Lewiston, Idaho and Clarkston, Washington are located at the confluence of the Snake and Clearwater Rivers at an elevation of 738 feet above mean sea level. Lower Granite Lake extends from the confluence of the two rivers, 32 miles downstream in the Snake River channel, to Lower Granite Dam. The valley is rather narrow with a range of hills to the north sloping abruptly to about 2,000 feet above the valley floor. To the south, the terrain rises more gradually to a more or less flat bench about 700 feet above the valley. The Weather Office is located on the bench at an elevation of 1,413 feet above sea level and about 2 miles south of Lewiston. Although Lewiston is at about the same latitude as Duluth, Minnesota, the climate, especially in the wintertime, is comparatively very mild. This mildness can be explained by its location with respect to the effects of Pacific air masses from the west and by the sheltering effects of the mountains that surround the valley in almost every direction.
Considerable variations in the climate are to be found within relatively short distances from the valley itself. On the prairies surrounding the valley, winter temperatures are much lower and the precipitation is normally almost double that recorded in the valley and at the airport location.
Snowfall in the valley averages about 18 inches during the year, concentrated mostly in the three months of December, January, and February, but in the higher country surrounding the valley the snowfall is much heavier.
Precipitation normally amounts to about 13 inches annually, which is rather evenly distributed through the year except for the months of July and August, which are characterized by infrequent thunderstorms that usually drop only small amounts of rain. Records show that several times during these two months not more than a trace of rain has been recorded and at times not even a trace. The thunderstorms on the prairie are, at times, accompanied by heavy hail and windstorms.
Most of the precipitation reaching this vicinity results from strong invasions of moist air from the North Pacific source region. Greatest amounts of both rain and snow occur when this moist air is overrunning a weak front that has become stationary along an east-west line a short distance south of the area.
Many winters have gone by without a temperature of zero being recorded in the valley, but the prairie sections usually experience lower temperatures. The summers experience hot and dry periods with as many as 10 consecutive days with afternoon temperatures reaching 100 degrees or more. Considerable cooling after sunset makes the nights very comfortable. Cold waves occur when arctic air, originating in the Yukon Territory, moves southward. Such cold waves are relatively infrequent when compared to the number of arctic outbreaks east of the continental divide in Montana only a short distance away.
Winds are lights, usually prevailing from the east, with occasional stronger winds accompanying the well-developed frontal systems from the west.
Relative humidity averages about 70 percent during the winter months and gradually lowers to about 40 percent during July and August.
The growing season of approximately 220 days in this part of the country, makes conditions favorable for the growing of many types of fruits, vegetables, and berries.
Valley Climate & Topography Today's Weather
Length of Growing Season
Annual Average Clear / Partly Cloudy Days
Total Square Miles
Winter....High 39.6Âº ~ Low 27.1Âº
Summer....High 89.0Âº ~ Low 59.2Âº
Normal monthly....Jan. 33.3Âº ~ July 74.1Âº