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<br />However, these results do not stand out clearly above the background variability <br />in precipitation. <br />4) During some hours on experimental days, blocking type flow prevented <br />tbe nucleant from the ground generator network from ascending over the carge~ <br />area (5.3% of the six-hour blocks). There vas total or partial blocking of <br />effective entrainment of nuclei in 12.6% of the six-hour blocks. Under these <br />conditions the nucleant was carried well to the west or northwest of the target <br />area. When the following day was a not-seeded experimental day, it was often <br />contaminated by nuclei returning from the west following the b~eakup of the <br />blocking flow regime (1.6% of the three-hour blocks). <br />5) Extra-area effects of seeding appear to have occurred over a broad area <br />lying up to 160 km from the generator sites. Much of this effect occurred as <br />expected, downwind of the target area, during within-boundS wind conditions. <br />Some of the effect occurred. not as expected, well to the west of the target area <br />as a result of the seeding under blocking flow conditions. Over the downvind <br />,rea (e.g.. San Lu~s Valley) apparent net increases in precipitation are small <br />in absolute value. No net effect appeared at the. abundant stations in the area <br />co the vest of the target instrumented as part of the original target ares. <br />6) Insufficient data were available in the stable cloud category to <br />establish empirically where the wa~ side l1mit (cloud top temperature) of the <br />positive effect seeding window lay~, In ,the unstable category the sounding <br />derived cloud top did not represent:,tbe -.true .top. the true tops developed <br />following the release of 1nstabili~y~~r:thecupwind'810pes of the barrier; <br />hence. no assessment of a warm side limit could be tDvestigated for this category. <br />7) Seeding during a frontal passage had no effect other than that accounted <br />for by the frontally associated distribution of stability and vind. <br />8) The- overall potential for precipitation enhancement was assessed in,this <br />evaluation study at around lOt. taking into account randomization bias. Thi~ <br />translates into an overall streamflow enhancement potential of around 19%. This <br />potential was not realized in the seeded 24-hour experimental days for several <br />reasons: a) the opportunIties lost with late starts in the fall season, b) sus- <br />pensions, c) missed forecasts of suitable days, d) losses in the acco~nting for <br />experimental day precitation ~hen strong blocking flow existed and seeding ~as <br />ineffective in the target area, e) excess accounting in not-seeded days when <br />contamination occurred 0:1 <1 not-se~deci experimental day following a seeded block- <br />ing flow day, f) losses when cold top stable orographic clOuds were seeded unde:- <br />strong wind - deep cloud condition$, and g) pOSsible moderate seeding-produced <br />losses on the upwind slopes \..'hen strong convection was seeded, <br /> <br />-22- <br /> <br />A-1l-6 <br />