WMOD00258

• Applicant: Western Weather Consultants
• Project Name: Vail & Beaver Creek
• Date: 11/1/1987
• Weather Modification - Doc Type: Application

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Hovevec. these results do not stand out clearly above the background variability
in precipitation.
4) During SOme hours on experimental days, blocking type flow prevented
the nucleant from the ground generator network from ascending over the target
area (5.3% of the six-hour blocks). There waS total or partial blocking of
effective entrainment of nuclei in 12.6% of the six-hour blocks. Under these

conditions the nucleant was carried well to the west or northwest of the target
area. When the following day was a not-seeded experimental day, it was often
contaminated by nuclei returning from the west following the breakup of the
blocking flow regioe (7.6% of the three-hour blocks).
5) Extra-area effects of seeding appear to have occurred over a broad area
lying up to 160 km from the generator sites. Much of this effect occurred as
expected, downwind of the target area. during within-bounds Wind conditions.
Some of the effect occurred. not as expected, well to the west of the target area
as a result of the seeding under blocking flow conditions. Over the downwind
~rea (e.g., San Lu~s Valley) apparent net increases in preeipitation are small
in absolute Value. No net effect appeared at the abundant stations in the area
to the west of the target instrumented as part of the original target area.
6) Insufficient data were available in the stable cloud categorY to
establish empirically where the warm side limit (cloud top temperature) of the
positive effect seeding vindow lay. In the unstable category the scunding
derived cloud top did not represent' the true tcp. The true tops developed
following the release of instability over the upwind ;lopes of the barrier;
hence, no assessment of a warm side limit could be investigated for this category.
7) Seeding during a frontal passage had no effect other than that accounted
for by the frontally associated distribution of stability and wind.
8) The overall potential for precipitation enhancement was assessed. in this
evaluation study at around'lO%, taking into account ranrlomi&ation bias. this
translates into an overall streamflow enhanceQent potential of around 19~. This
potential ~as not ~ealized in the seeded 24-hour experimental days for several
reasons: a) the opportunities lost with late starts in the fall season, b) sus-
pensions, c} missed forecasts of suitable days, d) losses in the accounting for
experimental day precitation ~hen strong blocking flow existed and seeding ~as
ineffective in the target ~rea, e) excess accounting in not-seeded days ~hen
contamination occurred .0:1 " not-$e~ded expt::ri!:".ental dOl}' follo1o.'ing a seeded block-
ing flov day, f) losses when cold top st~ble orographic clouds ~ere seeded unde~
strong ~lnd - deep cloud condition~, an~ g) possible moderate seeding-produced
losses on the up....inJ slopes \"'hen strong con....ection \"'as seeded.

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