The lowlands of the Inland Northwest should lose much of their snow this week as high temperatures slowly rise into the 40s. While some flooding may occur in small streams and low lying areas, larger rivers like the Umatilla, Walla Walla, and Yakima are forecast to remain below flood stage.
The snow certainly is deep, but the water content is not high. A decent average for snow water equivalent is a 10:1 ration (meaning 10 inches of snow equates to 1 inch of water). In the Columbia Basin, the ratio from the storms last weekend may be more like 15:1 or 20:1. This, combined with the relatively slow melt speed, will equate to not much more than if we got 0.25″ of rain a day for the next week or so.
When talking about flooding as a result of snow melt, one or more of the following conditions help to make a more extreme event:
- Frozen ground surface, forcing more water into streams and rivers
- Saturated ground, forcing more water into streams and rivers
- Rapid melt due to high temperatures
- Rapid melt due to heavy rain
None of the above conditions are expected over the next week. Much of the Columbia Basin and surrounding lowlands did not have 8″ soil temperatures drop below freezing during the recent three snow storms. This, coupled with the soil being unsaturated, means that a lot of the melted snow will enter the groundwater system.
Temperatures will be warming, but afternoon highs in the 40s don’t exactly cause a rapid melting scenario. Rain also accelerates melting, but only periodic light rain is expected. Snow levels should remain below pass level this week, so the precipitation type in the mountains is expected to remain snow. Even the higher parts of the Horse Heaven Hills may not be snow free by the end of the week.
This is in contrast to what caused the major flooding seen in the Umatilla and Walla Walla Rivers in February 2020 when a warm atmospheric river event took aim directly at the Blue Mountains. This raised snow levels well above pass level to drop up to 10″ of rain in just a few days on top of several feet of snow.
While widespread flooding is not likely this week, folks living in flood prone areas should not let their guard down. While there is nothing in the models to suggest this as of writing, it is never out of the question to have warmer temperatures bring heavy rain on the many feet of snow that have accumulated in the Blues and Cascades. Floods from spring melting have occurred well into April and May.
Also keep in mind that rivers will be higher and swifter than they are now despite not reaching flood stage. Be sure to watch children and pets closely when you’re by the water!
The featured image is the river forecast graphic for the Walla Walla River near Touchet, Washington from the National Weather Service.
