2020 Monsoon Review

Review of the 2020 Monsoon Across the Southwest U.S.
Compiled by NOAA/NWS Weather Forecast Office Phoenix, AZ and CLIMAS. Published Jan. 6, 2021.
Note: click on images for full-resolution versions.

Overview

On the heels of one of the drier monsoons on record, this year's established a new record for hottest and driest across the Southwest US (here defined as Arizona, New Mexico, and far west Texas; Figure 1) based on monthly data (Jun-Sep, since 1895) from the NOAA National Centers for Environmental Information (NCEI, link). Monthly temperature values were derived from the daily average temperature values (high plus low, divided by two) at climate stations across the region, averaged into sub-regional values then compiled into one average regional value. The four monthly values were then averaged for a final seasonal value. Similarly, daily precipitation data from individual stations are summed and aggregated into sub-regional than regional monthly values.

Fig 1. Areal extent of “Southwest” for purposes of seasonal precipitation and temperature rankings. 

Rainfall across the Southwest during the monsoon months of Jun-Sep in 2020 was the driest on record (since 1895), with a value of 2.97” (average is 6.68”). The previous driest season was 3.40” in 1956. Last year (2019) was the eighth driest at 4.69”. For temperature, the 2020 monsoon was the hottest on record for the Southwest with an average temperature of 77.1 °F, significantly beating the previous record of 76.8 °F in 2011 (average is 74.3 °F). The full time series for precipitation and temperature values is shown in Figure 2.

Fig. 2. Average precipitation (top) across the Southwest during the monsoon months of Jun-Sep
by month/year and average temperature (bottom). 2020 was a record for both. Data source: NCEI.

Widespread thunderstorm activity is often associated with the monsoon. A metric to measure such activity is the number of lightning strikes across a region. Utilizing data from Vaisala’s National Lightning Detection Network (NLDN), 1.2 million cloud-to-ground strikes were observed during Jun-Sep 2020 broadly across Arizona, New Mexico, west Texas, and northern Mexico. This was the fewest on record (since the mid-1990s), with the previous low of 1.9 million occurring in 1995 (Figure 3).

Fig. 3. (Left) Total cloud-to-ground lightning strikes across the region (inset map, right) during
Jun-Sep by year. Map (right) displays the relative frequency for 2020, with red (blue) areas
observing less (more) lightning activity than usual. Notably, only above average activity
as observed closer to central Mexico and far northeast New Mexico. Data source: NLDN.

Typical Monsoon Pattern

During the monsoon, high pressure in the mid-levels of the atmosphere over northern Mexico (often referred to as the “monsoon high”) strengthens, expands, and drifts northward, causing a reversal in the flow of weather patterns across much of the Southwest (Figure 4). The switch from generally a west-to-east flow in the spring months to a southeast-to-northwest flow in the summer causes a significant increase in atmospheric moisture levels across the typically parched desert region. Combined with the heat of the summer, conditions become favorable for intermittent periods of showers and thunderstorms. For the region as a whole, nearly 50% of annual precipitation falls during the Jun-Sep period. That amount varies significantly though, from near 30% across northwest Arizona to near 60% across much of New Mexico and west Texas (Figure 5). The increase in clouds, showers, and thunderstorms also provides relief from the region’s relentless heat by blocking out sunshine and spreading rain-cooled air across the surface.

Fig. 4. Average 500 mb heights (m) (black contours and color shading) and precipitable water (in)
levels (blue/purple contours) during the monsoon months Jun-Sep. Note the increase in strength
northward push of the 500 mb high from June to July and the subsequent northward increase in
moisture. Such conditions begin to quickly erode in September.
Data source: NCEP North American Regional Reanalysis (NARR).

Fig. 5. Percent of average annual rainfall (based on 1981-2010 data) that falls during the primary monsoon
months of Jun-Sep. Regional average is approximately 50%, though ranges from roughly 30% in
northwest Arizona to nearly 60% for much of New Mexico and far west Texas. Data source: NCEI.

The 2020 Monsoon Weather Patterns

Through June and into early July, the monsoon high struggled to push north as it typically does due to a semi-persistent trough of low pressure off the West Coast (Figure 6). This low would periodically push inland at times throughout July, suppressing the monsoon high and continuing to limit subtropical moisture from flowing north.

By late July and into August, the monsoon high did make several attempts to build north across the Southwest, but typically stalled just south and west of the climatologically favored position over the Four Corners. This resulted in the center of the monsoon high positioned over central Arizona, helping to suppress showers and thunderstorms while increasing surface temperatures. Deep subtropical moisture generally was still constrained to the south, with few “gulf surge” events (a mechanism where moisture “surges” into Arizona from the Gulf of California) occurring. 

A brief window of enhanced tropical activity in the East Pacific Ocean did spawn several tropical cyclones in late August that helped bring some moisture into Arizona. In particular, very moist air briefly moved into the Southwest at the end of August due to influences from Tropical Storms Hernan and Iselle, resulting in widespread precipitation.

By September, the monsoon high, which struggled all summer to bodily move northward, was already sinking south. A strong front passed through the region on September 8th and 9th, bringing smoke from wildfires in California and pushing any remaining moisture back to the south.

Fig. 6. Five day moving average of 500 mb height (m) and daily average
precipitable water anomaly (mm) for Jun-Sep 2020. Data source: NARR.

Fig 7. Percent of average precipitation (Jun 15 - Sep 30, 2020). Data source: OSU PRISM.

Precipitation anomalies (Figure 7) were especially low across northern and western Arizona, with many areas along the Colorado River observing no precipitation at all during the 2020 monsoon. Some locations from far southern Arizona through northeast New Mexico observed near average precipitation amounts (75-125%) but were isolated in nature and associated with locally heavy thunderstorm events. Overall, more than 90% of Arizona and New Mexico finished the season at less than 75% of average precipitation (Figure 8).

Fig 8. Daily coverage of precipitation anomaly categories across Arizona and New Mexico. Data source: OSU PRISM.

Daily monsoon activity in terms of coverage of precipitation across Arizona was generally suppressed through the whole season with only brief periods of widespread rain occurring in late July and again in late August (Figure 9). New Mexico observed more widespread precipitation events overall, though they were limited to similar periods in late July and late August.

Fig 9. Daily percent coverage of Arizona (left) and New Mexico (right) observing
>0.01” of precipitation (Jun 15 - Sep 30, 2020). Data source: OSU PRISM.

Overall, areas in eastern New Mexico and upper elevation locations in western New Mexico and eastern Arizona saw the most frequent precipitation activity throughout the season with a third to almost half of the days observing at least 0.01” (Figure 10). Low desert areas in Arizona observed the least activity with precipitation occurring on less than 10% of days through the season.

Fig 10. Percent of days with precipitation >0.01” (Jun 15 - Sep 30, 2020)
across the Southwest. Data source: OSU PRISM.

Analysis of Factors in the 2020 Monsoon Weather Patterns

In a review of meteorological data, it was found that the following factors played some role in the weather patterns and outcomes during the 2020 monsoon:
 

• Unfavorable evolution and strength of the monsoon ridge.

• Abnormally strong northerly winds.

• Unusually low levels of moisture.

• Average tropical cyclone activity in the Eastern Pacific Ocean

• Extended periods of opaque smoke at high altitudes due to extensive wildfire activity across the West.

Unfavorable Evolution and Strength of the Monsoon Ridge

To put the 2020 monsoon in a historical context, an analysis of 500 mb heights was conducted. This represents approximately the middle of the atmosphere, a standard metric used by meteorologists. Utilizing gridded reanalysis data, the frequency of 500 mb heights above 5900 m, which is the average height for the region during the monsoon months Jun-Sep, was calculated. It was found that roughly 65% of Jun-Sep 2020 saw heights at or above this threshold (Figure 11), which was near record levels but similar to the past several seasons. Assessing the day-by-day coverage of these elevated heights, it was found that extensive periods of 100% coverage occurred. This suggests that stronger-than-usual high heights were very common this summer, which can act to suppress thunderstorm activity across the region. Visualizing the spatial extent of these higher heights, the relative frequency was found to be much higher on the western and southern fringes of the region, indicating the enhanced potential for thunderstorm activity to be suppressed on the fringes of the monsoon high.

Abnormally Strong Northerly Winds

An analysis of wind data at 500 mb found persistent and unusually strong northerly winds across the Southwest (Figure 12). This is reflective of the several weather systems that skirted the region to the northeast, moving from the northern mountain states toward the central and southern plains states. Related, enhanced westerly winds were found across northern Arizona and New Mexico with enhanced easterly winds across northern Mexico, forming an anticyclonic signature indicative of the atypical position of the monsoon high.

at 500 mb for Jun-Sep 2020. Seasonal average wind (barbs) also plotted. Data source: NARR.
Fig. 12. Anomalies (shading/contours) for east-west (left) and north-south (right) wind components

Unusually Low Levels of Moisture

Taken as a whole, atmospheric moisture levels were very low across Arizona, New Mexico, and the northern areas of the Gulf of California during the 2020 monsoon (Figure 13). Further south, toward central Mexico, moisture levels were unusually high. This was likely a result of the wind patterns experienced during the 2020 monsoon (see previous section). With limited moisture, atmospheric instability levels were also depressed, resulting in less potential for showers and thunderstorms.

Fig. 13. Standardized anomaly values of precipitable water. Data source: NARR.

Typical Tropical Cyclone Activity

There were 17 tropical cyclones in the Eastern Pacific Ocean from June through September in 2020 (Figure 14). This was slightly ahead of 2019 (16) and average compared to the past 50 years. When looking at tropical cyclones that moved into a favorable area to potentially impact the Southwest as depicted by the red box in Figure 14 that includes the Gulf of California and Baja California peninsula, there were two such storms in 2020 - on par with the past several years. Given this, tropical cyclone activity was likely not a significant factor in this monsoon.

Extended Periods of Opaque Smoke at High Altitudes

During calendar year 2020, the southwest region saw the most acres of land burned due to fires this century (Figure 15), with over one million acres burned for the first time since 2002. Several extended periods during the 2020 monsoon were punctuated with notable layers of elevated thick smoke. Although not quantified in this analysis, published research has shown smoke particulate matter can inhibit the process in which water droplets form and grow. This in turn reduces the ability for raindrop formation and ultimately can lower rainfall amounts. Smoke can also reduce the amount of sunshine reaching the earth’s surface, which lowers the heating of the near-ground air and thus reduces the amounts of atmospheric instability. Therefore, it is reasonable to assume that smoke ultimately played some role in reducing rainfall amounts.

Fig. 15. Acres of land burned across the southwest (Arizona, New Mexico, and west Texas) by year. The 
highest amount in the past twenty years was observed in 2020 (1.07 million, previous high 1.05 in 2002).
Data from the NIFC Southwest Coordination Center.