WMOD00593

• Contract/Permit #: 14-06-D-7052
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains, Colorado
• Date: 3/1/1976
• State: CO
• Weather Modification - Doc Type: Report

Table 10. Fall precipitation (September, October)

% of Total Fall Precipitation
a) Durango b) Silver ton

Synoptic Percent of Days
Type Type Occurs
20 18
110 11
111 11
120 19
210 8
220 12
230 7
320 5
330 4
331 2
341 3
TOTAL 100

Main types responsible for precipitation in fall months:
20 (B) )
111 (Bn_b) ) 67% of precipitation at Durango
) 69%
120 (A) ) of precipitation at Silverton
220 (B ) )
n-c )
These four types occurred on 60% of days from 1928 to 1958.

15.8
10.4
10.7
26.5
6.6
13.6
8.9
5.6
0.3
0.3
1.3

17.3
9.4
13.4
26.8
5.2
11.6
8.5
4.9
0.7
0.8
1.4

100

100

10

:, ,~
/\ :\ :\ I
f \ !\ ~ ~ 1\ ,/ 8
" \\iN+j-V~:/)d-\I--';j

o

100

~80
070
'0
060
z

50

-j(

40

1928- 30- 32- 34- 36- 38-40- 42- 44- 46- 48- 50- 52- 54- 56- 1958-
29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59

Figure 30. Winter Frequency of Synoptic Types A,
F/CL, Eu & EL (lower line) and Winter
Precipi~ation at Durango (upper line).

Temperature

Temperature differences between types are generally
small and standard deviations large, making gen-
eralizations difficult. In winter months, types E
and ~ (important for bringing precipitation to th~
area) are warmer than the other main types A and
~/CL; minimum temperatures in particular are higher
1n Ehe former cases, resulting in higher mean daily
temperatures. During summer months temperature
differences between types are small but in general,
minimum temperatures during the rainy types are
lower than during other synoptic situations.

In conclusion, the synoptic-type classification has
been shown to have some value in indicating those
synoptic situations important in bringing precipi-
tation to the San Juan area. However, its use in
explaining observed fluctuations in precipitation on
an individual season basis has not been demonstrated'
more detailed studies of types occurring during each'

8} /" ^
,,^.... ' \
~ 6 / \ / \ / \ " ", t'\
~ I \ I \ /' \ I , :'
u ---:.j.--\--7'---~---""<;;;,--;or---J-- --'~ --~--X
-= 4 \...- \,", / \,' \:
, I "\ I \ I
'\ : \,' \!
2 ,,'-_J V

80
70

en
'"
g 60

050
o
z
40

30
1928 30 32 34 36 38 40 42 44 46 48 50 52 1954

Figure 31. Summer Frequency of Synoptic Types B,
Bn_a' and A (lower line) and Summer
Precipitation at Durango (inches).

season are necessary to clarify the details of year-
to-year variation. Temperature variations between
types are slight, making a synoptic interpretation of
observed temperature fluctuations difficult. Further
analysis of the synoptic types in relation to cloud
seeding days is discussed in the next section.

Synoptic types and cloud seeding events, 1970-72

In the two winter seasons (1970-71 & 1971-72) that
cloud seeding was conducted during this study, a
total of 59 days were determined to be "seedable."
The synoptic type of each "seedable" day has been
noted for the first two winters' seeding operations,

63