In the Wood River Valley and across Blaine County, lush, green grasses and vibrant wildflowers blanket the landscape. The Big Wood River—already swollen from runoff of an above-average winter snowpack—is enhanced by occasional downpours. In towns and in the backcountry, residents and visitors find themselves running for shelter from heavy rains or hail, or dashing to their car to roll up the windows.
Though spring rains are common in south-central Idaho, especially in May, many people who are used to seeing more sunshine in the region are asking different variations of the same question: “What’s up with the weather?”
Carter MacKay, a meteorologist for the National Weather Service in Pocatello, said Wednesday that much of the Northwest has been under the influence of a parade of “very active” weather systems moving into the region from the Southwest, where a “nearly stationary” low-pressure system has turned clouds and moisture northward. In the absence of a high-pressure system over Idaho and other parts of the Northwest to push the unsettled weather away, the storms—some moving quite slowly—have had a relatively open path to move to the north, he said.
“Once we see that high-pressure ridge, we can start thinking about drier conditions,” MacKay said.
One way meteorologists monitor weather, MacKay said, is through measuring “precipitable waters”—the amount of water vapor in a column extending from the landscape to the top of the atmosphere. Precipitable water amounts in a vast swath from Southern California to Montana have recently been 120% to 180% of normal, MacKay said.
“It has been an anomalously wet May,” MacKay said, noting that while some areas are measurably wetter, others have been drier.
For example, MacKay said, Custer County, immediately to the north of Blaine County, has done “really well” in receiving spring precipitation, while the Snake River Plain to the south has been significantly drier.
From May 1 to May 30, precipitation in Blaine County has ranged from 110% to
150% of normal in northern parts of the county and has been around normal or slightly below in southern areas, Weather Service data indicates. Precipitation measured at weather stations in the area ranged from 2 inches to 4.5 inches in valley areas during that period, MacKay said, and 4 inches to 6.5 inches at higher elevations, where mountains create a “buoyancy”—or upward force—for storms to grow. In central Ketchum, the normal precipitation in May—the wettest month outside of the winter and fourth-wettest month of the year—is 1.78 inches.
The measured precipitation in Blaine County in May was similar last year, MacKay said. What casual observers might be noticing is that there have simply been fewer breaks with long periods of sunshine between storms, though dry, sunny periods have been more common to the north, in Stanley, MacKay said.
“It’s anomalous to have constant moisture, to have active weather every day,” he said.
Though the weather pattern is not predicted to change dramatically in the coming days, a high-pressure ridge is building to the west of Idaho, MacKay said. Days with fewer clouds and rain could be ahead, he noted, though a significant amount of moisture remains in the regional atmosphere.
Most of Idaho is likely to have normal precipitation over the next three months, the National Weather Service has predicted.
Graphic courtesy of National Weather Service
The National Weather Service’s seven-day forecast on Thursday predicted unsettled weather through next Wednesday.
As for the summer, the weather in south-central Idaho is expected to be about normal, MacKay said.
The National Weather Service’s three-month precipitation outlook for the country for June, July and August predicts below-normal rainfall in the Idaho Panhandle but equal chances of above- or below-normal precipitation in the rest of the state. With northern Idaho, Washington, northern Oregon and parts of the Southwest predicted to likely have below-normal precipitation, much of the East and upper Midwest is expected to have above-normal rainfall.
The agency’s three-month temperature forecast for Idaho predicts that conditions are “leaning above” normal. The West, South and the entire East Coast are predicted to likely be above normal, while the Midwest is predicted to have equal chances of above- or below-normal temperatures.
After what MacKay called a “nonstop barrage of systems over the winter” that brought Blaine County’s snowpack well above average, the county and most of southern Idaho has moved out of various stages of drought, according to the National Oceanic Atmospheric Administration’s U.S. Drought Monitor. This week, only two areas of far southern Idaho and the northern part of the state registered on the agency’s five-point drought scale. 
