Warm temperatures in the Inland Northwest this week have accelerated snowmelt. Broadly speaking, the snowpack in the Columbia River watershed remains healthy though it has dropped rapidly relative to climatological norms the last few days. Rapid melt periods are common during Spring and early Summer and the rate is likely to decrease into the weekend as temperatures moderate.
An upper level low located near San Francisco is forecast to move northeast across the Pacific Northwest beginning Thursday, bringing scattered rain showers. Strong and slow moving thunderstorms with heavy rain from copious subtropical moisture are forecast in mountainous regions from Northern Nevada to the Idaho Panhandle, including the Blue and Wallowa Mountains of Washington and Oregon on May 4-5.
The Weather Prediction Center, a department of the National Weather Service, notes that on their own the thunderstorms would not be enough to trigger the marginal risk (level 1/4) for excessive rainfall causing flash flooding. This time of year, however, strong snowmelt will add to the water coming off the mountains. The risk of flash flooding, landslides, and debris flows is especially notable near recent wildfire burn scars.
The flash flood risk is limited to under/near strong thunderstorms but river flooding on some smaller rivers is also forecast. Minor to moderate flooding is common every spring as snow melts in higher elevations. The National Weather Service in Pendleton has issued a flood warning for the Imnaha RIver in Wallowa County where minor flooding is forecast.
As of Wednesday morning, other alerts are not in effect for rivers and streams in the NWS Pendleton’s forecast area but moderate flooding is forecast on a stretch of the John Day River with minor flooding forecast on the Naches and Grande Ronde Rivers. Others, such as the Yakima River, will run high but are not expected to reach flood stage.
River levels are forecast to peak late Friday or early Saturday. Rain chances and temperatures decrease into the weekend which will slow the amount of water coming off mountains in the Inland Northwest.
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The featured image is the WPC excessive rainfall outlook for May 4, 2023. It shows a level 1/4 risk of excessive rainfall from Northern Nevada north to the Idaho Panhandle.
