More Wave Clouds

A few days ago there was a great example of trapped lee waves (also known as trapped mountain waves). These waves occur when the wind speed increases rapidly with height and the atmospheric stability decreases above a mountain-top or ridge-top stable layer. This results in a series of lee waves (and clouds) downstream of the mountain. This wind and stability situation is fairly common—especially in the winter.

Panoramic image of Altocumulus Standing Lenticular (ASCL) just before sunset.
Panoramic image of Altocumulus Standing Lenticular (ASCL) just before sunset.
Cigar-shaped lee wave clouds.
Cigar-shaped lee wave clouds.
Another example of lee wave clouds.
Another example of lee wave clouds.
GOES-16 visible satellite imagery shows numerous wave clouds across northern Arizona
GOES-16 visible satellite imagery shows numerous wave clouds across northern Arizona

Towards sunset some higher-level Altocumulus Standing Lenticular (ACSL) clouds became more prominent and as the sun set became quite colorful. The image at the top of this post was taken a few minutes before sunset and is a panorama composed of five individual images taken with an ultra-wide 16 mm lens.

Mountain-Generated Snow Squalls

A few days ago we had an impressive snow squall that formed downwind of the San Francisco Peaks. Watching this evolve on radar was fascinating and I decided to drive to a location where I would have a good view to the east.

Low clouds and fog brightly lit by the sun
Low clouds and fog brightly lit by the sun
Annotated image showing cloud motions in the snow squall.
Annotated image showing cloud motions in the snow squall.

Clouds in the west were blocking most sunlight but there was a narrow gap that allowed the sun to brightly illuminate the low clouds and fog associated with this event.

Radar depiction of a snow squall.
Radar depiction of a snow squall.

The radar image shows the precipitation from the snow squall while the large white dot is my location. The overall motion of the low cloud was to the south-southeast (left-to-right in the photograph)—however, the motion at the top of the low cloud was in the reverse direction (i.e., right-to-left) and there was some rotation along the cloudy/clear interface. This is fairly typical of a density current with cold air sliding under warmer air and shear/rotation being present at the interface.

Another fun day of cloud photography!

Skiing in Skunk Canyon

In late January we had many days of snow with over 40″ falling in about 11 days. What a great chance to ski down Skunk Canyon. I’ve been here many times walking, running, and mountain biking—but never skiing.

Skiing down Skunk Canyon near Flagstaff.
Skiing down Skunk Canyon near Flagstaff.

In the morning, it was quite cold—the overnight low temperature was +9°F—and the snow was very dry; Swix Blue Extra Wax was perfect for getting a bit of grip and a good glide. There was a broken trail part of the way into the canyon but mostly we had untracked snow and broke our own trail.

Fisher Point with snow.
Fisher Point with snow.

At the point where Skunk Canyon briefly gets very narrow everyone but me turned around. I had never been to Fisher Point in the snow and I was probably not going to get another chance for a while so I continued. There were some old, snow-covered ski and snowshoe tracks here and folks had probably come in via Walnut Canyon and the Arizona Trail. But no one else was there while I visited.

Ski tracks in Skunk Canyon.
Ski tracks in Skunk Canyon.

I turned around to retrace my steps and within a few minutes was having issues with the rapidly warming snow sticking to my skis. I was no longer able to glide and several inches of heavy snow were clumping to the bottom of the skis. Wow! This was going to be a long slog back to the car. I was pretty tired when I finished and the deep powder was already a distant memory.

And that’s what skiing in Flagstaff is like. Once the storm ends and the sun comes out there is only a very brief window of a few hours. Get it while you can.

Thundersnow!

Thundersnow. Just the sound of it is exciting—snow accompanied by thunder and lightning. Although it is fairly rare east of the Rocky Mountains it is a bit more common across the west.

And we had multiple rounds of thunderstorms producing snow yesterday (Saturday, 13 February 2021) in Flagstaff. It was pretty amazing to see heavy snow falling and then a sudden flash of lightning followed by the muffled sound of thunder a few seconds later.

Radar depiction of precipitation across northern Arizona at 1:51 P.M. on 13 February 2021. Note the small lightning symbols near Flagstaff showing locations of cloud-to-ground lightning.
Radar depiction of precipitation across northern Arizona at 1:51 P.M. on 13 February 2021. Note the small lightning symbols near Flagstaff showing locations of cloud-to-ground lightning.

There were several clusters of thunderstorms that moved across the area during the afternoon. None of these were recorded by the ASOS at KFLG—and it’s not obvious why they were missed. The ASOS certainly captured the start of snow and then the moderate (SN) and heavy snow (+SN). Perhaps the lightning was just far enough away to not register.

KFLG 132307Z VRB06G25KT M1/4SM +SN FZFG VV004 M01/M02 A2960 RMK AO2 P0006 T10111017
KFLG 132257Z 29009G27KT 260V340 1/4SM SN FZFG VV005 M01/M01 A2961 RMK AO2 PK WND 24031/2239 SLP027 P0007 T10061011
KFLG 132243Z 27012G31KT 1/4SM SN FG BKN008 OVC029 00/M01 A2959 RMK AO2 PK WND 24031/2239 P0004 T00001006

Here is a loop of the visible channel from the GOES-17 satellite. Superimposed on the clouds are color blobs showing Group Flash Count Density from the satellite lightning mapper. The next image shows lighting from surface-based lightning detectors and shows numerous cloud-to-ground strikes across northern Arizona.

 

Locations of cloud-to-ground lightning across northern Arizona on the afternoon of 13 February 2021.
Locations of cloud-to-ground lightning across northern Arizona on the afternoon of 13 February 2021.

Model-generated soundings from the HRRR showed that there was very slight convective instability with surface-based CAPE values of around 100 J/kg.

Model-derived sounding for Flagstaff at 20 UTC 13 February 2021 (1300 MST)
Model-derived sounding for Flagstaff at 20 UTC 13 February 2021 (1300 MST)

That is a shallow layer of instability and very weak instability—but was enough to generate thundersnow.

Wave Clouds Over the San Francisco Peaks

A few weeks ago I captured these images of wave clouds over the San Francisco Peaks. At first, there was a “short stack” of lenticular clouds, specifically Altocumulus Standing Lenticularis (ACSL).

Wave clouds over the San Francisco Peaks.
Wave clouds over the San Francisco Peaks.

I took several photographs looking toward the peaks from the Bonito Park area near the west entrance of Sunset Crater Volcano National Monument. I alternated between wide-angle shots showing the snow-covered flats and zoomed-in images of the stack of clouds. After a few minutes, I was ready to leave.

Wide-angle view of wave clouds over the San Francisco Peaks.
Wide-angle view of wave clouds over the San Francisco Peaks.
Zoomed-in crop showing structure of the wispy filaments with ACSL above.
Zoomed-in crop showing structure of the wispy filaments with ACSL above.

But before I did leave, new clouds began to form beneath the stack of ACSL. These clouds were quite different and appeared as long, wispy filaments or rope-like clouds. Again, I took photographs ranging from wide-angle shots to zoomed-in shots. After about 8–10 minutes the delicate filaments began to take on more of an ACSL shape similar to the already-present ACSL above.

I was intrigued by the shapes of these clouds so I posted a comment with photographs to a weather discussion group with many atmospheric scientists far more aware of the dynamics and details of wave clouds than I. It turned into a fascinating discussion with links to journal articles, modeling studies and, inevitably, YouTube.

I do not think we reached a consensus on the dynamics and evolution of these cloud filaments but all agreed it was a worthwhile discussion.