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.

Morning Fog at Mormon Lake

A few days of tropical moisture—remnants of Tropical Depression Nora—brought widespread rainfall to much of northern Arizona. As the moisture was replaced by drier air in the middle- and upper-levels but was unchanged in the near-surface layer, we were left with a situation in which fog might form in the early morning hours.

 

So, up before dawn to drive out to Mormon Lake and the surrounding basin. And there was fog—at least in one corner of the basin. Here is a 30-minute time lapse compressed down to 17 seconds showing the movement of the fog as it sloshes back and forth.

Sunrise through morning fog.
Sunrise through morning fog.

As this was taking place, the sun rose through a shallow layer of fog.

Sunset at Grand Canyon.
Sunset at Grand Canyon.

Bonus: on the previous evening the sun was setting through layers of smoke from the western wildfires.

Edit: updated image.

Lightning and Reflections

We have been in a “monsoon break” in northern Arizona for a week or so, but there can still be some convection during these periods. A few days ago, some isolated showers and thunderstorms developed to the north of the San Francisco Peak in the late afternoon and early evening.

Lightning reflected in the ponds of the Kachina Wetlands.
Lightning reflected in the ponds of the Kachina Wetlands.
Lightning reflected in the ponds of the Kachina Wetlands.
Lightning reflected in the ponds of the Kachina Wetlands.

This was a good opportunity to capture images of thunderstorms and lightning with reflections in the ponds of the Kachina Wetlands.

Radar image of storms north of the San Francisco Peaks.
Radar image of storms north of the San Francisco Peaks.

Above is an image showing the thunderstorms and lightning north of the peaks.

One more thing: the mosquitos were pretty intense—but it was worth it.

Is the Monsoon Over Already?

An early-season trough of low pressure moved across the west on Wednesday and Thursday disrupting the winds associated with the North American Monsoon. As the trough moved across the area, copious amounts of sub-tropical moisture surged northward across Arizona and surrounding areas. Widespread rain with embedded thunderstorms occurred across the area with areas of heavy rainfall and localized flash flooding.

Water vapor satellite imagery for 0210 UTC 20 August 2021.
Water vapor satellite imagery for 0210 UTC 20 August 2021.
500-mb analysis for 0000 UTC 20 August 2021 (image courtesty NCAR/RAL).
500-mb analysis for 0000 UTC 20 August 2021 (image courtesty NCAR/RAL).

But now the trough has moved northeastward across the Rocky Mountains and into the northern Plains states. The moisture across the southwest has been swept away. This can be seen in the water vapor satellite image. The warm colors are associated with a drier air mass and this has overspread the southwest.

Medium range models such as the GFS (Global Forecast System) suggest that moisture and showers will not return to the area before the end of the month leaving us with an extended break in rainfall.

Hurricane Grace at 1800 UTC 20 August 2021. (Image courtesy TropicalTidbits.com)
Hurricane Grace at 1800 UTC 20 August 2021.
72-h forecast for the remnants of tropical cyclone Grace.
72-h forecast for the remnants of tropical cyclone Grace. (Image courtesy TropicalTidbits.com)

But there are always some features that can result in different outcomes. For example, Hurricane Grace is moving westward across the Gulf of Mexico and is expected to make landfall in Mexico. The remnants of this tropical cyclone will continue to move to the west and may emerge in the Pacific south of the Gulf of California. If this happens, there might be a surge of moisture moving northward toward Arizona that could bring a return of showers and thunderstorms sooner than models currently suggest.

Sunset Convection and Lightning

It was a pleasant evening in Sedona watching thunderstorms as the sun sank lower in the western sky. It was mostly clear in that direction allowing sunlight to illuminate storms in the east. This is one of my favorite setups: clear in the west and stormy in the east.

Early evening sunlight illuminates thunderstorms and Cathedral Rock in Sedona.
Early evening sunlight illuminates thunderstorms and Cathedral Rock in Sedona.
Early evening sunlight illuminates thunderstorms.
Early evening sunlight illuminates thunderstorms.
Lightning from a distant storm after sunset.
Lightning from a distant storm after sunset.
Lightning from a distant storm after sunset.
Lightning from a distant storm after sunset.

The setting sun produced wonderful pastel colors on the clouds and occasionally illuminated the rock spires and buttresses in the middle distance. And after sunset, distant storms showed large anvils along with occasional bolts of lightning.