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 7. Winter precipitation (November - March)
Synoptic Percent of Days % of Total Winter Precipitation
Type Type Occurs a) Durango b) Silverton
20 (B) 8 5.8 6.9
110 (Bn_a) 9 4.5 4.8
111 (Bn_b) 9 3.9 4.9
120 (A) 15 22.7 20.2
210 (F IS) 10 11.6 9.6
211 1 1.5 1.7
220 (Bn_c) 7 7.5 7.9
230 (D) 4 4.1 4.2
240 (Ej) 0.8 0.6
320 (Em) 10 16.6 15.0
321 (Eh) 2 3.4 4.0
330 (EL) 11 10.9 12.1
331 (En) 8 4.7 5.8
340 (Bs) 3 1.4 2.1
341 (Bs_b) 2 0.6 0.4
TOTAL 100 100 100
Main types responsible for precipitation in winter months:
120 (A) 62% of precipitation at Durango
210 (F/CL)
320 (E ) 57% of precipitation at Silverton
330 (Em)
L
These four types occurred on 47% of winter days from 1928 to 1958

develops off the west coast of the U.S. with cold air
moving south along the Pacific coast and then south-
east across the Great Basin. Type F/CL occurs when
a high pressure cell over the Great Basin deflects
depression tracks both north and south causing heavy
precipitation along both axes of movement (see
Elliott 1949, for more complete discussion of the
synoptic types).

-Spring (April, May): Approximately 69 percent of
precipitation at the two stations resulted from four
synoptic situations; these synoptic types occurred
on 58 percent of spring days (Table 8). Two of the
types were also important for winter precipitation
(Types A and Em)' Types Bn_ and B are similar to
the Flc types noted above wIth an anticyclone
dominant over the Great Basin forcing storm tracks
both north and south. In type Bn_c situations a low
pressure center may develop in Colorado resulting in
locally severe weather.

I
'f

-Summer (June, July, August): Summer months were
dominated by three major types, B, Bn_a' and A
(Table 9). These occurred on 70 percent of summer
days resulting in 72-73 percent of precipitation.
Type frequency is closely related to percentage of
precipitation during summer months. Precipitation
may be frontal or convectional during these situ-
ations, though the latter is of most importance.

-Fall (September, October): Four types accounted
for approximately 68 percent of precipitation
during fall months, though they occurred on 60 per-
cent of days (Table 10). Type Bn_b was important

in fall months and is characterized by cold air
moving southward from Canada around a Great Basin
anticyclone. Elliott (1949) notes that these types
frequently represent the first cold waves of the
winter season.

It is clear from an examination of these synoptic
types that certain types are of primary importance
in all seasons, notably types A and B. However, it
is interesting that winter season precipitation
results from types of little importance at other
times of the year. Whereas in winter, types F/CL,
E and EL accounted for almost 40 percent of pre-
cTpitation, they contribute less than 5 percent of
summer precipitation. Presumably this reflects the
great contrasts between the dominant cyclonic
situations of winter and the much more local con-
vectional activity of summer months.

Changing frequencies of the dominant types were
examined in relation to seasonal precipitation totals.
Figure 30 shows the frequency of the main winter
types from 1928-9 to 1957-8 compared with winter
precipitation at Durango over the same period. Be-
low normal frequencies of these types concurrent
with the dry period of the early 1930's are illus-
trated. However, other years do not show such a
clear relationship; in particular, above average
frequencies of the synoptic types in the 1940's are
not paralleled by above normal precipitation at
Durango (similarly at Silverton). For the 30 year
period, the correlation coefficient (r) is 0.33 and
is not significant at the 5 percent level. Figure
31 shows the frequency of synoptic types dominant

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