PROBABILISTIC FORECASTS OF QUANTITATIVE PRECIPITATION FROM THE NCEP GLOBAL ENSEMBLE

In these charts, the probability that 24-hour precipitation amounts 
over a 2.5x2.5 lat-lon grid box will exceed certain threshold values is given. 
The forecast probability is estimated directly from the 17-member global 
ensemble. At each gridpoint the number of ensemble members having a 24-hour 
precipitation amount greater than the limit considered is counted (M) and the 
probability is expressed as 100*(M/17). In addition to the color shading, the 
5, 35, 65 and 95% probability isolines are also drawn. 
If the NCEP MRF model has a bias or the perturbed ensemble forecasts do 
not span the range of uncertainty perfectly, the raw probability values from 
the ensemble may be biased as well. So the products should be used and 
evaluated with this in mind. We plan to calibrate the forecast probabilities 
based on observed rainfall data after which the new, more reliable probability 
values will be displayed. In particular we expect that the calibrated 
probability forecasts will be less sharp, i. e., the probabilities close to 0 
and 100 will be used less frequently. For the purpose of comparison, plots of 
the MRF precipitation forecasts are included for the US region.
With question or suggestions, please contact: 
Yuejian Zhu Yuejian.Zhu@noaa.gov or 
Zoltan Toth Zoltan.Toth@noaa.gov