Credit: Seattle NWS
Mt. Rainier has a habit of creating its own weather. At 14,411 feet tall, it is the second-tallest peak in the lower-48 states, and it is more prominent than K2, the second tallest peak in the WORLD. And when the light winds of summer give way to the powerful atmospheric rivers that define our storm seasons around here, the mountain blesses the denizens of Western Washington to a most majestic sight.
On Friday, we saw some spectacular lenticular clouds surrounding Mt. Rainier. Lenticular clouds are a type of lens-shaped cloud that form on the lee side of large mountains when a stable, moist air mass with laminar flow (little change in wind velocity with height, meaning there is little shear/turbulence) flows into a mountain, creating a series of oscillating “waves” in its wake. When the air rises, it cools via the process of adiabatic expansion, and if it cools to saturation, a cloud forms. As the air sinks down the trough of the wave, it may warm past the dew point, resulting in clear skies. Lenticular clouds can exist for miles downwind of large mountains like Rainier, and they are often neatly spaced apart at a regular interval. The clouds themselves don’t tend to move much, even though air is constantly flowing through them.
If you look in the distance, you can see that these clouds exist far downwind of Mt. Rainier. Also, they sometimes occur at multiple levels due to Mt. Rainier creating lee waves at various layers of the atmosphere.
As is almost always the case, skies gradually darkened and the scattered lenticular clouds gave way to a solid nimbostratus cloud deck with moderate-to-heavy rain on Saturday, especially over the Central Cascades. Note the profound shadowing over Eastern Washington.
Credit: NWS
It’s been a while since the lenticular clouds were out in force like this on Mt. Rainier, and that’s because it has been a while since we saw such a strong, moist, westerly laminar flow into our region. Take a look at the graphic below showing the total precipitable water over the Eastern Pacific, and it’s pretty easy to see why we had so many lenticular clouds on Friday. A strong atmospheric river originating all the way past the International Date Line was headed right into our area, giving several inches of rain to the mountains and nearly an inch for some lowland locations.
Monday and Tuesday look unsettled, with showers at times. We dry out midweek, but another weak system comes in Friday evening, opening up the door for another system Saturday into Sunday. We aren’t in full-blown “storm season” yet, but our days in the 90s are certainly over!
