Climatology of the Interior

Columbia River Basin

United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

United States

Department of the

Interior

Bureau of Land

Management

General Technical

Report

PNW-GTR-445

March 1999

Sue A. Ferguson
Author

SUE A. FERGUSON is an atmospheric scientist, Forestry Sciences Laboratory, 4043

Roosevelt Way NE, Seattle, WA 98105.
Climatology of the Interior

Columbia River Basin

Sue A. Ferguson

Interior Columbia Basin Ecosystem

Management Project: Scientific Assessment

Thomas M. Quigley, Editor

United States Department of Agriculture

Forest Service

Pacific Northwest Research Station

Portland, Oregon

General Technical Report PNW-GTR-445

March 1999
Abstract

Ferguson, Sue A. 1999. Climatology of the interior Columbia River basin. Gen. Tech.

Rep. PNW-GTR-445. Portland, OR: U.S. Department of Agriculture, Forest Service,

Pacific Northwest Research Station. 31 p. (Quigley, Thomas M., ed.; Interior Columbia

Basin Ecosystem Management Project: scientific assessment).

This work describes climate means and trends in each of three major ecological zones and

13 ecological reporting units in the interior Columbia River basin. Widely differing climates help define each major zone and reporting unit, the pattern of which is controlled

by three competing air masses: marine, continental, and arctic. Paleoclimatic evidence and

historical weather records show that the region has undergone significant fluctuations in

temperature and precipitation as air masses alternate dominance over the basin. The major

change in climate occurred near the time of western settlement with the end of the “Little

Ice Age.” Since then there have been numerous annual cycles in climate that may be

related to the Pacific Decadal Oscillation. During the last 50 years, winter precipitation

has decreased slightly and summer precipitation has increased throughout most of the

basin. At the same time, winter temperatures have increased and summer temperatures

have slightly decreased. Some impacts of changes in climatic means and trends on ecological conditions in the basin are discussed.

Keywords: Climate, Columbia River basin, climatology, climate variability, temperature,

precipitation, snowfall.
Interior Columbia

Basin Ecosystem

Management Project

United States United States

Department of Department

Agriculture of the Interior

Forest Service Bureau of Land

Management

Preface

The Interior Columbia Basin Ecosystem Management Project was initiated by the Forest

Service and the Bureau of Land Management to respond to several critical issues including,

but not limited to, forest and rangeland health, anadromous fish concerns, terrestrial

species viability concerns, and the recent decline in traditional commodity flows. The

charter given to the project was to develop a scientifically sound, ecosystem-based strategy

for managing the lands of the interior Columbia River basin administered by the Forest

Service and the Bureau of Land Management. The Science Integration Team was organized

to develop a framework for ecosystem management, an assessment of the socioeconomic

and biophysical systems in the basin, and an evaluation of alternative management strategies. This paper is one in a series of papers developed as background material for the framework, assessment, or evaluation of alternatives. It provides more detail than was possible to

disclose directly in the primary documents.

The Science Integration Team, although organized functionally, worked hard at integrating

the approaches, analyses, and conclusions. It is the collective effort of team members that

provides depth and understanding to the work of the project. The Science Integration

Team leadership included deputy team leaders Russel Graham and Sylvia Arbelbide; landscape ecology–Wendel Hann, Paul Hessburg, and Mark Jensen; aquatic–Jim Sedell, Kris

Lee, Danny Lee, Jack Williams, and Lynn Decker; economic–Richard Haynes, Amy

Horne, and Nick Reyna; social science–Jim Burchfield, Steve McCool, and Jon Bumstead;

terrestrial–Bruce Marcot, Kurt Nelson, John Lehmkuhl, Richard Holthausen, and Randy

Hickenbottom; spatial analysis–Becky Gravenmier, John Steffenson, and Andy Wilson.
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Introduction

The Columbia River basin in the northwestern

United States is in a transition-type climate

zone. It is influenced by three distinct air

masses: (1) moist, marine air from the west that

moderates seasonal temperatures; (2) continental

air from the east and south, which is dry and

cold in winter and hot with convective precipitation and lightning in summer; and (3) dry,

arctic air from the north that brings cold air to

the basin in winter and helps to cool the basin

in summer.

The timing and extent of competing air masses

is controlled mainly by synoptic weather patterns

and local terrain features that vary across the

basin. Prolonged periods of drought occur

when storms off the Pacific Ocean are deflected

around the region, thereby preventing the

intrusion of moist, marine air. At such times,

dry, continental conditions prevail. Damaging

frosts and freezing conditions commonly occur

when arctic air invades the basin either in autumn

before winter hardening or after budbreak in

spring. Cold damage also may occur in winter if

a warm, marine intrusion is followed by a sweep

of arctic air. In addition, the unique interplay

between air mass types results in dramatic

changes during transitions. The most unique of

these is rain-on-snow flooding that occurs when

warm, wet marine air displaces cold, arctic

conditions in winter. Lightning and gusty

winds also occur during transitions between

continental and marine air masses, mainly in

spring and summer.

This paper summarizes the background of

climatological studies in the basin and then

describes analyses of available data for a detailed

description of regional climate patterns.

Emphasis is on aspects of climate that influence

ecological processes because the major impetus

for this work came from the Interior Columbia

Basin Ecosystem Management Project

(ICBEMP).

Previous Climate Studies

Every decade new normals (30-year averages)

from thousands of weather observation stations

throughout the United States are computed as

part of an international effort led by the World

Meteorological Organization. These normals

are summarized in several documents available

from the U.S. Department of Commerce,

National Climatic Data Center (NCDC), in

Asheville, North Carolina. The summaries list

normals, means, and extremes for National

Weather Service first-order and cooperative

observation stations that have long-term

weather records. Also, they include a narrative

describing important features of distinct climate

zones and summaries of climate divisions

within each state. In addition to these official

summaries, others have summarized various

aspects of weather and climate in the Columbia

River basin and the Western United States (for

example, Pacific Northwest River Basins Commission 1968, 1969a, 1969b; Phillips 1962;

Rumney 1968).

An interesting analysis of temperature shows

two major frontal zones in the Columbia River

basin (Mitchell 1976). A Pacific air mass

boundary, which dominates during summer,

extends diagonally across the basin from northwest California to northwestern Montana.

Relatively moist, marine air exists north of the

boundary, and drier, continental air is common

south of the boundary. Mitchell notes that the

Pacific boundary coincides with the eastward

extension of coastal vegetation found in northern

Idaho and northwestern Montana. During

summer the polar jet stream is usually too far

north to bring many storms into the basin,

except north of the boundary. The thermal

trough and intermittent marine pushes continue

to play a role, but the westerlies are much
2

weaker than in winter. Strong convection in

southeastern Idaho and western Montana continue through the summer but lack moisture

because the prevailing northwesterly flow aloft

is relatively dry. Only when the southwestern

monsoon extends northward does the basin

experience any appreciable summer precipitation.

Mitchell (1976) describes a westerlies-anticyclone

boundary, which dominates during the winter,

stretching east-west along the Oregon-Nevada

border and across northern Utah. It marks the

boundary between a region of prevailing westerly

winds in the north and southerly winds in the

south, which is caused by circulation around a

persistent center of high pressure (anticyclone)

over southern Nevada. This frontal zone also

coincides with the northern or southern extent

of several tree species.

In the transition period between winter and

summer, a thermal trough migrates north from

California. When combined with a marine

push, convective precipitation and thunderstorms are possible, especially in Oregon and

southern Idaho. In Washington, northern

Idaho, and northern Montana, Pacific storms

remain influential during spring. In all seasons,

marine air flows through the Columbia Gorge

into the Snake River valley and other valleys

adjacent to and oriented toward the oncoming

Pacific air mass.

Bryson and Hare (1974) note that precipitation

variability in the Western United States is

caused by numerous small-scale climatic controls

(primarily the result of complex topography)

embedded within large-scale climatic controls.

Mock (1996) was able to determine the hierarchy

of climatic controls that operate at different

spatial scales by analyzing month-to-month

precipitation patterns. He found that while

large-scale climatic controls (such as the polar jet

stream, Pacific subtropical high, and subtropical

ridge) play important roles in precipitation

variability, small-scale climatic controls (such as

complex topography, thermal troughs, confined

mixing heights, and convective systems) can

dominate. In particular, winter precipitation is

dominated by orographic lifting of strong westerlies

associated with the mean position of the polar

jet stream over Oregon and Washington.

Data

As in most climate studies, the type and availability of data can be limiting factors. Although

the amount of weather data may seem quite

large, it usually is not sufficient to adequately

describe climate patterns in regions of complex

terrain. This is because terrain strongly influences the atmosphere, and measurements in one

location often do not represent measurements

in another location if surrounding terrain

changes between the stations. Value can be

found in existing data records, however, and

various sources are described below.

Observational Data

Recorded weather observations began in the

Western United States in the mid 1800s.

Because population was relatively sparse at the

time, only eight stations within the interior

Columbia River basin are identified as having

consistent, quality controlled weather records

approaching 100 years: Spokane Weather

Station Office (WSO), Washington; Dufur,

Oregon; Fortine, Kalispell WSO, and Haugan,

Montana; and Priest River Experiment Station,

Caldwell, and Saint Ignatius, Idaho. Daily

summaries of precipitation and temperature

from these stations have been adjusted for

station relocations, changes in instrument

heights and types, changes in observing times,

and increases in urbanization as part of the

Historical Climatology Network (HCN;

Hughes and others 1992, Karl and others

1990).
3

The HCN data are a subset of the National

Weather Service, Cooperative Observer Network

(COOP; National Climatic Data Center 1991).

Data from COOP stations provide the highest

spatial resolution of daily temperature and precipitation measurements. Because many observers in

the COOP network are volunteers, however,

consistency and quality can be problematic. These

data have undergone some quality control, but

thorough review to locate those sites with reliable

records for each period of interest is lacking.

High-quality measurements of multiple parameters are available from the National Weather

Service first-order stations that are operated by

trained observers. Hourly temperature, dew point,

relative humidity, wind, precipitation, and radiation data from these sites are available through the

Solar and Meteorological Surface Observation

Network (SAMSON) data set developed by the

National Renewable Energy Laboratory (1992).

In addition, the hourly data are summarized each

day and added to the COOP database.

The HCN and SAMSON observation sites usually

are located near population centers or airports

away from the wild land areas of forests and

mountains. The COOP sites have more diverse

locations, but most are near homes or businesses

that commonly exist in valleys or away from wild

land areas.

Significant additions to high-elevation data occurred in the mid-1930s and again in late 1970s

and mid-1980s. The U.S. Department of Agriculture, Soil Conservation Service (now Natural

Resources Conservation Service), added many

observations in the mid-1930s (at the height of a

nationwide drought) to measure snow depth and

snow water equivalent along a transect of several

elevations (snow course) in the headwaters of

major river basins (USDA Soil Conservation

Service 1988). Snow course measurements are

acquired about every month during winter and

spring. In the late 1970s, an automatic system,

snowpack telemetry (SNOTEL), was added. The

SNOTEL sites transmit temperature, accumulated

seasonal precipitation, and total snow water

equivalent about once each day (Barton 1977). In

the late 1970s, remote automated weather systems

(RAWS) also were installed by the USDA Forest

Service and Bureau of Land Management

(Redmond 1991; U.S. Department of Interior,

Bureau of Land Management 1995). Most of the

network was complete by 1985. These stations are

designed to support fire weather forecasting and

thus operate mainly during summer and are located in forest clearings on hill slopes and ridges.

Those that operate year-round usually are not

outfitted with de-icing equipment, so much of the

winter data from RAWS sites are of poor quality.

The RAWS stations transmit hourly information

on air temperature, precipitation, fuel temperature, relative humidity, and wind. There are about

200 RAWS, 200 SNOTEL, and 200 snow course

sites in the interior Columbia River basin.

A northwest cooperative agricultural weather

network (AgriMet) is maintained by the Bureau of

Reclamation in Boise, Idaho, as part of the Pacific

Northwest Hydrometeorological Network for river

and reservoir management.1

 Historical data since

1983 include daily summaries of soil moisture, soil

and air temperatures, crop water use, and evapotranspiration.

A consistent network of radiosonde observations

(RAOBs) began in the mid-1930s (U.S. Department of Commerce 1964) with electronic files

available since 1948. The RAOB sensors measure

wind, temperature, dew point, and height at

mandatory atmospheric pressure levels (surface,

850 hectoPascals [hPa], 700 hPa, 500 hPa, etc.)

and other significant levels twice a day, at 0000

Greenwich Mean Time (GMT) and 1200 GMT.

1Bureau of Reclamation, 1150 N. Curtis Road, Boise, ID

83706.
4

Only two stations within the basin regularly report

upper air data: Spokane, Washington, and Boise,

Idaho.

A summary of surface and upper air climatic data,

which are in accessible electronic formats, is

shown in table 1. Other local and regional meteorological observations exist within the region; for

example, the Hanford meteorological station,

which includes a 125-meter (410-ft) tower, has

been recording observations on a plateau in southwestern Washington since 1945 (e.g., Stone and

others 1983). The Columbia River Operational

Hydromet System (CROHMS) organizes data

from the Bureau of Reclamation, U.S. Geological

Survey, Natural Resources and Conservation

Service, USDA Forest Service, National Weather

Service, and British Columbia Hydro and Power

Authority.2

 State highway departments began

installing automated weather systems in the early

1990s. Also, regional avalanche centers and ski

areas maintain automated weather stations (for

example, Ferguson and others 1990). Other

meteorological data, such as the forest fire lookout

observations, remain in paper form but are no less

valuable.

Simulated Data

Over 600 mountain weather and snowpack observations in the interior Columbia River basin

would seem like enough to describe mountain

climate. These stations, however, are spaced too

far apart (50 to 150 kilometers [30-90 mi]) to

represent small-scale climate caused by complex

topography in the region. Horizontal resolution

of at least 10 kilometers (6 mi) is needed to resolve

many of the mesoscale features influencing climate

in the Pacific Northwest (Doran and Skyllingstad

1992). In addition, the limited amount and

quality of data from each station make complete

analysis rather cumbersome. It is because of these

network limitations that model-generated data

play an increasing role in climate analyses of the

mountainous west. Three sets of model-generated

data have been completed for the U.S. portion of

the Columbia River basin: (1) historical means of

monthly and annual precipitation at 2.5-minute

(about 5 kilometers [3 mi]) latitude-longitude

spatial resolution from the PRISM model (Daly

and others 1994); (2) average daily temperature

and precipitation for three characteristic years

(1982, 1988, and 1989) at 2-kilometers (1.2 mi)

resolution from the MTCLIM-3D model

(Thornton and Running 1996); and (3) monthly

mean winds at 5-minute latitude-longitude resolution from the WINFLO3

 model. Output from

these models was used to help describe major

climatic features of each ecological zone, as discussed in the following sections.

Regional Climate Patterns

To help assess ecological conditions in the Columbia

River basin, ICBEMP required a description of

climatic means and trends in each of 3 major

ecological zones and 13 ecological reporting units

(ERUs). The major zones are (1) east side

(“Eastern”) Cascades, (2) Northern Rockies, and

(3) Central Columbia and Snake River Plateaus

(fig. 1). The ERUs, which exist in each zone, do

not exactly match National Weather Service climate division boundaries for which 30-year averages are calculated. Therefore, new summaries

were calculated from available COOP and HCN

data within each ERU (tables 2-14).

To increase representation of variability caused by

terrain in the climate, all records with data for

more than 10 years were used. This means that

the ERU summaries may not match NCDC

calculated normals (30-year averages) for the same

area. Whenever possible, ERU summary data

were compared with climate division summary

data to ensure that magnitudes and trends of ERU

calculations were within reason. Because the

spatial density of stations increases over time, ERU

summary data may be slightly biased toward the

most recent 10 years.

2Contact individual agencies for historical records.

3

 Ferguson, S.A.; Rorig, M.L.; Hayes, P.S.; Ruthford, J.

Surface wind patterns in the Pacific Northwest. Manuscript in

preparation.
5

Table 1—Summary of climate data sets

Dataset Beginning Basin Time

name year sites step Duration Measurement

HCN 1895 8 daily annual T,a pptb

Coop varies >300 daily annual T, ppt

>1895

SNOTEL 1978 45 daily annual T, ppt, SWEc

Snow course 1930 50 monthly winter H,d SWE

RAWS 1985 ~200 hourly summer T, Tf,e RH,f ppt, Wg

SAMSON 1961 10 hourly annual T, Td,h RH, W, P,i Qj

RAOBS 1948 2 2 per annual T, Td, W, P

day (multiple heights)

a Dry-bulb temperature f Relative humidity

b Precipitation g Wind

c Snow water equivalent h Dew point temperature

d Snow depth i Atmospheric pressure

e Fuel temperature j Solar radiation

Despite the increased spatial density of weather

observing stations, these data still do not represent

the variability of climate in highly complex terrain.

To quantify this concern, 30-second digital elevation model (DEM) data were analyzed for each ERU

(fig. 2). Only ERUs with reasonably accessible

terrain (for example, those dominated by rolling

hills and plateaus as in ERUs 3, 4, 5, and 10 in fig.

2) have average weather station elevations in the

midrange of terrain elevations. Inaccessible mountain terrain in all other ERUs cause most weather

stations to be sited at elevations well below the

mean terrain elevation. This can make it difficult

to describe climate influences on vegetation within

the basin because of the elevational extent of many

species (Daubenmire and Daubenmire 1968, Fowells

1965, Franklin and Dyrness 1988, Johnson and

Simon 1987, Shantz and Zon 1924, Volland

1976) is well above the average elevation of

weather observing stations.

The elevational distribution of weather observations

would improve by including data from snow

course, SNOTEL, and RAWS stations. Unfortunately, the different reporting formats, inadequate

metadata, insufficient station identification, lack

of maintenance, and difficulty in obtaining records

covering a long period from these observing networks prevented their use in ERU summary calculations. Data from these stations were used,

however, to help verify model results and improve

qualitative descriptions. Elevation-regression

models such as PRISM (Daly and others 1994)

and MTCLIM-3D (Thornton and Running

1996) improve representation of high-elevation

climate by interpreting available observations

through “topographically intelligent” techniques.

Both methods are relatively consistent, differing

only in local detail.
6

Figure 1—The interior Columbia River basin. Each ERU is numbered. The Eastern Cascades zone (ERUs 1-3) has no shading,

the Northern Rockies zone (ERUs 6-9 and 12-13) is dark shaded, and the Central Columbia and Snake River Plateaus zone

(ERUs 4, 5, 10, and 11) is lightly shaded. Stations selected to represent climate trends in each ERU are labeled.
7

ERU 01 ERU 02 ERU 03

ERU 04 ERU 05 ERU 06

ERU 07 ERU 08 ERU 09

ERU 10

ERU 13

Elevation increments (meters)

Percent of total in ERU

0 500 1000 1500 2000 2500 3000 3500

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0

10

20

30

40

50

60

Elevation increments (meters)

0 500 1000 1500 2000 2500 3000 3500

Elevation increments (meters)

0 500 1000 1500 2000 2500 3000 3500

ERU 11 ERU 12

Figure 2—Terrain distribution and average weather station elevations

in each ERU within the interior Columbia River basin. Terrain elevations were divided into 500-meter (1600-ft) increments, beginning

with 0 to 500 meters and ending with 3500 to 4000 meters (11,500 to

13,000 ft). The percentage of terrain in each elevation increment is

shown by the height of the bar. The terrain increment, which includes

the average station elevation for COOP and HCN weather observation

stations, is marked with an arrow.
8

Eastern Cascades

The Cascade Range blocks most marine air from

entering the basin. The eastern slopes of the

mountains (ERUs 1, 2, and 3) lie in a rain shadow

from oncoming Pacific storms. During winter,

however, when westerly winds are strongest,

enough moisture spills over the crest to cause this

region to remain wetter than other parts of the

basin (fig. 3). This region also receives significant

quantities of snowfall. Seasonal totals typically

range from about 200 centimeters (78 in) in the

south (ERU 03) to over 300 centimeters (120 in)

in the north (ERU 01), with greater amounts at

higher elevations (over 2000 centimeters [790 in]

of snowfall has been recorded at Crater Lake,

Oregon). During summer, when westerly winds

are weak, the rain shadow effect of the Cascades is

most apparent and this region becomes the driest

in the basin (fig. 3).

Chinook winds can cause occasional warm, dry,

and windy conditions that rapidly can melt snow

or initiate blow-down. Strong winds also are

common within mountain gaps as air flow is

channeled from both east and west directions.

The westerly gap winds, most common in summer, are strongest as they flow into the Eastern

Cascade region. The principal mountain gap is

the Columbia River Gorge, just east of Portland,

Oregon. Tornadoes and funnel clouds have been

observed near the outflow of the gorge’s westerly

winds. In addition, the persistence of its summer

westerlies allow the gorge to be one of the Nation’s

principal wind surfing recreation areas. Strong

southerly winds also are common, mainly during

winter, on several east-west oriented ridges that

protrude into the basin from the Cascade crest.

Although this region often is under continentaltype climate conditions (with cool, dry winters

and hot, dry summers), marine air spilling over

the mountain crests and through mountain gaps

moderates both summer and winter temperatures

and allows coastal vegetation species to coexist

among interior species. In addition, arctic air

often pools in the basin and is pulled against the

Cascades, causing a persistent temperature

inversion to about 1200 meters (4,000 ft),

especially in winter, and elevational gradients in vegetation are diffused or inverted.

Sharp contrasts in temperature occur when

arctic air is displaced by marine air, and

rain-on-snow flooding is common at all

elevations.

Northern Rockies

The Canadian Rockies and other mountain

ranges in British Columbia block most

arctic air from entering the basin. The

deep Okanogan, Columbia, and Pend

Oreille valleys, however, can funnel the

dry arctic air into the basin where it often

stagnates, especially during winter when

it is cold and dense.

The Rocky Mountains on the eastern

border of the basin intercept continental air

masses that rise over the imposing mountains,

favoring thunderstorm development around

-15

-10

Temperature (°C)

Precipitation (mm)

-5

0

5

10

15

20

0 ERU 01 ERU 02 ERU 03 ERU 04 ERU 05 ERU 06 ERU 07 ERU 08 ERU 09 ERU 10 ERU 11 ERU 12 ERU 13

20

40

60

80

100

120

140

Figure 3—Seasonal values of temperature and precipitation for

each ERU. The solid squares ( ) mark July average temperatures

and the solid triangles ( ) mark January average temperatures.

The hatched bar shows January precipitation, and the hollow bar

shows July precipitation.
9

the edge of the basin, especially in western

Montana and southern Idaho. A thermal trough

that migrates northward during spring and summer

also can cause thunderstorms, mainly in eastern

Oregon around the Blue Mountains (ERU 06).

The convection causes an increase in precipitation

during spring, with 24-hour accumulations often

greater than 25 millimeters (1 in). Drier lightning

is more common during summer and fall. Most

of the convection and lightning occurs in the east

and southeast units of this region (ERUs 9 and 12)

nearest the Continental Divide.

Blow-down in this region is common. Strong

down-burst winds associated with convective cells

can cause blow-down, most commonly during

spring. Blow-down also occurs in the elongated

north-south valleys that channel strong southerly

storm winds, mainly during winter. The high and

contrasting topography also favors accelerated

storm winds near ridgetops during winter and

persistent slope winds during summer.

The northern Rockies are the coldest part of the

basin with mean winter temperatures between -4.2

and -10.2  ̊C (-40 to -50  ̊F). Snowfall amounts

range from about 200 to over 300 centimeters (78

to 120 in), with greater amounts at higher elevations. Note that although winter precipitation in

the northern Rockies is less than along the east

side of the Cascades, snowfall amounts are comparable. There are two reasons for this: (1) cold

winter temperatures cause relatively low-density

snow to fall in the northern Rockies, so snowfall

amounts appear greater even though snow water

equivalents may be less than along the east side of

the Cascades; and (2) colder spring and fall temperatures in the northern Rockies allow more

snowfall during those seasons than in the warmer

Cascades.

Despite cold winter temperatures in the northern

Rockies, occasional marine intrusions enter the

northern part of the zone, mainly at lower elevations and in those places exposed to flow from the

Columbia Gorge: northeastern Oregon, eastern

Washington, the Idaho panhandle, and northwestern Montana. The increased exposure to maritime

air masses allow a somewhat moister vegetation

regime. Also, in summer the marine intrusions

moderate summer high temperatures and add

moisture to the convective cycle, thereby increasing the chance that lightning will be accompanied

by precipitation. This helps to reduce the threat of

wildfire somewhat in this northern section of the

zone. In winter, marine intrusions can cause

sudden warming and initiate rain-on-snow floods.

Central Columbia and

Snake River Plateaus

This is the driest part of the basin (fig. 3) and

supports various desert landscapes. The Columbia Plateau and Snake River Valley, however, are

susceptible to marine intrusions that can relieve

summer hot spells and winter cold spells. The

accompanying moisture can put out summer

wildfires or cause winter rain-on-snow floods.

Although rain-on-snow floods are rare in this

region, when they occur they are more destructive

and of much greater magnitude than spring

floods. Typical seasonal snowfall totals range from

40 to 80 centimeters (15 to 32 in).

The upper plateaus (ERUs 4, 10, and 11) experience

a moderate spring cycle of convective precipitation,

with lightning most common in ERU 11. The

convection can be caused by the northward migration of a thermal low pressure center, especially in

ERU 05, ERU 04, and the western part of ERU 10.

Also, hot unstable air from the Great Salt Lake

region can increase thunderstorm and lightning

development over ERU 11 and eastern ERU 10.

The strongest sustained winds in this region occur

during summer from the west at the eastern outflow

of the Columbia Gorge (ERU 05) and during

winter from the south and west along ridgetops.

Because of the volcanic soils and significant agricultural tilling in this zone, the wind can cause

significant soil erosion and degradation in visibility.

Although climate in this region is marked by few

extremes, long periods of stagnation occur during

winter in the central Columbia basin (ERU 05),
10

the Snake River Valley (ERU 10), and high, isolated basins (ERUs 4, 10, and 11). The stagnation events cause this region to be the most

susceptible to air pollution concerns.

Trends in Regional

Climate Patterns

For the years prior to about 1850, climatic trends

in the interior Columbia River basin can be determined only by using proxy data such as that evident from tree rings, glacier fluctuations, and

various sorts of geologic evidence. The tree-ring

chronologies in the Pacific Northwest have helped

to describe climate patterns from about the mid1600s. Because tree-ring data must be adjusted

for growth, some information about century-scale

climate variability is damped out. Tree-ring data

are being used, however, to help identify annual

and decadal patterns in climate variability (for

example, Briffa and others 1992). The data suggest periodic warm periods in 1630s, between

1640 to 1660, 1790s, and the 1920s, with temperatures about 0.5  ̊C (0.9  ̊F) greater than the

1881-1982 average. Alternating cold periods also

are apparent in the chronologies, with the most

significant from 1870 to 1900 (near the end of the

“Little Ice Age,” which occurred from about A.D.

1550 to 1850) having temperatures 0.2 to 0.5  ̊C

(0.5 to 0.9  ̊F) less than the 1881-1982 average.

The historical extent of mountain glaciers can

provide some information on climate, but data

are complicated owing to fluctuations caused by

differences in winter accumulation or summer

melt, or both. Also, the dynamics of glacier

movement complicate the time response to

climate changes; therefore, climate patterns

related to glacier fluctuations usually are described

in decades or centuries. Most glaciers in the

region significantly advanced during the “Little Ice

Age.” Since about 1850, the glaciers have been

retreating (Ferguson 1992, Meier 1984), suggesting that the region has been experiencing warmer

winters, cloudier summers, or both.

Geologic evidence includes lake levels, fossil pollen, and borehole measurements, which have

shown major epochs in Northwest climate, such as

a relatively cold and wet period between 12,000

and 9,000 years before present (B.P.), a warm and

dry period between 9,000 and 6,000 years B.P.,

and a cool, moist period since 6,000 years B.P.

(Whitlock 1992, 1993; Whitlock and Bartlein

1993). The spatial heterogeneity of these largescale features is controlled primarily by topography (Davis and others 1986, Whitlock and

Bartlein 1993).

Historical weather observations within the Columbia River basin began in the late 1800s around the

time that the Little Ice Age ended. A large number of weather stations were added during the

1930s drought to help monitor mountain snowpack and agricultural water conditions. Therefore,

many trends seen in available data could show

slight cooling from that drought period. Drought

returned in the 1950s and 1980s, but cooling

comparable to the late 1800s has not occurred.

There appear to be decadal trends in regional

climate that could be related to the El NiñoSouthern Oscillation (ENSO) and Pacific Decadal

Oscillation (PDO) indices (Cayan and Peterson

1989, Mantua and others 1997, Redmond and

Koch 1991, Ropelewski and Halpert 1986, Yarnal

and Diaz 1986). These trends are most obvious in

streamflow fluctuations, which aggregate precipitation and temperature signals over large areas of the

catchment and drainage basins of the stream.

To analyze climatic trends in each ERU of the

interior Columbia River basin, representative

stations were selected that have weather records

longer than 40 years and that lie near the

midrange of station elevations for that unit. A

simple 10-year moving average of monthly mean

values was used to evaluate trends and to help

smooth year-to-year variability. Historical trends

in precipitation are apparent, but trends in temperature are not obvious.
11

A notable maximum in snowfall and winter precipitation occurred in all areas during the mid

1970s and is clearly seen in the precipitation traces

at Moscow, Idaho (fig. 4). This coincides with a

step-wise change in climate observed in other

environmental variables during 1976 (Ebbesmeyer

and others 1991). Several new ski areas were

developed and existing areas expanded during that

period. Previous maximums in winter precipitation occurred in the mid-1950s (when most alpine

ski areas in the Western United States began), early

1940s, and mid-1930s, but these were not as

pervasive throughout the basin as the mid-1970s

maximum.

Winter precipitation since the mid-1970s has

decreased by 30 to 80 percent in all areas to a level

comparable to historical means, except in ERUs 3,

8, 9, and 10 where current winter precipitation

trend is slightly lower than any in previous history;

for example, at Lakeview, Oregon, shown in figure 5.

In contrast to winter trends, all areas showed an

increasing trend (30 to 80 percent) in summer

precipitation during the last 30 years, and figure 6

shows the summer trend at McCall, Idaho. Comparable highs in summer precipitation occurred

around 1910. Since the mid-1980s, about half the

basin (ERUs 3, 6, 10, 12, and 13) has shown a

20- to 70-percent decrease in summer precipitation

but remains at 40 to 80 percent above the 1960s

Figure 4—January precipitation and snowfall data from Moscow, Idaho, that

represent winter climate trends in ERU 05. Shaded bars show monthly precipitation totals in millimeters. Hollow bars show monthly snowfall totals in centimeters. The thick line shows the moving 10-year averages of monthly precipitation,

and the thin line shows the moving 10-year averages of snowfall.

0 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

Precipitation (mm)

0 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

1894 1903 1912 1921 1930 1939 1948 1957 1966 1975 1984 1993

Year
12

Figure 6—July precipitation data from McCall, Idaho, that represent summer climate

trends in ERU 13. See figure 4 for definitions of bars and lines, except no snowfall.

Figure 5—January precipitation and snowfall data from Lakeview, Oregon, that represent winter climate trends in ERU 03. See figure 4 for definitions of bars and lines.

0 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

Precipitation (mm)

0 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

Snowfall (cm)

1928 1934 1940 1946 1952 1958 1964 1970 1976 1982 1988

Year

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

55 

60 

65 

Precipitation (mm)

193019351940 19451950195519601965 19701975198019851990

Year
13

levels. Periods of drought throughout the basin

since 1988 appear to be related to the overall

decrease in winter precipitation and beginning

signs of decreasing summer precipitation.

Trends in annual precipitation over the last 100

years show significant variability around the basin

(Karl and others 1996). The central basin (Washington, northeastern Oregon, and eastern Idaho)

showed 5- to 10-percent increases. Elsewhere,

annual precipitation decreased 5- to 10-percent,

with ERU 03 showing a 20-percent decrease. It is

difficult to compare the 100-year trends with

those calculated from stations within each ERU

that have records of only 50 to 70 years. Also,

Mock (1996) illustrates the highly seasonal nature

of precipitation in the Western United States.

Therefore, changes in spring and autumn months

that influence annual trends are not shown in the

January and July records analyzed for each ERU.

Although there has been almost no change in

measured winter temperatures for the east side of

the Cascades, there has been a notable increase in

the percentage of water in the winter precipitation;

for example, since the early 1970s, snowfall at

Bend, Oregon, has decreased at a more rapid rate

than total precipitation (fig. 7). This could indicate somewhat higher snow levels with less snow

accumulating at lower elevations. In the same

region, a 1 to 2  ̊C (2 to 4  ̊F) decrease in the

diurnal range of summer temperatures may suggest slightly increasing summer cloudiness (fig. 8).

Other regions have less obvious changes, but the

overall trends appear to be toward slightly cooler

summers and slightly warmer winters.

Mean temperature has increased 1 to 2 ̊C (2 to 4  ̊F)

throughout the basin over the last 100 years (Karl

and others 1990). Note again that many of the

stations used to analyze ERU trends have records

Figure 7—January precipitation and snowfall data from Bend, Oregon, that represent

winter climate trends in ERU 02. See figure 4 for definitions of bars and lines.

0 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

Precipitation (mm)

0 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

Snowfall (cm)

1928 1934 1940 1946 1952 1958 1964 1970 1976 1982 1988

Year
14

that began during the 1930s drought era. Therefore, their trends are smaller than the 100-year

trends and are slightly negative.

The persistent drought throughout the basin in

the 1930s may relate to warm winter temperatures

and associated high snow levels. Many areas also

experienced a decrease in summer precipitation

during the 1930s, which added to severe drought

conditions. Periods of significant drought in a few

areas of the central basin and plateau regions

during the 1950s appear to have been driven more

by warmer than normal temperatures rather than

by changes in precipitation.

Conclusions

Climate of the interior Columbia River basin can

be characterized by three distinct air mass types

that interact with each other in a region of complex topography. Most precipitation accumulates

during winter (75 to 125 cm [30 to 50 in] in the

Eastern Cascades, 25 to 95 cm [10 to 37 in] in the

Northern Rockies, and 20 to 40 cm [8 to 16 in] in

the Central Columbia and Snake River Plateaus).

The mountain snowpack acts as a natural reservoir

and supplies the basin with most of its usable

water. Only the east and south ERUs of the basin

have summer maximum precipitation, which is

associated with significant thunderstorm activity.

Summer precipitation throughout the basin ranges

Figure 8—July temperature data from Bend, Oregon, that represent summer climate

trends in ERU 02. Shown are monthly average temperatures ( ), monthly average range

between daily maximum and minimum temperatures ( ), with moving 10-year averages

of each (thick and thin lines, respectively).

10 

15 

20 

25 

Temperatue (°C)

1928193319381943194819531958196319681973197819831988

Year
15

from about 20 to 50 cm (8 to 20 in). Trends in

the last 50 to 100 years indicate a general decrease

in winter precipitation and increase in summer

precipitation.

Temperatures are generally mild in the basin

because of periodic influxes of moderating Pacific

moisture. Winter mean monthly temperatures

range from -10 to -3  ̊C (-50 to 27  ̊F), and summer temperatures range from 10 to 15  ̊C (50 to

59  ̊F). Trends in the last 50 to 100 years indicate

a slight increase in winter temperatures and slight

decrease in summer temperatures.

Because most weather observation stations are

located near towns that are in valleys or away from

mountains, there is an elevational bias in the

instrument data. More effective analysis of the

Columbia River basin climate would include highelevation data from snow course, SNOTEL, and

RAWS sites. To do such analyses requires a significant amount of data processing to check and

adjust for quality. In addition, outfitting RAWS

sites with winter-durable sensors would improve

their reliability as climatic indicators. Developing

a unified database from all available data sources

also would be a great asset. Exploring new methods of analyzing spatial and temporal variability

would significantly improve our understanding of

climate patterns in regions of complex topography,

such as the Columbia River basin.

In addition to an elevational bias in this instrumental analysis, there also was a temporal bias

because the number of weather observations has

increased over time. Determining better ways of

gathering and interpreting paleoclimatic data

would significantly improve our understanding of

climate variability and trends. Of particular interest is the “Little Ice Age,” when Northwest climate

was significantly colder than now and when many

tree species in Northwest forests were establishing.

No weather measurements exist for this critical

time period (approx. A.D. 1550 to 1850), and few

proxy data are available. The climatological

evidence in proxy data that do exist has not been

fully realized. There are robust methods of climate

analysis, however, that can re-create regional climate patterns from sparse proxy data and from

correlations with long-period records in other

parts of the world. These methods can be used to

develop a detailed description of Northwest climate during a “presettlement” era when the impacts of climate variability and trend were not

complicated by management practices.
16

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19

Appendix

Table 2a—Daily climate summary for ERU 01 (26 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT b Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 -1.12 -8.61 7.49 -4.87 124.69 78.03 0.374

2 3.21 -5.92 9.14 -1.36 88.17 52.28 0.407

3 7.44 -3.55 10.98 1.95 69.87 36.15 0.483

4 12.44 -0.52 12.96 5.96 41.74 10.77 0.742

5 16.01 2.10 13.91 9.04 29.86 2.41 0.919

6 20.93 6.50 14.43 13.70 30.55 0.15 0.995

7 25.30 8.69 16.60 16.99 12.90 0.04 0.997

8 25.01 8.35 16.66 16.68 18.28 0.00 1.000

9 20.91 4.64 16.27 12.76 29.86 0.18 0.994

10 13.66 0.35 13.31 6.99 62.27 4.43 0.929

11 4.95 -3.18 8.12 0.87 114.12 36.14 0.683

12 0.43 -6.37 6.80 -2.97 135.49 77.14 0.431

Annual 12.43 0.21 12.22 6.31 63.15 24.81 0.746

a Station elevations: minimum = 195 meters, maximum = 1207, average = 603, range = 1012.

b PPT = precipitation.

Table 2b—Daily seasonal trends for ERU 01a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter -0.5 0 0 -- -35 --

Summer -2.5 0 +50 +50 -- +10

-- = negligible.

a Represented by data from Cle Elum, Washington (588 meters elevation and 64 years of record).
20

Table 3a—Daily climate summary for ERU 02 (18 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 2.42 -6.29 8.71 -1.95 108.68 62.77 0.422

2 5.30 -4.57 9.87 0.36 74.80 35.86 0.521

3 7.27 -3.71 10.98 1.76 61.68 26.40 0.572

4 11.10 -1.85 12.95 4.61 34.09 7.92 0.768

5 15.35 0.97 14.38 8.17 27.16 1.76 0.935

6 19.01 3.98 15.03 11.48 23.16 0.15 0.994

7 25.41 7.07 18.35 16.23 8.35 0.01 0.999

8 24.97 6.74 18.23 15.85 14.86 0.00 1.000

9 21.06 3.63 17.43 12.35 20.14 0.18 0.991

10 13.27 -0.57 13.84 6.35 44.24 2.54 0.943

11 6.98 -2.68 9.67 2.14 93.70 22.35 0.761

12 3.21 -5.06 8.27 -0.94 114.26 48.36 0.577

Annual 12.95 -0.20 13.14 6.37 52.09 17.36 0.790

a Station elevations: minimum = 30, maximum = 1475, average = 848, range = 1445.

Table 3b—Daily seasonal trends for ERU 02a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter -0.5 0 0 -- -40 --

Summer -1 -1 0 +75 -- +10

--- = negligible.

a Represented by data from Bend, Oregon (1116 meters elevation and 67 years of record).
21

Table 4a—Daily climate summary for ERU 03 (12 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 2.35 -8.24 10.60 -2.95 72.60 41.69 0.426

2 4.89 -6.39 11.28 -0.75 58.34 33.73 0.422

3 7.51 -4.81 12.32 1.34 55.82 34.20 0.387

4 11.78 -2.89 14.67 4.43 32.91 14.73 0.553

5 16.42 0.25 16.18 8.31 33.02 5.85 0.823

6 20.68 3.42 17.26 12.04 26.75 1.47 0.945

7 25.67 5.89 19.79 15.78 9.42 0.07 0.993

8 25.18 5.05 20.13 15.08 13.46 0.03 0.997

9 21.55 1.85 19.70 11.66 19.03 0.71 0.963

10 15.38 -1.52 16.90 6.92 41.68 6.39 0.847

11 7.35 -4.67 12.02 1.33 69.93 25.33 0.638

12 3.11 -7.30 10.41 -2.09 84.60 46.16 0.454

Annual 13.49 -1.61 15.10 5.92 43.13 17.53 0.704

a Station elevations: minimum = 1231, maximum = 1972, average = 1370, range = 741.

Table 4b—Daily seasonal trends for ERU 03a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter -0.5 +1 10 -- -70 --

Summer -3 0 +50 +50 -- -30

-- = negligible.

a Represented by data from Lakeview, Oregon (1457 meters elevation and 67 years of record).
22

Table 5a—Daily climate summary for ERU 04 (22 weather stationsa)

 Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 2.68 -8.32 11.00 -2.83 27.59 15.90 0.424

2 5.68 -5.96 11.64 -0.15 21.95 10.10 0.540

3 8.68 -4.31 12.99 2.17 25.30 9.35 0.630

4 13.12 -2.37 15.49 5.35 20.96 3.70 0.824

5 17.91 1.11 16.80 9.50 28.46 1.75 0.939

6 22.60 4.75 17.86 13.67 25.31 0.48 0.981

7 27.78 7.36 20.41 17.57 10.17 0.00 1.000

8 27.00 6.59 20.41 16.79 14.20 0.01 1.000

9 22.57 2.42 20.15 12.48 13.94 0.13 0.990

10 16.29 -1.48 17.77 7.37 20.20 1.53 0.924

11 7.81 -4.74 12.55 1.52 29.69 8.54 0.712

12 3.35 -7.80 11.15 -2.25 31.65 16.10 0.491

Annual 14.62 -1.06 15.68 6.77 22.45 5.63 0.788

a Station elevations: minimum = 1253, maximum = 1713, average = 1355, range = 460.

Table 5b—Daily seasonal trends for ERU 04a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter 0 +1 0 -- -30 --

Summer -2 0 +10 +50 -- 0

-- = negligible.

a Represented by data from Sqaw, Oregon (1420 meters elevation and 58 years of record).
23

Table 6a—Daily climate summary for ERU 05 (87 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 1.75 -6.37 8.12 -2.32 39.13 19.46 0.503

2 5.63 -3.85 9.48 0.89 28.60 10.10 0.647

3 10.24 -1.61 11.85 4.31 30.20 6.14 0.797

4 14.91 0.98 13.93 7.94 25.78 2.17 0.916

5 19.72 4.54 15.19 12.12 28.60 0.47 0.984

6 23.89 8.00 15.89 15.94 28.40 0.03 0.999

7 28.61 10.39 18.22 19.49 10.33 0.00 1.000

8 27.96 9.95 18.01 18.95 13.04 0.00 1.000

9 23.22 6.14 17.08 14.67 16.37 0.05 0.997

10 15.80 1.47 14.34 8.63 24.47 0.78 0.968

11 7.17 -2.18 9.35 2.49 40.05 6.93 0.827

12 2.73 -4.94 7.68 -1.12 41.69 16.15 0.613

Annual 15.14 1.88 13.26 8.50 27.22 5.19 0.854

a Station elevations: minimum = 58, maximum = 1902, average = 566, range = 1844.

Table 6b—Daily seasonal trends for ERU 05a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter -0.5 0 0 -- -35 --

Summer +2 -1 +30 +60 -- -10

--= negligible.

a Represented by data from Moscow, Idaho (810 meters elevation and 95 years of record).
24

Table 7a—Daily climate summary for ERU 06 (34 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 0.66 -9.03 9.69 -4.20 41.76 10.90 0.337

2 5.05 -6.54 11.59 -1.11 31.39 6.13 0.504

3 9.07 -3.82 12.89 2.61 34.71 4.33 0.683

4 13.97 -1.11 15.07 6.42 30.61 1.39 0.884

5 18.62 2.34 16.29 10.46 39.47 0.27 0.983

6 22.93 5.63 17.31 14.27 35.04 0.02 0.998

7 28.31 7.81 20.50 18.04 14.23 0.00 1.000

8 27.78 7.10 20.68 17.43 18.47 0.00 1.000

9 22.97 3.08 19.89 13.02 19.21 0.11 0.986

10 16.19 -0.86 17.05 7.65 26.23 0.41 0.960

11 7.25 -4.23 11.48 1.50 39.57 4.08 0.738

12 1.80 -7.62 9.42 -2.93 44.38 9.21 0.473

Annual 14.55 -0.60 15.16 6.93 31.26 3.07 0.795

a Station elevations: minimum = 363, maximum = 1506, average = 959, range = 1143.

Table 7b—Daily seasonal trends for ERU 06a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter +0.5 0 0 -- -30 --

Summer -0.5 0 +50 +45 -- -15

--= negligible.

a Represented by data from Union, Oregon (844 meters elevation and 67 years of record).
25

Table 8a—Daily climate summary for ERU 07 (52 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 -1.57 -9.48 7.91 -5.54 57.15 45.28 0.208

2 2.27 -7.18 9.45 -2.46 41.39 24.34 0.412

3 6.87 -4.58 11.44 1.13 38.36 14.39 0.625

4 12.78 -0.94 13.72 5.92 34.17 3.30 0.903

5 18.02 2.93 15.10 10.46 45.34 0.67 0.985

6 21.91 6.35 15.56 14.12 49.73 0.08 0.998

7 25.90 7.84 18.06 16.86 26.83 0.01 1.000

8 26.13 7.70 18.42 16.90 28.80 0.01 1.000

9 19.75 3.40 16.35 11.57 31.54 0.19 0.994

10 12.27 -0.66 12.93 5.80 36.31 2.27 0.937

11 3.83 -4.09 7.93 -0.13 58.77 18.14 0.691

12 -0.56 -7.58 7.02 -4.07 60.98 41.02 0.327

Annual 12.30 -0.52 12.82 5.88 42.45 12.48 0.757

a Station elevations: minimum = 250, maximum = 1795, average = 796, range = 1545.

Table 8b—Daily seasonal trends for ERU 07a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter -2 +0.5 -25 -- -40 --

Summer -2 +1 0 +30 -- na

--= negligible.

na = not available.

a

 Represented by data from Fortine, Montana (719 meters elevation and 95 years of record).
26

Table 9a—Daily climate summary for ERU 08 (29 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 -1.36 -8.88 7.52 -5.13 94.84 54.51 0.425

2 2.06 -6.88 8.94 -2.41 67.44 32.48 0.518

3 5.42 -5.28 10.69 0.07 59.80 20.71 0.654

4 9.56 -2.82 12.38 3.38 45.81 4.09 0.911

5 15.44 0.87 14.58 8.15 51.54 0.98 0.981

6 20.61 4.90 15.70 12.74 54.33 0.05 0.999

7 24.19 5.78 18.41 14.98 23.20 0.00 1.000

8 24.91 6.15 18.76 15.53 29.54 0.00 1.000

9 20.56 2.77 17.79 11.04 39.60 0.28 0.993

10 11.02 -1.53 12.54 4.73 52.34 2.99 0.943

11 2.82 -4.76 7.58 -0.98 78.69 20.07 0.745

12 -0.67 -7.33 6.65 -4.00 100.53 52.82 0.475

Annual 11.21 -1.42 12.63 4.84 58.14 15.75 0.804

a Station elevations: minimum = 658, maximum = 1817, average = 882, range = 1159.

Table 9b-Daily seasonal trends for ERU 08a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter -2 +2 -25 -- -50 --

Summer -2 +0.5 +40 +40 -- na

-- = negligible.

na = not available.

a Represented by data from Haugan, Montana (890 meters elevation and 43 years of record).
27

Table 10a—Daily climate summary for ERU 09 (21 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 -3.55 -13.07 9.53 -8.31 25.65 27.61 -0.076

2 -0.66 -11.12 10.46 -5.90 17.49 17.95 -0.026

3 2.56 -8.66 11.22 -3.05 19.06 18.04 0.054

4 7.51 -5.16 12.67 1.16 22.41 7.63 0.659

5 11.86 -1.84 13.70 5.00 38.53 2.59 0.933

6 15.46 1.17 14.30 8.31 44.36 0.27 0.994

7 19.83 2.64 17.19 11.23 23.03 0.00 1.000

8 19.32 1.98 17.34 10.64 25.53 0.09 0.997

9 15.84 -0.57 16.41 7.61 26.41 1.48 0.944

10 10.03 -4.17 14.20 2.93 21.37 4.80 0.775

11 1.24 -8.62 9.85 -3.69 22.07 15.05 0.318

12 -2.85 -11.97 9.12 -7.43 23.74 24.86 -0.047

Annual 8.05 -4.95 13.00 1.54 25.80 10.03 0.544

a Station elevations: minimum = 1030, maximum = 1847, average = 1367, range = 617.

Table 10b—Daily seasonal trends for ERU 09a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter 0 0 -50 -- -45 --

Summer 0 0 0 +35 -- 0

-- = negligible.

a Represented by data from Butte, Montana (1689 meters elevation and 95 years of record).
28

Table 11a—Daily climate summary for ERU 10 (62 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 1.23 -8.81 10.03 -3.81 31.41 17.21 0.452

2 4.94 -6.21 11.15 -0.66 24.89 12.97 0.479

3 9.47 -3.57 13.04 2.93 27.09 7.93 0.707

4 14.77 -0.39 15.16 7.19 24.17 3.95 0.836

5 19.80 3.62 16.18 11.69 28.94 1.50 0.948

6 24.52 7.34 17.19 15.91 34.18 0.27 0.992

7 29.81 10.47 19.34 20.13 8.01 0.00 1.000

8 28.93 9.45 19.49 19.18 10.64 0.04 0.997

9 23.58 4.76 18.82 14.17 14.52 0.49 0.966

10 6.94 0.06 16.89 8.49 19.32 2.05 0.894

11 7.84 -4.27 12.11 1.78 30.18 6.88 0.772

12 2.18 -7.85 10.03 -2.86 30.39 12.41 0.592

Annual 15.34 0.38 14.95 7.85 23.65 5.47 0.803

a Station elevations: minimum = 652, maximum = 2146, average = 1124, range = 1494.

Table 11b—Daily seasonal trends for ERU 10a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter -1 -0.5 -25 -- -60 --

Summer -2 +2 0 +50 -- -70

-- = negligible.

a Represented by data from Caldwell, Idaho (722 meters elevation and 90 years of record).
29

Table 12a—Daily climate summary for ERU 11 (22 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 -1.88 -11.76 9.89 -6.83 23.31 20.46 0.122

2 1.42 -9.14 10.56 -3.89 19.92 14.04 0.295

3 5.96 -5.82 11.79 0.06 21.90 8.78 0.599

4 12.04 -2.08 14.12 4.95 22.32 3.66 0.836

5 17.06 2.00 15.06 9.50 32.48 1.09 0.966

6 21.74 5.54 16.19 13.64 27.44 0.02 0.999

7 26.71 8.60 18.11 17.65 13.44 0.00 1.000

8 25.96 7.51 18.45 16.74 14.67 0.00 1.000

9 20.74 3.01 17.73 11.88 16.60 0.15 0.991

10 14.24 -1.65 15.89 6.30 18.38 1.69 0.908

11 5.08 -6.19 11.27 -0.57 22.87 8.33 0.636

12 -0.46 -10.36 9.91 -5.42 23.08 18.37 0.204

Annual 12.38 -1.70 14.08 5.34 21.37 6.38 0.713

a Station elevations: minimum = 1268, maximum = 1798, average = 1409, range = 530.

Table 12b-Daily seasonal trends for ERU 11a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter 0 0 0 -- -75 --

Summer -2 -1 0 +80 -- +10

-- = negligible.

a Represented by data from Aberdeen, Idaho (1344 meters elevation and 81 years of record).
30

Table 13a—Daily climate summary for ERU 12 (12 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 -3.83 -16.42 12.59 -10.21 51.22 71.29 -0.392

2 -0.67 -14.50 13.83 -7.65 42.33 51.59 -0.219

3 2.89 -11.35 14.24 -4.25 40.36 46.55 -0.153

4 8.32 -6.00 14.32 1.15 40.17 23.86 0.406

5 14.31 -1.48 15.79 6.39 53.49 7.43 0.861

6 19.56 1.77 17.79 10.66 43.39 0.53 0.988

7 24.18 4.07 20.11 14.10 29.99 0.00 1.000

8 23.41 3.23 20.18 13.34 32.79 0.02 0.999

9 18.44 -0.73 19.17 8.81 38.16 2.04 0.946

10 12.05 -5.10 17.15 3.46 34.08 10.10 0.704

11 2.40 -10.25 12.65 -3.94 48.79 45.20 0.074

12 -3.18 -15.60 12.42 -9.46 50.29 66.05 -0.313

Annual 9.82 -6.03 15.85 1.87 42.09 27.06 0.408

a Station elevations: minimum = 1603, maximum = 2487, average = 1959, range = 884.

Table 13b—Daily seasonal trends for ERU 12a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter 0 0 0 -- -50 --

Summer -2 +1 +50 +80 -- -35

-- = negligible.

a Represented by data from Ashton, Idaho (1603 meters elevation and 47 years of record).
31

Table 14a—Daily climate summary for ERU 13 (51 weather stationsa)

Temperature Water content

Month Max. Min. Daily range Avg. PPT Snowfall of snow

.................................... °C .................................... Mm Cm Percent

1 -1.17 -12.48 11.31 -6.87 70.07 44.12 0.370

2 2.51 -10.29 12.80 -3.90 42.09 28.03 0.334

3 6.53 -7.05 13.59 -0.27 40.85 20.79 0.491

4 11.98 -3.06 15.04 4.45 37.02 7.99 0.784

5 17.31 0.81 16.49 9.05 43.48 2.11 0.952

6 21.86 4.29 17.57 13.07 44.06 0.13 0.997

7 27.37 6.54 20.84 16.93 19.22 0.00 1.000

8 26.78 5.68 21.10 16.22 21.84 0.00 1.000

9 21.36 1.63 19.73 11.48 26.91 0.33 0.988

10 13.76 -2.85 16.61 5.45 30.35 2.87 0.906

11 4.94 -6.74 11.68 -0.92 48.71 20.53 0.578

12 -0.49 -11.31 10.82 -5.94 65.76 41.51 0.369

Annual 12.73 -2.90 15.63 4.90 40.86 14.03 0.731

a Station elevations: minimum = 485, maximum = 2225, average = 1371, range = 1740.

Table 14b—Daily seasonal trends for ERU 13a

Temperature trend Precipitation trend

Season Daily range Avg. All years Since 1960 Since 1975 Since 1985

.................. °C ................ ....................................... Percent ...................................

Winter -1 +2 0 -- -50 --

Summer +2 -0.5 +50 +60 -- -20

-- = negligible.

a Represented by data from McCall, Idaho (1533 m elevation and 65 years of record).
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Ferguson, Sue A. 1999. Climatology of the interior Columbia River basin. Gen. Tech. Rep.

PNW-GTR-445. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific

Northwest Research Station. 31 p. (Quigley, Thomas M., ed.; Interior Columbia Basin

Ecosystem Management Project: scientific assessment).

This work describes climate means and trends in each of three major ecological zones and 13 ecological

reporting units in the interior Columbia River basin. Widely differing climates help define each major

zone and reporting unit, the pattern of which is controlled by three competing air masses: marine,

continental, and arctic. Paleoclimatic evidence and historical weather records show that the region has

undergone significant fluctuations in temperature and precipitation as air masses alternate dominance

over the basin. The major change in climate occurred near the time of western settlement with the end

of the “Little Ice Age.” Since then there have been numerous annual cycles in climate that may be

related to the Pacific Decadal Oscillation. During the last 50 years, winter precipitation has decreased

slightly and summer precipitation has increased throughout most of the basin. At the same time, winter

temperatures have increased and summer temperatures have slightly decreased. Some impacts of changes

in climatic means and trends on ecological conditions in the basin are discussed.

Keywords: Climate, Columbia River basin, climatology, climate variability, temperature, precipitation,

snowfall.

The Forest Service of the U.S. Department of

Agriculture is dedicated to the principle of multiple

use management of the Nation’s forest resources for

sustained yields of wood, water, forage, wildlife, and

recreation. Through forestry research, cooperation with

the States and private forest owners, and management

of the National Forests and National Grasslands,

it strives—as directed by Congress—to provide

increasingly greater service to a growing Nation.

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contact USDA’s TARGET Center at (202) 720-2600

(voice and TDD).

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Director, Office of Civil Rights, Room 326-W, Whitten

Building, 14th and Independence Avenue, SW, Washington,

DC 20250-9410 or call (202) 720-5964 (voice and TDD).

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Portland, Oregon 97208-3890
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