in the available forecast tools are made.
The monthly technical discussion may be found by clicking here while the seasonal discussion is here.