Climate

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Union County offers four distinct seasons. The County, 2,038 square miles in size, consists of snow-capped peaks, timbered slopes, fertile valleys, and a multitude of streams. The Grande Ronde Valley, 35 miles long and 15 miles wide, is the defining feature of the landscape. The climate in Union County is as diverse as it topography. Elevation is a key factor in temperature and precipitation.

The elevation of the floor of the Grande Ronde Valley is about 2,700 feet. On the horizon of the valley one can see Mt. Fanny to the east (7,136 feet), Mt. Harris to the northeast ( 5,335 feet) and Mt. Emily northwest (6,110 feet). The Blue Mountains ring the valley and the Wallowa Mountains and the Eagle Cap Wilderness Area define the southeast boundary of the valley.

Elevations of the communities in Union County are:

The Grande Ronde Valley itself exhibits a relatively mild climate – not too cold in the winter or too hot in the summer. Climate Data La Grande, OR 1981-2011

Climate Data La Grande, OR  1981-2011
J F M A M J J A S O N D Average
Average Maximum F 38.4 43 51.1 58.3 66.7 74.8 85.5 85.7 76.2 62.3 46.3 37.5 60.5
Average Minimum F 24.7 25.8 30.7 35.3 42.2 48.8 53.9 52.4 44 35.4 30 23.8 37.2
Average Temperature F 31.5 34.4 40.9 46.8 54.4 61.8 69.7 69.1 60.1 48.9 38.1 30.7 48.9
Precipitation (in.) 1.69 1.18 1.48 1.56 2.08 1.61 0.68 0.84 0.66 1.28 2.03 1.65 16.74
Average Snowfall (in.) 5.9 1.9 1.1 0.1 0 0 0 0 0 0 2.1 4.4 15.6
Average Snow Depth (in.) 1.4 0.4 0 0 0 0 0 0 0 0 0.2 0.7 0.2 Season
Heating Degree Days (base 65) 1036 838 732 532 338 146 30 34 176 486 798 1050 6196 Season
Cooling Degree Days (base 65) 0 0 0 0 13 54 182 167 39 1 0 0 456 Annual
Growing Degree Days (base 50) 0 0 8 51 184 365 612 598 318 76 5 0 2217 Annual
Highest Maximum F 58 66 73 87 95 100 108 104 100 89 71 59
Lowest Maximum F 11 4 20 35 38 43 53 54 46 27 6 -3
Highest Minimum F 45 48 52 53 61 69 75 68 66 59 53 48
Lowest Minimum F -17 -14 9 19 25 30 37 34 23 11 -14 -18
Source: National Weather Service,  NOAA

 

The differential between the average maximum and minimum temperatures during the winter season – November through February – range about 13 to 17 degrees. From March to October the differential ranges from 20.4 degrees to over 30 degrees. The relatively mild climate supports a broader range of commodities that can be grown by area farms. Comparing the highest maximum temperatures in August with the highest low temperatures, one can occasionally experience a temperature variation of up to forty degrees in one day, from the low triple digits to the mid to lower 60s.

These temperature differentials in the summer months contribute to the very low cooling degree days registered in the Valley. Degree Day Definitions

Precipitation is fairly evenly distributed through the seasons in La Grande. The average rainfall during the four seasons:

December-February (winter):    4.52 in.
March-May (spring):     5.12 in.
June-August (summer):   3.13 in.
September-November (fall):  3.97 in.

The mild winter temperatures mean that snowfall does not generally accumulate in the Valley. The average snow depths do not exceed 1.5 inches on average for any of the winter months.

Pertinent weather data from the National Weather Service: (PDF)

Calendar Days Extremes – Highest High Temperature (F)
Calendar Days Extremes – Lowest High Temperature (F)
Calendar Days Extremes – Highest Low Temperature (F)
Calendar Days Extremes – Lowest Low Temperature (F)
Extremes – Highest Daily Precipitation (in.)
Extremes – Highest Daily Snowfall (in.)