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18 In figures 2.4a to e, the seasonal trends in the 1- and 3-day data are comparable with some exceptions, most notably between October and February in northwest Nevada (fig. 2.4e) in which the trends appear opposed. Some rather large differences occur for specific months as in September in figures 2.4a, c, d, and e, and February in figures 2.4a and c. All five figures show the seasonal tendency of the 0.02 probsbility values to generally follow the trends in the 1- and 3-day data. A large exception for one month appears in the 0.02 probability peak in February in figure 2.4d. In additon to the maximum rainfall data, an index to moisture potential was considered for additional input to the seasonal variation problem. Potential moisture in the form of precipitable water associated with the maximum l2-hr persisting dew points was determined. The dew points were read from the analyses developed for the Southwest general storms (Schwarz and Hansen 1978) at mid-points of each subregion. These data have been entered on figures 2.4a to 2.4e in percent of maximum precipitable wster amount (dash triangle curve). All five subregions show late summer maxima (July or. August) with broad minimums through the winter months, extending into spring. Figures 2.4c and 2.4e, also show seasonal curves of 24-hr lOOO-mb (lOO-kPa) convergence PMP (alternate long-short dashes) taken from HMR No. 43 at the southern edge of the region of that report. Although HMR No. 43 covers only the months of October to June, the data were extended through the remaining months by simple extension of smoothed curves. Table 2.3 gives the smoothed values considered at these two locations. Table 2.3.--Seasonal variations of 1000-mb (lOO-kPa) convergence PMP for 24 hrs, from HMR No. 43 (U. S. Weather Bureau1966a). Location Jon Feb Mar Apr Hay Jun July Aug Sept Oct Nov Dec 420N 1180W in. 8.60 8.45 8.37 8.46 8.50 8.70 (8.93) (9.18) (9.30) 9.20 9.00 8.75 (Northwest Nevada) ... 218 215 213 215 216 221 ( 227) ( 233) ( 236) 234 229 222 420N 1120W in. 8.30 8.15 8.40 9.25 10.30 11.80 (12.72) (12.80) (11.70) 10.50 9.28 8.5S ('Northern Utah) ... 211 207 213 235 262 300 ( 323) ( 325) ( 297) 267 236 217 Values in parentheses estimated from interpolation. based on smooth seasonal distribution. 2.2.5 PMP Storm Prototypes Another consideration before we can develop mid-month convergence PMP maps is to determine what type(s) of stormes) is (are) likely to produce general- storm PMP in the Southwest, and the seasonal and regional variations of the general storm. An extensive review of the meteorology of Southwestern storms is presented, with examples, in the companion volume (Schwarz and Hansen 197B). Neverthe- less, brief comments are included here to establish the trend in storms that are considered representative of producing rainfall of PMP magnitude.