Thunderstorms and Tornadoes in Arizona

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.

Radar-estimated rainfall amounts across Arizona on Tuesday morning.
Radar-estimated rainfall amounts across Arizona on Tuesday morning.

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.

Severe thunderstorm with a Three-Body Scatter Signature (TBSS; also known as a "hail spike").
Severe thunderstorm with a Three-Body Scatter Signature (TBSS; also known as a “hail spike”).
Severe thunderstorm with a hook echo (left) and strong velocity couplet (right). The box indicates that a Severe Thunderstorm Warning is in effect for this storm.
Severe thunderstorm with a hook echo (left) and strong velocity couplet (right). The box indicates that a Severe Thunderstorm Warning is in effect for this storm.
Severe thunderstorm continues to show a hook echo (left); on the right is Correlation Coefficient where low values may indicate tornadic debris. The warning has been upgraded to a Tornado Warning.
Severe thunderstorm continues to show a hook echo (left); on the right is Correlation Coefficient where low values may indicate tornadic debris. The warning has been upgraded to a Tornado Warning.

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.

Multi-Radar Multi-Sensor (MRMS) depiction of large hail swaths.
Multi-Radar Multi-Sensor (MRMS) depiction of large hail swaths.
MRMS rotation tracks associated with the tornado.
MRMS rotation tracks associated with the tornado.

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.

Monsoon Transition Season Is Here

It is now September and we are in the so-called “transition season” as the North American Monsoon weakens and stronger baroclinic weather systems begin to traverse the southwestern states. From a paper presented at the 23rd Severe Local Storms Conference (2006):

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.

Composite 500-mb height field for the 19 tornado event days.
Composite 500-mb height field for the 19 tornado event days.
Composite 700-mb v-component of the wind field.
Composite 700-mb v-component of the wind field.
Composite surface-based Lifted Index field.
Composite surface-based Lifted Index field.

Will we have a closed-low, transition-season type of event this year?

References

Blanchard, D. O., 2006: A cool season severe weather episode in northern Arizona. Preprints, 23rd Conference on Severe Local Storms, St. Louis, MO., Amer. Meteor. Soc.

Blanchard, D. O., 2008: Synoptic environments associated with tornadoes in northern Arizona. Preprints, 24th Conference on Severe Local Storms, Savannah, GA., Amer. Meteor. Soc.

Blanchard, D. O. 2013: Comparison of wind speed and forest damage associated with tornadoes in northern Arizona. Wea. Forecasting, 28, 408–417.

Yet another tornado for Northern Arizona

Last week another tornado moved across northern Arizona. The damage produced by this tornado was confined to a small area and was nothing like the tornado outbreak of 06 October 2010.

What makes this tornado interesting is where it occurred. The tornado formed just west of the San Francisco Peaks and then moved up the west flank of Humphreys Peak. Damage was isolated along the early portion of the path at around 9000 feet. As it moved up the mountain, damage increased in intensity and coverage with the peak damage occurring near 9800 feet. Farther up the mountain isolated damage continued until the tornado dissipated near 10500 feet.

The worst damage occurred as the tornado crossed the Humphreys Trail just a short distance from the trail register. In the first few days after the event travel along this section of the trail can best be characterized as challenging. Climbing over and under downed trees –  and around others — certainly slows the hiking pace.

 

Somewhere under all those trees is the Humphreys Trail.
Somewhere under all those trees is the Humphreys Trail.Â

This is not the highest elevation tornado of record. That belongs to a tornado that occurred in the Sierra Nevada mountains a few years ago with evidence of damage at elevations above 12000 feet.

The  limited damage inflicted upon the mountain might never have been noticed if it had not passed over a heavily used hiking trail. Now that’s serendipity. Or bad luck.

Nobody saw the tornado so all we have to go on is the radar data. And according to that information the event occurred between 5:25 and 5:35 pm on 14 September 2011.

Tornado outbreak in northern Arizona

While tornadoes are not as common in Arizona as in, say, the mid-western United States, neither are they rare. In an average year, about six tornadoes are reported in Arizona. It’s a near certainty that this number is an undercount of the actual number of tornadoes that occur. Because much of Arizona is sparsely populated, it’s possible — likely, even — that many tornadoes are not seen and thus, not reported.

So, while it may seem like the tornado outbreak in northern Arizona that occurred on October 6, 2010, was an extreme event, we can not be certain that it was. We can only be certain that it was one of the most extreme “reported” events.

At least eight tornadoes have been confirmed. Based on a review of the Doppler weather radar data there is a reasonable probability that additional tornadoes occurred in the vast Ponderosa forests of northern Arizona and the damage paths have not yet been noted and surveyed. While conducting a formal damage survey on October 7 and 8 for two of the already confirmed tornadoes, we found additional tornado damage paths that had not yet been previously identified.

Recent research has identified a repeatable weather pattern that is responsible for a significant fraction of tornadoes in northern Arizona. As that weather pattern began to develop, warning forecasters began to carefully monitor Doppler radar looking for signatures of severe thunderstorms. And when these signatures appeared, they quickly issued Tornado Warnings. It is almost certain that these warnings minimized injuries in the paths of these tornadoes.

Damages homes from the Bellemont, Arizona, tornadoes on October 6, 2010.
Damaged homes from the Bellemont, Arizona, tornadoes on October 6, 2010.

Flattened forest of Ponderosa Pine in northern Arizona from the tornadoes on October 6, 2010.
Flattened forest of Ponderosa Pine in northern Arizona from the tornadoes on October 6, 2010.
Forest Service crews quickly cleared treefall from Forest roads.
Forest Service crews quickly cleared treefall from Forest roads.

Some of the research on tornadoes in northern Arizona can be found at these links sponsored by the American Meteorological Society:

Blanchard, D.O., 2006: A cool season severe weather episode in northern Arizona. Preprints 23nd Conference on Severe Local Storms, St. Louis, MO., Amer. Meteor. Soc.

Blanchard, D.O., 2008: Synoptic environments associated with tornadoes in northern Arizona. Preprints 24th Conference on Severe Local Storms, Savannah, GA., Amer. Meteor. Soc.

Blanchard, D.O, 2010: Forest damage associated with tornadoes in northern Arizona. Preprint 25th Conference on Severe Local Storms, Denver, CO, Amer. Meteor. Soc.