WMOD00277

• Contract/Permit #: 14-06-D-6467
• Title: An Operational Adaptation Program for the Colorado River Basin
• Prepared By: Lewis O. Grant, Chappell, Crow, Mielke Jr., Rasmussen, Shobe, Stockwell, Wykstra
• Date: 10/1/1969
• State: CO
• Country: United States
• Weather Modification - Doc Type: Report

increased daily snowfall under these conditi~ons.
A decrease of daily snowfall to below the overall
average is indicated for the temperature class from
from -16.50 C to -20.50 C, as well as for all
colder temperature classes.

(2) Daily snowfall related to upper level airflow
Figure 41 shows the distribution of
snowfall and occurrences at Climax as a function
of the 700 mb wind directio_n. The distributions

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QUENCY OF CCCURREf\CE

~ENil\GE OF TOTAL
-, SNOWFALL

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700MB WIND DfRECTKlN (degrees)

Figure 41. --(a) Mean Daily Snowfall at Climax as a
function of the 700 mb wind direction. (b) Distribu-
tionS of total snowfall and total occurrences as :'l
function of the 700 mb wind direction computed over
20 degree class intervals.

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FREQUENCY OF OCCURRENCE

PERCENTAGE OF TOTAL
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5 10 15 20 25
700MB WIND SPEED (MPSI

Figure 42. --(a) Mean Daily Snowfall at Climax as a
function of the 700 mb wind speed. (b) Distributions
of total snowfall and total occurrences as a function
of the 700 mb wind speed computed over 4 mps
class intervals.

are determined by utilizing 20 degree class
intervals for the wind direction.

The greatest frequency of daily snowfall
and the main contribution to total snowfall occurs
in the 280-290 degree class interval. It is
quite evident that snowfall is more frequent and
heavier with northwest flow at Climax. About
500/0 of the total snow occurs with the 700 mb wind
between 280 and 330 degrees while only about
120/0 of the total snow occurs with the flow from
180 to 250 degrees, The lack of snowfall with
southwest flow is apparently due to the presence
of two major mountain massifs to the southwest
of Climax. These are the San Juan Mountains
and the Sawatch Range. The greater frequency
and larger snowfalls with northwest flow at
Climax attest to the pronounced orographic
influence on wintertime precipitation since this
type of flow exists after the upper trough has
passed the area, and the stronger synoptic scale
upward motions are well to the east.

Figure 42 shows the distribution of
natural snowfall and occurrences at Climax as a
function of the 700 mb wind speed computed using
4 mps class intervals. The relative percentage
of the total snowfall is higher than the relative
frequency for the higher wind speeds suggesting
again an orographic influence on the precipitation.
A slight increase of the mean daily snowfall with
wind speed above 6 mps is noted for the Climax
area.

Figure 43 shows the distribution of
natural snowfall at Wolf Creek Summit during the
two unseeded winter seasons as a function of the
700 mb wind directlon. The distributions are
determined by utilizing 20 degree class intervals
for the wind direction. The frequency of occur-
rence of daily snowfall is also included. It is
readily apparent that the occurrences of snowfall
are distributed nearly symmetrical about the
prevailing westerlies, indicating that the migrat-
ing disturbances in the mean westerly flow play
an important part in ,determining the occurrence
frequency. Snowfall amounts, on the other hand,
are highly skewed toward southwest flow, a
direction normal to the mountain barrier. This
indicates that the mean daily snowfall is a
maximum under southwest flow conditions where
favorable orographic effects are superimposed
upon the stronger synoptic scale upward motions.

Wind flow at 700 mb from 1800 to
2700 accounts for 81 % of the total snowfall on
Wolf Creek Summit but represents only 530/0 of
the total occurrenceS. Wind flow from 2800
to 3500 represents 370/0 of the total occurrences
but contributes only 160/0 of the total snowfall.

The influence of the orography upon
the mean daily snowfall is striking. Northwest
and northerly flow snowfall generally averages
around O. 10 inch per day (water equivalent) or
less, while south and southwesterly flow snowfall
averages in excess of 0.50 inch per day (water
equivalent). Snowfall occurring with the 700 mb
wind flow within 250 to a normal to the mountain

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