#
PROBABILISTIC FORECASTS OF QUANTITATIVE PRECIPITATION FROM THE NCEP GLOBAL
ENSEMBLE

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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