In this study, we first compute the annual maximum
(AM) precipitation accumulations from running totals
of 1-, 2-, 3-, 5-, 7-, and 10-day time series for all years
when 3B42RT data were available (2000–13) for each
grid box. Choosing the AM series limits the sample size
but is a required procedure for computing the ARI. The
three-parameter GEV distributions are derived by fitting
the AM series using open source MatLab GEV software
(http://www.mathworks.com/help/toolbox/stats/gevfit.
html), which uses maximum likelihood method to fit
the parameters (Kotz and Nadarajah 2000). The best-fit
GEV curves and thresholds for 2–100-yr ARI are computed
with the derived statistical parameters. As an
example, Fig. 1 shows a sample plot of the PDF curves
and return thresholds for different accumulation days
from a randomly picked single grid point located in the
Cascade Range near the border of the states of Washington
and Oregon. The PDFs of multiday precipitation
accumulation not only shift toward larger values as
compared with that of 1-day accumulation as expected,
but also show much wider spread with longer tails at
high accumulations (Fig. 1a).