multnomah county climate

Introduction

Multnomah County lies along the Columbia River in the Willamette Valley. It is wholly within Climate Division 2 (Willamette Valley) established by the National Climatic Data Center. Below is a description of the climate of Division 2 followed by specific descriptions of Multnomah County. Climate tables for various parameters, as observed at long-term climate stations in Multnomah County, are included below.

Climate Division 2 — Willamette Valley

The Willamette Valley is the most diverse agricultural area in the state of Oregon, and also the home of the majority of the population. Oregon's three largest cities, Portland, Salem, and Eugene, are located in the north, central, and south portions of the Valley, respectively. The urban areas are surrounded by varied and productive ranches, orchards, and farms. Among the crops grown in significant quantities are tree fruits, nuts, berries, mint, grains, and hay. Livestock operations are also common, including the dairy and poultry industries.

The climate of the Valley is relatively mild throughout the year, characterized by cool, wet winters and warm, dry summers. The climatic conditions closely resemble the Mediterranean climates, which occur in California, although Oregon's winters are somewhat wetter and cooler. Growing seasons in the Willamette Valley are long, and moisture is abundant during most of the year (although summer irrigation is common).

Like the remainder of western Oregon, the Valley has a predominant winter rainfall climate. Typical distribution of precipitation includes about 50 percent of the annual total from December through February, lesser amounts in the spring and fall, and very little during summer. Rainfall tends to vary inversely with temperatures -- the cooler months are the wettest, the warm summer months the driest. Figure 1 shows NOAA climate stations in Zone 2, which were in operation during the 1961-1990 period. Figure 2 shows the Multnomah County region from the Oregon annual precipitation map.

There is considerable variation in precipitation in the Valley, ranging from annual totals below 40 inches in the Portland area to upwards of 80 inches in the Cascade and Coast Range foothills. Elevation is the single most important determinant of precipitation totals. Table 1 shows a plot of monthly & annual average precipitation versus elevation for stations in the Valley, and indicates a strong correlation between the two. Even in the lower sections of the Valley the effects of elevation are pronounced. Portland, for example, at 21 feet above sea level, receives an average of 37.4 inches (30-year normal), while Salem (196 feet) receives 40.4 inches and Eugene (359 feet) receives 46.0 inches. Thus, a change of only 338 feet of elevation produces an increase of 23 percent above Portland's total. Table 2a and 2b list the average number of days with precipitation amounts exceeding certain thresholds.

Table 3 lists normal monthly temperature at stations in the area. Extreme temperatures in the Valley are rare. Days with maximum temperature above 90 deg F occur only 5-15 times per year on average, and below zero temperatures occur only about once every 25 years. Mean high temperatures range from the low 80's in the summer to about 40 deg F in the coldest months, while average lows are generally in the low 50's in summer and low 30's in winter. The mean growing season (days between 32 deg F temperatures) is 150-180 days in the lower portions of the Valley, and 110-130 days in the foothills (above about 800 feet). Table 6 lists the mean growing season for Zone 2.

Although snow falls nearly every year, amounts are generally quite low. Valley floor locations average 5-10 inches per year, mostly during December through February, although higher totals are observed at greater elevations in the foothills. Table 4 lists average monthly and annual snowfall totals for various stations.

Table 5 lists the median frost dates for Zone 2. Severe storms are rare in the Valley. Ice storms occasionally occur in the northern portions of the Valley, resulting from cold air flowing westward through the Columbia Gorge. High winds occur several times per year in association with major weather systems.

Relative humidity is highest during early morning hours, and is generally 80-100 percent throughout the year. Humidity is generally lowest during the afternoon, ranging from 70-80 percent during January to 30-50 percent during summer. Annual pan evaporation is about 40 inches, mostly occurring during the period April - October.

Winters are likely to be cloudy. Average cloud cover during the coldest months exceeds 80 percent, with an average of about 26 cloudy days in January (in addition to 3 partly cloudy and 2 clear days). During summer, however, sunshine is much more abundant, with average cloud cover less than 40 percent; more than half of the days in July are clear.

Tables 7 and 8 list average monthly and annual heating and growing degree days, respectively.

County Description

Established: Dec. 22, 1854
Population: 666,350
Area: 465 sq. mi.
Economy: Manufacturing, transportation, wholesale and retail trade and tourism.
County Seat: Portland

Lewis and Clark made note of the Indian village of Multnomah on Sauvie Island in 1805, and applied that name to all local Indians. The name is derived from nematlnomaq, probably meaning "downriver." Multnomah County was created from parts of Washington and Clackamas Counties by the Territorial Legislature in 1854, five years before Oregon became a state, because citizens found it inconvenient to travel to Hillsboro to conduct county business. The county is both the smallest in size and largest in population in Oregon. Over 50 percent of its people live in Portland, a busy metropolis dominated by rivers and greenery. The remaining area includes picturesque rural land, from pastoral farms on Sauvie Island to the rugged Columbia River Gorge and the western slopes of Mt. Hood.

(County information obtained from Oregon Blue Book)

Climate Tables (Multnomah County, Oregon)


Table 1. Precipitation, Monthly and Annual Averages (1971-2000) (back to top)
Name
Number
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
Bonneville Dam
897 11.20 9.83 7.96 6.02 3.96 3.01 1.03 1.35 3.02 5.73 12.09 12.77 77.97
Portland WB City
6749 6.24 5.07 4.51 3.10 2.49 1.60 0.76 0.99 1.87 3.39 6.39 6.75 43.16
Portland WSO AP
6751 5.07 4.18 3.71 2.64 2.38 1.59 0.72 0.93 1.65 2.88 5.61 5.71 37.07
Troutdale Substation
8634 6.09 5.16 4.40 3.65 2.83 2.20 0.94 1.10 2.00 3.34 6.53 6.61 44.85

Table 2a. Average number of Days with Selected Precipitation Amounts, Bonneville Dam, 1971-2000 (back to top)
Threshold
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
.01"or more
19.9 18.3 19.3 17.8 14.7 10.8 5.3 5.5 8.8 12.8 21.4 20.8 174.8
.10"or more
15.9 14.2 15.0 12.5 9.6 7.0 2.5 3.1 6.0 9.9 17.1 16.2 129.4
.50"or more
8.0 6.8 6.1 4.6 2.4 1.7 0.6 0.9 2.2 4.0 9.3 8.8 55.4
1.00"or more
3.6 2.9 1.7 0.8 0.5 0.5 0.1 0.2 0.8 1.5 3.6 4.4 20.2
Table 2b. Average number of Days with Selected Precipitation Amounts, Portland WSO AP, 1971-2000 (back to top)
Threshold
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
.01"or more
17.2 15.9 17.2 15.3 12.8 8.8 4.4 4.8 7.5 11.4 18.9 18.3 152.5
.10"or more
11.5 10.5 10.4 7.9 7.0 4.5 2.0 2.4 4.0 7.0 12.8 11.9 91.8
.50"or more
3.2 2.2 1.7 0.9 1.2 0.7 0.4 0.5 0.9 1.7 3.3 3.9 20.5
1.00"or more
0.8 0.5 0.1 0.2 0.1 0.1 0 0.1 0.2 0.3 0.9 1.1 4.4

Table 3. Monthly and Annual Average Temperatures (deg F), Portland WSO AP (1862), 1971-2000 (back to top)
Parameter
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Mean max
45.6 50.3 55.7 60.5 66.7 72.7 79.3 79.7 74.6 63.3 51.8 45.4 62.1
Mean min
34.2 35.9 38.6 41.9 47.5 52.6 56.9 57.3 52.5 45.2 39.8 35.0 44.8
Mean temp
39.9 43.1 47.2 51.2 57.1 62.7 68.1 68.5 63.6 54.3 45.8 40.2 53.5
Extreme max
63 71 77 90 100 100 104 107 105 92 73 65 107
Extreme min
12 9 19 30 35 41 45 44 37 26 13 8 8
Mean number of days
Max 90 or more
0 0 0 0 0.4 1.1 4.5 4.6 2.1 0.1 0 0 12.9
Min 32 or less
11.6 7.1 3.0 0.7 0 0 0 0 0 0.4 4.4 9.8 37.0
Max 32 or less
1.3 0.3 0 0 0 0 0 0 0 0 0.2 1.2 3.1
Min 0 or less
0 0 0 0 0 0 0 0 0 0 0 0 0

Table 4. Snowfall, Monthly and Annual Averages (1971-2000) (back to top)
Name
Number
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
Bonneville Dam
897 6.9 3.7 0.9 0 0 0 0 0 0 0 1.2 3.5 16.6
Portland WB City
6749 1.2 0.9 0.1 0 0 0 0 0 0 0 0.4 1.0 3.0
Portland WSO AP
6751 1.1 1.4 0.1 0 0 0 0 0 0 0 0.6 1.3 4.5
Troutdale Substation
8634 1.5 0.6 0.5 0 0 0 0 0 0 0 0.5 0.7 4.5

Table 5. Median Spring and Fall Frost Dates, Portland WSO AP, 1971-2000 (back to top)
Percentile
Last Date in Spring of Low Temperatures (deg F) First Date in Fall of Low Temperatures (deg F)
24 28 32 36 24 28 32 36
10
16-Jan 29-Jan 20-Feb 28-Mar 22-Nov 7-Nov 28-Oct 14-Oct
20
28-Jan 4-Feb 10-Mar 7-Apr 2-Dec 11-Nov 30-Oct 18-Oct
50
8-Feb 16-Feb 26-Mar 13-Apr 30-Jun 3-Dec 14-Nov 27-Oct
80
1-Jan 7-Mar 5-Apr 26-Apr 31-Dec 18-Dec 28-Nov 5-Nov
90
1-Jan 15-Mar 13-Apr 29-Apr 31-Dec 23-Dec 6-Dec 15-Nov

Table 6. Average Growing Season, Portland WSO AP, 1971-2000 (back to top)
Percentile
Length of Time (Days) Between Occurrence of Temperatures ( deg F)
24 28 32 36
10
270 240 206 176
20
299 255 222 178
50
355 285 240 195
80
695 313 255 207
90
701 341 263 221

Table 7. Monthly and Annual Average Heating Degree Days (base 65°F), 1971-2000 (back to top)
Name
Number
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
Bonneville Dam
897 824 664 576 418 259 122 32 26 94 306 580 796 4705
Portland WB City
6749 734 579 512 343 240 105 31 23 80 299 546 731 4212
Portland WSO AP
6751 766 605 532 391 234 93 21 15 78 317 562 756 4369
Troutdale Substation
8634 781 617 531 385 227 102 31 27 100 318 568 770 4531

Table 8. Monthly and Annual Average Growing Degree Days (base 50°F), 1971-2000 (back to top)
Name
Number
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
Bonneville Dam
897 1 3 20 83 226 .369 548 561 410 179 21 3 2424
Portland WB City
6749 4 14 41 113 249 396 559 571 435 181 29 6 2598
Portland WSO AP
6751 3 9 30 96 248 402 581 594 425 166 27 5 2586
Troutdale Substation
8634 4 8 33 101 258 401 561 558 398 165 25 4 2515