OVERVIEW
Winter Precipitation Variability in Three Western US Mountain Ranges
Winter precipitation accumulations are shown for three western US mountain ranges in localized regions of the
Cascades (NOC), Sierra Nevada (NSN), and Rockies (NCR), for the period 1937-2000.
Winter precipitation at C1 and D1
Potential impacts of Climatic Change
Precipitation is compared by elevation for both the east (leeward) and west (windward) slopes in each region.
Similarly, two sites in the Colorado Rockies, separated by only six kilometers, are examined to show
a much finer spatial scale.
Winter Precipitation (mm)
Mark Losleben*, Nicholas Pepin+, Kurt Chowanski*
* Mountain Research Station, University of Colorado
+Dept of Geography, University of Portsmouth, UK
Very poor coherence was found between the precipitation amounts at lower vs. higher elevations regardless
of slope aspect on both the inter-mountain and intra-mountain range scales.
The greatest variability was found at the Cascade site for snowpack, and at the Sierra Nevada site for
lowland precipitation. In contrast, the Rockies site showed the least variability at both elevations.
Winter Precipitation Variability in Time, Space, and by Elevation in Three Mountain Regions
April 1 Conditions on Western Slopes
Study Sites
Lowlands
LWP
Highlands
SWE
25
900
700
25
600
20
700
500
600
10
SWE (mm)
500
400
LWP300
(mm)
200
5
15
LWP (mm)
LWP (mm)
15
SWE (mm)
Northern Colorado Rockies
NCR
Lowlands
LWP
800
20
C1 Ridge top site in the NCR at 3030 meters
D1 Ridge top site in the NCR at 3750 meters
Highlands
SWE
SWE (mm)
NOC The localized region in the Northern Oregon Cascades.
NSN Similarly, the Northern Sierra Nevada local.
NCR Similarly, the Northern Colorado Rockies local.
April 1 Conditions on Eastern Slopes
10
1950
1960
April1SWE
1970
1980
MA9
1990
0
1940
2000
1950
1960
1970
PPT
Linear (April1SWE)
1980
MA9
y = -0.0085x + 32.065
R2 = 0.0023
1990
0
1940
2000
Winter precipitation at C1(red) and D1 (blue) is diverging with time.
Less than half the common variance is explained (r=0.41)
100
1950
1960
April1SWE
Linear (PPT)
300
LWP200
(mm)
SWE (mm)
5
100
0
1940
400
y = 0.5885x - 634.18
R2 = 0.0077
1970
1980
MA9
1990
0
1940
2000
1950
PPT
Linear (April1SWE)
1960
1970
MA9
1980
1990
2000
Linear (PPT)
Ranking of winter precipitation at C1 and D1 by year.
y = 1.1828x - 2025.9
R2 = 0.0408
y = -0.1279x + 265.94
R2 = 0.2673
Definitions
50
30
SWE (mm)
20
200
100
0
1940
10
0
1940
1950
1960
April1SWE
1970
1980
MA9
1990
2000
1950
Overall high snowpack is relatively rare, but widespread low snowpack is more common, and often the result of a blocking high.
1940
2000
1800
1600
1400
1200
1000
800
LWP 600
(mm)
400
200
0
1940
1950
April1SWE
1960
1970
1980
MA9
1990
2000
Linear (April1SWE)
B
1970
1980
MA9
1990
0
1940
2000
1950
1960
April1SWE
Linear (PPT)
1970
1980
MA9
1990
50
20
0
1940
1950
1960
1970
1980
1990
2000
2000
PPT
Linear (April1SWE)
MA9
Linear (PPT)
y = 0.0639x - 76.289
R2 = 0.003
y = -0.0533x + 131.67
R2 = 0.0057
40
300
50
45
40
250
200
35
30
25
20
SWE15(mm)
SWE (mm)
1950
PPT
1960
1970
1980
MA9
1990
2000
Linear (PPT)
10
5
0
1940
150
100
LWP (mm)
50
0
1940
1950
April1SWE
y = -0.149x + 316.4
2
R = 0.0593
A
1960
LWP (mm)
Northern Oregon Cascades
NOC
50
45
40
35
30
25
20
SWE15(mm)
10
5
0
60
40
LWP (mm)
y = -0.0097x + 482.29
R2 = 1E-06
SWE (mm)
Widespread highest (A) and lowest (B,C,D) snowpack.
40
10
y = 0.0327x - 25.777
R2 = 0.0018
700 mb Anomaly Maps
80
50
30
SWE (mm)
20
PPT
Linear (April1SWE)
100
60
500
400
LWP 300
(mm)
40
70
60
Rank (1=driest)
LWP (mm)
Northern Sierra Nevada
NSN
SWE (mm)
60
120
LWP (mm)
SWE Snow water equivalent in the high elevations
LWP Lowland cumulative winter precipitation
80
LWP (mm)
70
1000
900
800
700
600
SWE (mm)
80
y = 4.2561x - 7424.5
R2 = 0.083
1960
1970
MA9
1980
1990
1950
PPT
2000
Linear (April1SWE)
1960
1970
1980
MA9
1990
2000
Linear (PPT)
20
y = 0.4708x - 771.28
R2 = 0.0311
y = -0.1125x + 234.72
R2 = 0.0797
D
C
30
10
Lowland precipitation (LWP) and highland snowpack (SWE) trends often differ, even compensating for slope aspect. Increasing lowland precipitation trends are
generally not likely to offset decreasing snowpack as a water supply source because of snowpacks relatively slow release over time. The NOC is particularly
vulnerable in this regard.
Minimum
0.28
0.33 / 0.23
361
343 / 378
+ 54 %
NCR
- 50 %
Mean
East / West
0.44
0.46 / 0.43
798
665 / 930
+ 110 %
- 68 %
NOC
Mean
East / West
0.64
0.62 / 0.66
425
328 / 523
+ 111 %
- 86 %
CV
LWP
Median
LWP (mm)
0.18
0.37 / 0.21
407
280 / 505
NCR
NSN
NOC
Maximum
Minimum
Mean
East / West
+ 87 %
- 32 %
Mean
East / West
0.39
0.42 / 0.40
227
48 / 406
NCR
+ 123 %
- 65 %
NSN
NOC
Mean
East / West
0.25
0.30 / 0.25
581
160 / 965
+ 67 %
- 60 %
Maximum
E/W SWE (mm)
Minimum
Mean SWE (mm)
Minimum
E/W SWE (mm)
627 / 556
[1943 / 1965]
180
152 / 208
[1981]
148 / 208
[1977 / 1981]
1511 / 2000
[1952 / 1983]
259
220 / 297
[1988]
220 / 297
[1988 / 1988]
2 / 114
[1963 / 1981]
Minimum
E/W LWP (mm)
Mean
East / West
Year
556
627 / 485
[1943]
Mean
East / West
Year
1676
1353 / 2000
[1983]
Mean
East / West
Year
897
665 / 1128
[1956]
711 / 1804
[1952 / 1950]
58
2 / 116
[1963]
Maximum
Mean LWP (mm)
Maximum
E/W LWP (mm)
Minimum
Mean LWP (mm)
602 / 777
[1984 / 1993]
277
152 / 402
[1966]
142 / 368
[1967 / 1964]
104 / 926
[1969 / 1995]
80
41 / 118
[1995]
12 / 118
[1950 / 1976]
260 / 1725
[1997 / 1974]
236
42 / 430
[1977]
42 / 43
[1977 / 1977]
Site
NOC
October to March
(1967, 1982, 1956, 1952, 1986, 1958, 1969, 1973, 1983, 1995)
- (1976, 1977, 1972, 1990, 1957, 1994, 1950, 1960, 1955)
2004
Maximum
Maximum
Mean SWE (mm)
B
October to March
(1983, 1969, 1993, 1971, 1956, 1982, 1978, 1967, 1975, 1962)
- (1988, 1977, 1976, 1992, 1991, 1990, 1959, 1987, 1961, 1981)
2001
Mean
East / West
Site
October to March
(1965, 1986, 1957, 1987, 1949, 1993)
- (1966, 1950, 1972, 1967, 1954, 1964, 1968, 1969)
1998
Median
SWE (mm)
Site
C
October to March
(1952, 1951, 1965, 1996, 1957, 1956, 1970)
- (1961, 1977, 1966, 1964, 1969, 1989, 1991, 1973)
1995
CV
SWE
NSN
C
B
1992
Site
NCR
October to March
(1995, 1972, 1965, 1971, 1961, 1982, 1983, 1996, 1997, 1999)
- (1977, 1994, 1992, 1955, 1957, 1973, 1979, 1960, 1968, 1962)
1989
Spatially, SWE and LWP variability are different. NOC has the most variable SWE; NSN the most
variable LWP. NCR is the least variable in both SWE and LWP
NSN
October to March
(1956, 1950, 1952, 1949, 1974, 1999, 1971, 1951, 1969, 1975)
- (1963, 1981, 1992, 1966, 1978, 1973, 1977, 1994, 1991, 1996)
1986
April 1 conditions for the three mountain range sites for the full study period, 1937-2000 as a whole
(Mean) and by slope (East / West). The coefficient of variation (CV=SD/Mean), Median, and Departure
from Median ((Maximum-Median)/2 or (Minimum-Median)/2) are given.
April 1 Snowpack (SWE) and Lowland Cumulative Winter Precipitation (LWP) for the three mountain
range sites for the full study period, 1937-2000. The maximum and minimum SWE and LWP amounts
are given (mm) for the sites as a whole and by east (E) and west (W) slopes. The year of occurrence is
given in [brackets] beneath the amounts.
The highest SWE occurs at the NSN site, but the NOC site has the highest LWP.
The decreasing trend in NOC snowpack, and a minimum extreme of 2 mm SWE in 1963, could mean no snowpack in
some years to come on the east slope of the Cascade site!
A
A
1983
Shown are the top minus the bottom quartile years.
1979
SWE
Shown are the top minus the bottom quartile years.
1974
VARIABILITY
Extremes in lowland winter precipitation also show strong
700 mb signatures, but they are different compared to snowpack
extremes, particularly at the Cascade (A) and Rockies (C) sites.
Ex.: High LWP at the Cascades site occurs during a southwest flow.
C1 and D1 rankings are very different in most winters
EXTREMES
LWP
Highest snowpack years in the three regions have strong 700 mb
signatures in the Cascades (A), and Sierra Nevada (B) sites,
but not in the Rockies site (C).
Ex.: High snowpack at the Cascade site occurs during a northwest flow.
1971
October to March 1991
October to March 1981
1968
October to March 1977
October to March 1956
1965
C1 rank=blue
D1 rank=maroon
1962
1959
1956
1953
0
Mean
East / West
Year
552
327 / 777
[1993]
Mean
East / West
Year
507
88 / 926
[1995]
Mean
East / West
Year
970
215 / 1725
[1974]
RESULTS
Variability of winter precipitation in mountain areas is significantly different at multiple spatial scales. Both high and
low
elevation precipitation extremes can be explained by different synoptic scale atmospheric circulation patterns.
Widespread high snowpack years are rare and associated with a Pacific Northwest low. Widespread low snowpack
years are more frequent and characterized by some version of a blocking high pressure pattern.
The NCR winter precipitation is the least variable, at high and low elevations.
The NOC snowpack, and the NSN lowland precipitation is most variable.
Even at the fine spatial scale of six km, diverging trends were found in winter precipitation at the Rockies site.
CONCLUSIONS
Lowland winter precipitation would be a poor indicator of adjoining upland snowpack conditions,
as there is little correlation between the two on either the windward or leeward slopes of the three
mountain ranges in this study; the Oregon Cascades, Sierra Nevada, and Colorado Rockies.
This is also true at the finer spatial scale of six kilometers at two sites on the east slope of the Rockies.
The snowpack at the Cascades site may be most sensitive to future climatic change, while that at the
Rockies site may be the least.
Differences in sensitivity are rooted in atmospheric circulation, driven by closeness of current winter
temperatures to the freezing level, and reflect nearness to moisture sources.