A strong storm system approached the southwest yesterday and moved through the state overnight. Ahead of the storm, clouds began to increase just before sunset and I found myself at the Mormon Lake overlook. The cumulus clouds were fairly shallow so that the sunlight hitting the tops of the clouds was scattered downward and provided some diffuse lighting and color to the cloud bases.
There was also a thin gap between the clouds and the ground allowing for just a few minutes of direct sunlight.
Here are a few photographs of the sunset and clouds along with some sunlight spilling across the currently dry lakebed of Mormon Lake.
As the storm and cold front approached showers and thunderstorms quickly developed with a line of strong thunderstorms developing about an hour after sunset. There was quite a bit of lightning as the line of storms moved across the area. The sound of rain faded as it was replaced by snow falling in the wake of the front.
In the morning there was snow covering the ground. Measurements were difficult as strong winds during the night had blown the snow such that some areas had a few inches and other areas were bare.
The snow juxtaposed with autumn colors was very pretty but my favorite photograph was the snow on this cinquefoil that still has some flowers.
Late September provided a chance to photograph storms associated with the passage of an upper-level trough crossing the southwest. Ahead of the trough would be moderate-to-strong upper level winds along with deep-layer shear. So there was a possibility of a few strong thunderstorms that might develop supercellular characteristics. As well, there would likely be a line of storms that formed on the surface cold front pushing southeastward across the state.
The plan was to head north to Cameron, Arizona, then slowly work back to the south as the line of storms moved across the area. I went to Cameron but the storms to the west and northwest were already weakening while storms farther to the southwest were strengthening. So — back south I went with stops at Wupatki NM and Sunset Crater NM while watching the storms. A cluster of storms developed north of the San Francisco Peaks and eventually pushed a strong outflow across the mountains. The leading edge of this outflow had an interesting cloud structure and was briefly very photogenic.
After a few photos, I continued moving across Flagstaff on my way home. Suddenly, the sun dropped low enough to get below the clouds and for a few minutes there was great sunset light on the bottom of the clouds. I shot a few photos at Fort Tuthill County Park with the undersides of the clouds full of sunset colors. And, then just a few minutes later it was over and the light was gone.
So I could have just stayed at home and gone out shooting at the last minute of daylight.
The next day showed potential for a few interesting storms south of Flagstaff. I headed to Sedona and spent a few hours photographing storms.
A potent weather system moved across Arizona on Tuesday, October 5, 2021, and produced a variety of weather including tornadoes, large hail, heavy rain, and flooding. This system was well forecasted and the various models did a very good job depicting the possibilities.
Areas of moderate rain moved across Arizona in the pre-dawn hours with many locations receiving more and one inch of rain by sunrise.
There had been some expectations that the severe weather would commence before sunrise owing to the favorable vertical wind profiles. Both the low-level shear and deep-layer shear supported organized thunderstorms with the possibilities of rotating supercells and even tornadoes. That scenario did not manifest probably because the widespread overnight rain cooled the boundary layer enough to inhibit strong thunderstorm development.
The forecast models also suggested a second period of severe weather was possible in the afternoon as temperatures warmed and buoyant instability increased across the area.
Strong thunderstorms developed across the desert regions of central and southern Arizona in the afternoon. These moved quickly to the northeast and into the higher terrain of the Mogollon Rim and Tonto Rim. At least two of these thunderstorms began to acquire supercellular characteristics and began to show rotation signatures in the Dopper radar data. By mid afternoon, one storm had developed a well-defined hook echo and velocity couplet. This storm was already producing strong winds and hail and a Severe Thunderstorm Warning (SVR) was already in effect. As the rotation rapidly intensified, the SVR was replaced with a Tornado Warning (TOR). Polarimetric Dopper radar data indicated low values of Correlation Coefficient (CC). This often accompanies tornadic debris lofted into the air. This was clearly a dangerous storm and the TOR was justified.
Eventually, the rotation weakened but the storm was still producing large hail as it moved northeastwards. Other storms also produced large hail. The Multiple Radar Multiple Sensors (MRMS) data shows these long swaths of large hail across portions of Arizona.
Forecasters from the National Weather Service offices in Arizona did a great job of anticipating this severe weather event and issuing timely warnings.
Outbreaks of severe thunderstorms in northern Arizona are most likely to occur during the transition between the moist, tropical environment of the warm-season North American Monsoon regime and the first incursions of mid-latitude baroclinic systems in September. The presence of copious tropical moisture, combined with increased convective instability and deep-layer shear, is supportive of long-lived supercells which are responsible for most of the severe weather.
Supercells are more common during the transition season than other times of the year in northern Arizona. Tornadoes—especially long-lived, damaging tornadoes—are more likely to occur during this brief period. This is from a paper presented at the 24th Severe Local Storms Conference (2008):
The results show that more than half of the tornado days occurred during the approach of a closed low from the eastern Pacific with northern Arizona located in the warm sector of the northeast quadrant of the low. The closed lows produced environments with deep-layer shear and low-level shear comparable to the 3rd and 4th quartiles of tornadic environments discussed by Rasmussen and Blanchard (1998) while instability was small and was comparable to or less than their 1st quartile. These interesting results suggest that shear may be the more important factor and that instability need only be sufficient to initiate and maintain convection long enough for the shear to act upon the updrafts.
While tornadoes in northern Arizona are more likely to occur during the transition season they are still uncommon (a notable exception was the October 2010 outbreak) and several years may pass without any tornado events. Supercells, on the other hand, are more common and at least a few can be expected each year during the transition.
Below are composite means of 500-mb geopotential height, 700-mb v-wind component, and surface Lifted Index for the tornado events.
Will we have a closed-low, transition-season type of event this year?
It was only a few days ago that I photographed an evening barrage of lightning over the San Francisco Peaks and the Cinder Hills. Those photographs were taken from Wupatki National Monument—right at the entrance pullout off of Highway 89. And, now I found myself in this same location shooting lighting from an early afternoon series of thunderstorms—except looking in the other direction across vast grasslands.
The most amazing lightning strike of the day occurred before I had set up the gear. The bolt landed miles away from the storm in a sunny area. This was a classic “bolt from the blue” and would have surprised anyone in that location. The radar image below shows the bolt landing a fair distance from the storm.
Lightning continued in this direction for more than an hour. As the storms would move to the northeast newer storms would develop north of the San Francisco Peaks and move over Wupatki National Monument.
After shooting at this location for awhile I decided to reposition to Wukoki Pueblo with hopes that lightning to the north would continue and I could get photographs with lightning and the pueblo. I was not disappointed.
Again, storms would move to the northeast and then be replaced by new storms from the southwest. This went on for several hours. The lightning plot above shows how many lightning strikes there were in this region during the afternoon.
I had hoped that thunderstorms and lightning would continue through sunset and twilight but it was not to be. Eventually, newer storms stopped developing and the other storms moved far to the northeast.