manual for estimation of probable maximum precipitation - WMO

ESTIMATES FOR MID-LATITUDE NON-OROGRAPHIC REGIONS 15
storm area and the moisture source, the mean elevation of the ridge, or
cres t, is generally selected as the base of the moisture column. In mas t
cases, it is advisable to select the storm and maximum dew points between
the barrier and the storm location.
2.3.3 Applicability of persisting 12-h dew points for all storm
durations
The dew points from a set of stations used to obtain a
representative persisting 12-h storm dew point are unlikely to be in the
mas t intense moisture inflow for much more than 12 to 24 h, after which the
stations where the dew points were observed are very likely to be in the
cold air because of the displacement of the storm. The selection of
different representative 12-h dew points for every 12 h of a stann is a very
tedious task, especially for storm durations of 72 h and longer.
Comparisons of storm rainfall values adjusted on the basis of 12-h dew
points from different sets of stations and those from a single set indicate
that differences are too small to justify the additional time required for
obtaining representative 12-h dew points for different storm intervals.
It should be noted also that the use of different representative
dew points for a storm requires different maximum dew points for the
maximizing procedures described below. Tests of the use of representative
storm dew points over time intervals up to 72 h, e.g., 24-, 48- and 72-h dew
points, for adjusting storm rainfall values showed only small differences
from the results obtained from the use of the 12-h representative storm dew
point. The general practice is to use a single representative persisting
12-h dew point for adjusting the storm rainfall for all durations and sizes
of area.
2.3.4 Maximization of storm in place
Moisture maximization of storms in place, i.e., without change in
location, consists simply of multiplying the observed storm rainfall amounts
by the ratio (r ) of the maximum precipitable water (W m ) indicated for the
storm reference "location to the precipitable water estimated for the storm
(W s )' or
(2.1 )
Thus, for example, if the representative persisting 12-h 1000-hPa storm dew
point is·21°C and the maximum is 24°C and the rain area is at mean sea-level
(always assumed to be at 1000 hPa) with no intervening topographic barrier
between the rain area and moisture source, then the moisture maximizing
ratio (r m ) is computed from precipitable water values obtained fr.om the
Table A.1.1 in Annex 1:
The precipitable water values
moisture column with base at
Table A.l.3 were used instead
of r m would be unchanged.
used in determining Wand Ware for a
1000 hPa and top at 30"0 hPa. ff values in
of those in Table A.l.1, the resulting value