manual for estimation of probable maximum precipitation - WMO

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12 ESTIMATION OF PROBABLE MAXIMUM PRECIPITATION
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Hotel 20 July dew point found to have been observed
during fair weather situation and ia undercut.
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5 1525 5 1525 5 1525 5 1525 5 1525 5 1525 5 1525 5 1525 5 1525 5 1525 5 1525 5 15 25
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Figure 2.3-Enveloping maximum persisting 12-h dew points at a station
weather charts for the dates of highest dew points should be examined and
the dew points discarded if they appear to have occurred when the observing
station was clearly in an anticyclonic or fair weather situation rather than
in a cyclonic circulation with tendencies towards precipitation.
Another consideration is to avoid dew points that are too
extreme. With very long record lengths, values of dew points may occur that
are higher than optimum for the processes that produces extreme
precipitation amounts. If dew points are found that substantially exceed
100-y values, the weather situation accompanying them should be carefully
evaluated to ascertain that they are capable of causing ext reme
precipitation amounts.
All values of maximum persisting 12-h dew points selected
directly from surveys of long records are plotted against date observed, and
a smooth envelope drawn, as illustrated in Figure 2.3. When dew points from
short records are subjected to frequency analysis, the resulting values are
usually plotted agains t the middle day of the interval for which the series
is compiled. Thus, for example, if the frequency analysis is for the series
of semi-monthly maximum persisting 12-h dew points observed in the first
half of the month, the resulting 50- or 100)' values would be plotted
against the eighth day of the month.
The preparation of monthly maps of maximum persisting 12-h
1000-hPa dew points is advisable, especially where numerous estimates of PMP
are required. Such maps not only provide a ready, convenient source of
maximum dew points, but also help in maintaining consistency between
estimates for various basins. The maps are based on mid-month dew point
values read from the seasonal variation curves and adjusted to the 1000-hPa
level. These values are plotted at the locations of the observing stations,
and smooth isopleths are then drawn, as in Figure 2.4.
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