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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ,I II-3 2904 The Design Storm Probable Maximum Precipitation The first step in the derivation of a design storm is, or should be, to establish the upper limits of storm precipitation, or probable maximum precipitation (PMP). The magnitude of the design storm selected for final design of project features may be at the PMP level, but is often less. The estimation of PMP is particularly difficult, and the results least reliable, in mountainous regions because of orographic effects on pre- cipitation. These effects include stimulation of the precipitation-producing processes resulting from forced lifting by windward slopes of the moist air flow into the storm area, carry-over to lee slopes of precipitation formed over windward slopes, decreased precipitation in areas sheltered from moisture inflow by intervening mountains, and other more localized effects, such as funnelling of moist air through mountain passes. These effects are in addi- tion to the various meteorological factors, such as rate of moisture inflow and atmospheric instability, involved in the precipitation process. The raingage networks and procedures for evaluating precipitation in mountainous regions, whether in observed storms or for estimating PMP, are not adequate to permit accurate estimates, and any evaluation of PMP in such regions is, at best, a rough approximation. Confidence Bands for PMP Estimates A great deal of meteorological judgment is required in all proce- dures for estimating PMP. The estimates may be conservative or liberal, depending on decisions affecting the degree of maximization used in their derivation. The World Meteorological Organization PMP Manual1/ states: 1/ World Meteorological Organization, "Manual for Estimation of Probable Maximum Precipitation", Operational Hydrology Report No.1, WMO-No. 332, 1973, pp. 2-3. "c>'[i\\""f'jf~S; lIJ, lflfflOOr.'il:EE