WMOD00528

• Title: The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
• Date: 12/1/1991
• Weather Modification - Doc Type: Report

3.3 Analysis of Blowhard Mountain Weather Observations

It is infonnative to consider conventional weather observations from the Sevier River Drainage as they
indicate long-tenn averages and ranges of conditions. Moreover, as previously discussed, snowfall records
provide some indication of the frequency of seedable stonns.

While several weather stations exist at valley locations, infonnation from them has little relevance to
conditions high in the mountains where the seasonal snowpack accumulates each winter and melts into
streamflow each spring and summer. Weather observations are rare in the high altitude portions of the
Sevier Drainage because of difficulties of access. Fortunately, one climatological station has been
maintained next to a radar system on Blowhard Mountain in the Cedar Breaks region, located at the
headwaters in the extreme southwest end of the Sevier River Drainage. This station has pcoyided a fairly
long-tenn record of daily precipitation, snowfall, and temperature observa,tions which should be reasonably
representative of higher elevations in the drainage.

The data set was obtained from the National Oimatic Data Center in Asheville NC. Observations
consisted of 24 hour precipitation and snowfall totals, and daily maximum and minimum temperatures for
the Blowhard Mountain radar (BMR, 10,700 ft m.s.!.) for water years 1965 through 1981 and from 1986
through 1989, The years 1982 through 1985 were not included because of substantial missing data. This
is unfortunate in that 2 of these years were quite wet The months of October through May were included
to contain the entire snowfall season, even at high elevations.

3.3.1 Precipitation data. . As previously noted, precipitation does not relate directly to cloud
seed ability. However, over the daily period for which precipitation observations are available, the
existence of some precipitation implies the existence of some SLW. As discussed in section 3.2.1,larger
precipitation amounts imply larger fluxes of SLW. Because of this relationship, and also because
precipitation observations are available over a much longer period than radiometer measurements of SL W,
it is reasonable to use the snowfall data in estimating the long-tenn frequency of seed able <:onditions.

Figures 3-3 and 3-4 show the average monthly precipitation (melted snow water equivalent plus any
rainfall) and average monthly summations of daily snowfall amounts (depths) for BMR for the period of
record, The monthly averages are based on summations of the daily observations. It is seen that
precipitation gradually increases from October through March before beginning to decline. March has the
maximum precipitation with nearly 4.5 in. Snowfall depth has a corresponding March maximum of almost
45 in suggesting that essentially all March precipitation falls as snow (fresh snowfalls at mountain
locations have an average density of about 0.1 so. 1.0 in of snowfall represents approximately 0.1 in of
precipitation). Most precipitation is in the fonn of snow for all months shown except Octob~r and May.
A closer comparison of these two graphs indicates that the ratio of snowfall depth to water equivalent
decreases to about 13 to 1 in December and January as ambient temperatures drop and returns to about
10 to 1 in February, March, and April.

Figure 3-5 shows the average number of days per winter reporting over 0.1 in (2.5 mm) of precipitation.
As discussed in section 3.2.1, stonns with lesser precipitation amounts generally had little SLW flux so
days with less than 0,1 in are not included in any of the following analyses. The frequency of
precipitation days per month is rather consistent, averaging between 6 and 8 days except for March which
averages between 9 and 10 days. Since stonn duration varies from several hours up to a few days (table
3.1) daily precipitation measurements only approximate stonn frequency. Most stonns are less than one
day in duration although a stonn may continue across the daily observation time for pn~cipitation.
Consequently, stonn frequency would be expected to be somewhat less than the frequency of precipitation

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