WMOD00403

• Title: Special Regional Weather Modification Conference Augmentation of Winter Orographic Precipitation in the Western US
• Date: 11/11/1975
• Weather Modification - Doc Type: Report

Based upon the wind speed and directional profiles, atmospheric stability, terrain
influences, generator elevations and the cloud water content, a three dimensional
network of generators was selected for each experimental day. As additional mete-
orological data became available on de!clared seed days from the project rawinsonde
and the NWS circuits, the experimental day criteria were continually monitored and
any necessary adjustments were made to the generator network to insure effective ice
nuclei concentrations within the targe:t area. Generators were turned on one hour
prior to the start of the experimental period and shut off two hours prior to the end
of the period. This procedure was assumed to allow adequate transport and ventilation
times, respectively. In general, slightly fewer generators were used during the last
three operating seasons because a preliminary analysis of the seeding results of the
first two seasons by the Bureau of Rec:lamation suggested that oversee ding might have
occurred during that period. The gene:rator network by types, location and elevation
for the Pilot Project is presented in Figure 1.

A suspension appraisal of a potential experimental day was made each morning prior
to obtaining a random decision. This suspension appraisal was updated at other time
intervals during an experimental period frollI1 incoming Project data. In no case did
this time interval exceed twelve hours. Se1eding operations were suspended during
periods when any of the following cond.itions were exceeded or existed:

1) Above normal snowpack
2) Avalanche potential (16 IJiecemb,er through 31 January only)
3) Other hazards such as sev'ere wleather conditions.

Above normal snowpack was considered to be above 200 percent of the long term average
for the period 1 October to 1 February; from 200 percent to 175 percent of the long
term average for the period 1 February to 1 March; from 175 percent to 150 percent
for the period 1 March to 1 April and above 150 percent of the long term average
thereafter.

Avalanche suspension criteria consisted of three factors:

1) amount of snowpack above long-term average
2) maximum 24 hour forecast of natural precipitation within a 48-hour period
3) total precipitation accrued during the preceding 10 days

For the 1970-71 and 1971-72 seasons, depending on the suspension appraisal, seeding
was suspended for avalanche potential based on the following criteria:

1) There will be no suspension for avalanche when snowpack is less than 90
percent normal unless the Colorado State Highway Department advises that unusually
hazardous snow conditions exist.

2) Snowpack 91 to l~Q percent of normal, 24-hour maximum forecast precipi-
tation 2.0 inches or greater water content, and a preceding 10-day accumulation of
4.00 inches or greater water content.

3) Snowpack 121 to 150 percent of normal, 24-hour maximum forecast precipi-
tation, 1.0 inch or greater water content, and preceding 10-day precipitation accumu-
lation of 2.50 inches or greater water cont4:m t.

4) Snowpack 151 percent of normal or above, 24-hour maximum forecast preci-
pitation of 0.7 inch or greater, and a prec4~ding 10-day precipitation accumulation of
1.75 inches or greater water content. (NOtl~: When snowpack exceeds 200 percent of
normal February 1 to February 15, seeding w:ill be suspended due to above-normal snow-
pack criteria.)

Observations of avalanche occurrences during the 1971-72 season indicated that either
the 10-day total E.!:. the 24-hour maximum prec~ipitation amount was capable by itself of

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