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34 2.3 Effect of Barrier and Elevation The adopted convergence PMP is for 1000 mb (100 kPa) or sea level. For lo- cations at higher elevations or to the lee of mountain barriers, the 1000-mb (lOO-kPa) convergence PMP must be decreased. This is accomplished by reduc- tions for barrier and elevation. 2.3.1 Effective Barrier and Elevation Map During strong inflow of saturated or near saturated air, moisture is de- pleted on windward slopes by the higher elevations. Moisture is depleted for areas to the lee of upwind barriers by the effect of the barrier. Elevations used in this study were based on smoothed elevation contours ofa 1:1,000,000 scale topographic map. The smoothing moved the actual terrain elevation slightly upwind. This "effective" elevation, as differentiated from the actual elevation, provided for greater moisture into a region be- cause precipitation particles can be carried along by the wind to higher elevations. The "effective" barrier for the lee areas was determined from the height of the upwind barrier. These effective barriers may differ from the maximum elevation of the barrier since allowance was made for moisture flow through substantial breaks in the ridgeline. Inflows from southwest through south-southeast were of prime importance in deriving the effective barrier and effective elevation chart for a large por- tion of the Southwestern States. Winds from westerly to northwesterly direc- tions were involved near the northwest corner of the region. A reasonable. tie-in was maintained with the effective barrier and elevation charts of studies for adjoining areas. Also, inflow into southwestern Wyoming and northeastern Utah from the east to northeast resulted from the prototype storm for this portion of the study region. This is consistent with extreme rains to the east of the Continental Divide caused by easterly flow in late spring storms. With some variability permitted in the direction of moist inflow, isolated mountains and ridges less than 10 miles (16 km) long (measured at the base relative to the wind direction) are not effective in reducing moisture. The effective barriers were in many instances phased out, downwind, at a distance about 1 to 1.5 times their length, implicitly allowing recharge of moisture behind such obstacles. The amount of recharge is similar to that of border- ing generalized reports (HMR Nos. 36 and 43). Recharge toned down or eliminated effects of ridges somewhat longer than the initial 10-mi (16-km) criterion. Figure 2.17 shows the combined barrier/elevation map for the for the Southwest. 2.3.2 Reduction for Effective Barrier and Elevation Variation of nonorographic PMP with barrier height and elevation has been made proportional to the variation with elevation of precipitable water in a saturated column. It is the same as that used for convergence PMP in HMR No. 36 for California and for some of the variation in HMR No. 43 for the