On Thursday, Friday and Saturday of this week we had some days of thunderstorms in the Cascades and the Olympics, but little activity near sea level, something that is very evident in the satellite images that have visible early Friday evening (see below)
On Friday night, a Puget Sound resident could see a cumulus cloud towering over the Cascades, with a distinct arc-shaped anvil, which is relatively rare around here:
And let’s not forget our ability to map every lightning bolt from several lightning networks that sense the electrical impulse from each lightning. These are the lightning bolts on Thursday – the preference for higher terrain is obvious.
Around noon on Friday, I was flying back from a meeting in Boulder, Colorado (at the National Center for Atmospheric Research) and I took some pictures of cumulus clouds growing over the central Cascades. (see below).
Fly near the top of Cascade (south of Snoqualmie Pass)
Then look back from the lowlands, where the sky is clear.
So why are mountainous areas favored for thunderstorms but lowland areas are not?
And why are mountain thunderstorms so active this week?
One main reason is that the air in the mountains is more unstable than in the plains.
Thunderstorms are a form of convection in which air masses accelerate upward. An important requirement for convection is the large vertical variation in temperature. So overhead cooling or near-surface warming helps to increase the vertical temperature change (slope) that promotes convection. You all have experienced convection when reheating hot cereal, with the increased oatmeal flow that occurs when the bottom of the pan is heated.
In the mountains, the temperature changes with greater altitude because the mountains act as a source of elevated heat (see diagram below). You can consider the temperature to be the same at high altitudes in both the mountains and lowlands, but the mountains act as a heater at some altitudes (due to the warming of the sun during the day), forcing the region to change. convert the temperature to a narrower range of altitudes.
That creates a larger temperature change (slope) at the top, which increases instability which increases thunderstorms. Meteorologists have a preferred measure of instability called CAPE (Convection Available Potential Energy) and the Thursday afternoon CAPE map (5pm) confirms it is more abundant. in the mountains compared to the lowlands.
Thunderstorms are more frequent in the mountains for a different reason. Uphill currents develop on slopes during the day and such uphill flows converge at the top, creating a strong upward air current that can promote cumulus clouds and thunderstorms (see graph). .
Finally, why is this week so “favored” for thunderstorms.
We are now at the beginning of summer when the sun can effectively heat the slopes. Furthermore, the melting of snow at lower altitudes allows the ground to warm in a way not possible in the colder seasons.
But there’s something else. We have cooler-than-normal air overhead combined with a cold, low-pressure trough (see map 500 hPa, about 18,000 ft) on Friday at 2 p.m., below). Green and blue indicate cooler temperatures.
The cold depression will be active in the coming days, so expect more mountain thunderstorms. And don’t expect any future heatwaves: temperatures will stay above 70 degrees Celsius for the next few days. 😋😀
