SPOKANE, Wash. — It looks like an oxymoron, Spokane's 7-day forecast has chances for rain nearly every single day. Yet the total forecasted rainfall is close to nothing.
What's happening is that small waves of energy from the jet stream is causing a slightly unsettled weather pattern across the Pacific Northwest. However, due to the lack of deep atmospheric moisture and the result of our topography is preventing any significant or widespread rainfall from forming for the region.
In Spokane, each day from Saturday to Thursday will come with a 20% chance of rain. So it's reasonable to think that we might see a quick passing shower one or two of those days, with the rest being dry. Additionally, the region as a whole will likely see scattered showers develop each day, but what cities see rainfall will differ each day.
So when looking at the big picture, Spokane's total forecast precipitation is well under one-tenth of an inch. In fact, it's more likely that we get no rain at all than more than a tenth, as is the forecast situation over the next week.
Why can't we get good rainfall? Two reasons, the first being our geography. The Columbia Basin, being surrounded by mountains on all sides will always see a rain shadow effect, meaning that the mountains themselves will squeeze out most or all available moisture leaving none left for the areas that sit on the leeward side of the mountains.
Secondly, the total moisture in the atmosphere, known as precipitable water, values are extremely low. We would need deep moisture throughout the entire atmosphere to carry rainfall over the mountains and delivery widespread rain. This is just not the case for the slight disturbances that are forecasted.
