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

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inducing avalanche occurrences. Consequently, after the 1971-72 season, several
changes were made in the avalanche criteria. The primary changes were replacing
the predetermined 24-hour period of 1100 MST to 1100 MST with any consecutive
24-hour maximum precipitation forecast and to consider either the 10-day total or
the 24-hour maximum amount as individual rather than jointly occurring criterion.

Over the five operating seasons there were ten suspension requests that were granted
by the Bureau of Reclamation. Of the possible 910 operational days during the fi.ve
winter season, 117 days or 12.7 percent were lost due to conditions meeting the
suspension criteria.

OPPORTUNITY RECOGNITION AND FORECAST VERIFICATION

A full scale meteorological analysis and forecast center was located in the EG&G
Durango, Colorado central facility. The project meteorological network and the
National Weather Service teletype and facsimile circuits provide mesoscale and
synoptic scale input data for operational decision making and monitoring of field
operations. A time-share computer terminal was available with access to Bureau of
Reclamation programs and data along with National Weather Service data and analysis.

The recognition of experimental meteorological conditions over the Target area and
declaration of an experimental period were generally accomplished by 0900 MST using
the 0500 MST national Weather Service data. Occasionally, when possible experimental
conditions were forecast to develop late in the 24-hour period, the final declaration
of an experimental day was delayed until after the 1700 MST National Weather Service
data had been analyzed.

During the five operating seasons 147 experimental days (71 seed, 76 no-seed) were
declared, including two emergency decisions (flip of a coin) when the computer system
failed. A total of 148 experimental days were observed during the five seasons which
met the experimental criteria in effect at that time and when operations were not
suspended. Out of a total of 910 forecast for the occurrance of experimental con-
ditions, 849 days or 93.3 percent were correct. The percentage increased to 95.2
precent for the last four seasons when the precipitation verification criterion was
made more specific. Of the 147 experimental days that were forecast, 117 days or
79.6 percent verified as occurring. Again for the last four seasons this percentage
was higher at 86.7 because of the improved verification criterion.

To give the reader some feel for the precipitation distribution during the duration
of the Pilot Project operations, the precipitation at a site on Wolf Creek Pass~
located four miles west of the summit, was analyzed for the 910 possible operational
days. A total of 144.44 inches of precipitation was recorded during the 910 days over
the five seasons. During days that were declared non-experimental, 54.65 inches of
precipitation were recorded. During suspended operational days, 31.03 inches of pre-
cipitation were recorded. A total of 58.76 inches fell on experimental days of which
31.98 occurred on no-seed days and 26.78 inches occurred on seed days. Of the 26.78
inches that fell on seed days approximately 21.50 inches occurred while the cloud
nuclei generators were operating or within two hours after they had been turned off.

Sl~Y OF EQUIPMENT OPERATIONS

The central station computer and data telemetry system operated during the 910 days .
of the five operational seasons with approximately 10 days of downtime. The telemetry
system relayed data from the remote meteorological sites and provided monitoring and
control capabilities of the remote generator sites. From 17 to 20 remote sites were

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