2024 Monsoon Season Review

Review of the 2024 Monsoon Across the Southwest U.S.

 Compiled by NOAA/NWS Phoenix. Published December 31, 2024.

Note: Click on images for full-resolution versions. 

1. Overview

Similar to the 2023 Monsoon, a large portion of the Desert Southwest experienced below to well-below normal rainfall during the 2024 Monsoon (Fig. 1). In particular, most of south-central and western Arizona, southeast California, and the southern Great Basin were the areas that observed well-below normal rainfall. In fact, many of these areas received less than 30% of normal precipitation (Fig. 2). Areas that observed near to above-normal rainfall include southwest Colorado and northern New Mexico, as well as small portions of northeast and southeast Arizona. 

Fig. 1. Precipitation ranking for June-September 2024. Source: WestWide Drought Tracker.

Fig. 2. Percent of average precipitation between June 15th and September 30th, 2024. Source: CLIMAS/OSU PRISM.

The higher terrain areas in northern and eastern Arizona, as well as New Mexico and the Four Corners Region experienced the most frequent precipitation activity during the monsoon season with 30-60% of the 107 days observing at least 0.01”. On the other hand, most of south-central and southwest AZ, southeast CA, and the southern Great Basin observed less than 10% of days with measurable precipitation. Areas along the Lower Colorado River Valley observed nearly zero days of measurable precipitation during the 107-day stretch (Fig. 3). 

Fig. 3. Percent of days with measurable precipitation (>0.01 in) between June 15th and September 30th, 2024. Source: CLIMAS/OSU PRISM.

Lightning data is one way of assessing the activity of a particular Monsoon season. Not surprisingly, given the mostly negative rainfall anomalies, lightning activity for 2024 was below average.  Data from the Vaisala National Lightning Detection Network (NLDN) shows the 2024 Monsoon having the seventh fewest total lightning strikes across Arizona since 1990 with around 350,000 total strikes (Fig. 4).

Fig. 4. Total lightning strikes across Arizona for the monsoon season for each year between 1990 - 2024. Source: NLDN.

Given the drier than normal conditions observed across the majority of the Desert Southwest, temperatures were significantly elevated. In fact, the majority of Arizona through the southern third of California and the southern Great Basin observed the warmest mean temperatures on record (Fig. 5). The main culprit for the record breaking temperatures was an anomalously strong area of high pressure centered over the southern Great Basin through the summer months (Fig. 6). This generally resulted in sinking motion across much of the region, limiting thunderstorm activity, and increasing surface temperatures due to the overall lack of cloud cover and rainfall. 

Fig. 5. Temperature ranking for June-September 2024. Source: WestWide Drought Tracker.

Fig. 6. 500 mb Geopotential Height Composite Anomaly for June-August 2024. Source: NCEP/NCAR Reanalysis.

Thanks to a wetter than normal 2023-24 winter season across much of the western U.S., the overall coverage of more significant drought (i.e. D1 or greater) remained confined mostly over New Mexico, as well as portions of central and southeastern Arizona prior to the start of the monsoon season. Unfortunately, due to the record hot temperatures and drier than normal conditions during the summer, the overall drought intensity increased in coverage to include much of the southwestern U.S.. By early October, a large area of moderate (D1) to severe (D2) drought conditions were observed across much of the southern Great Basin, southeast California, and western and south-central Arizona. Conversely, much of New Mexico through the Four Corners region observed an overall reduction in drought conditions due to above normal monsoon rainfall (Fig. 7).  

Fig. 7. Depiction of drought conditions via the U.S. Drought Monitor prior to and after the monsoon.

2. Typical Monsoon Pattern

During the monsoon, the subtropical high pressure (often known as the “Monsoon High”) strengthens, expands, and drifts northward (Fig. 8). As it drifts northward during the course of the summer months, it will cause a reversal in the overall wind pattern from west-to-east to east-to-west. This reversal in the wind pattern results in a significant increase in atmospheric moisture across the southwestern U.S. This moisture combined with the summer heat results in a favorable pattern for periodic episodes of shower and thunderstorm activity across the region. 

Fig. 8. Animation showing the typical gradual development, northward migration, and ultimate demise of the “monsoon high” through the summer. The black contours and red/blue colorfill represent the height (in meters) of the 500 mb pressure level, representing the vertical middle of the atmosphere. The cool/blue (warm/red) colors in the north (south) represent lower (higher) density air; temperature is directly proportional to pressure, thus blue (red) colors are associated with cool (warm) air. The blue/magenta/cyan lines represent precipitable water values (PWAT, the amount of water vapor in the atmosphere if condensed into liquid) of 1.0/1.5/2.0”. As the monsoon high builds, greater PWAT values move north. The inset trace is for Phoenix, AZ, showing the seasonal moisture trend. Graphic produced by Paul Iñiguez, former Science and Operations Officer at NWS Phoenix. Data source: NCEP NARR.

For most of the region as a whole, approximately 50% of the annual precipitation occurs during the June to September period. However, that amount varies significantly from less than 20% across extreme western Arizona to almost 70% in the mountains of southern New Mexico and far west Texas (Fig. 9). The increase in clouds, showers, and thunderstorms provide relief from the region’s relentless heat by limiting the surface heat and spreading rain-cooled air down to the surface. 

Fig. 9. The percent of annual rainfall which arrives during the core monsoon months of June through September. Typically the season’s rains are more notable across New Mexico and far southeast Arizona. Graphic produced by Paul Iñiguez, former Science and Operations Officer at NWS Phoenix. Data source: NCEI NClimGrid.

3. The 2024 Monsoon Pattern

During July and August, the time frame when the bulk of the monsoon thunderstorm activity occurs over the southwestern U.S., the subtropical high was situated mostly near the Arizona/New Mexico border (Fig. 10). Even though this overall positioning was not enough to suppress thunderstorm activity over the mountains, it was enough to mostly suppress activity over the lower elevations of south-central and western Arizona through the southern Great Basin on most days due to strong subsidence. The overall positioning of the high also maintained the deeper moisture south of the international border through northern Mexico and thus the overall pattern was not favorable for rainfall-efficient thunderstorm activity (Fig. 11). 

Fig. 10. 500 mb Geopotential Height Composite Mean for the period of July 1st to August 31, 2024. Source: NCEP/NCAR.

Fig. 11. Precipitable Water Composite Anomaly for the period of July 1st to August 31st, 2024. Source: NCEP/NCAR. 

One of the mechanisms to aid in increasing moisture content over the southwestern U.S. during the monsoon season, and enhance shower and thunderstorm activity originates from moisture remnants of tropical systems that develop over the eastern North Pacific. In particular, any tropical system that tracks near the vicinity of the Baja California area will have the best potential to influence rainfall activity over the southwestern U.S.. During the June to September period, there were only 10 tropical cyclones that developed over the eastern North Pacific (Fig. 12). Only one tropical cyclone, Tropical Storm lleana that developed in mid-September, managed to track close to the Baja California area. However, most of its moisture remnants remained south of the region. Thus, tropical cyclone activity in the eastern North Pacific was a non-factor in the monsoon precipitation across the southwestern U.S. in 2024. 

Fig. 12. Tropical Cyclone Activity over the Eastern North Pacific for the period of June-September 2024. Source: NOAA NWS National Hurricane Center 

Significant Events During Monsoon 2024:

Despite there being a low frequency of thunderstorm days compared to average, there were some notable high impact events that occurred during this monsoon season in the NWS Phoenix county warning area (CWA), mainly across the Phoenix metro area from microburst wind damage:

July 24, 2024 - Phoenix Microburst Event

• An isolated severe thunderstorm near central Phoenix produced several damaging microbursts. One of the microbursts resulted in the complete collapse of a warehouse, resulting in a fatality. A full event writeup can be accessed here: July 24: Phoenix Microburst Event

August 6, 2024 - Scottsdale Severe Downburst

• During the evening hours of August 6, 2024, an isolated severe thunderstorm near the McCormick Ranch Neighborhood of Scottsdale resulted in significant tree damage.  A full event writeup can be accessed here: August 6: Scottsdale Severe Downburst

August 22, 2024 - Phoenix Metro Severe Thunderstorms and Dust Storm

• During the mid to late-afternoon hours of August 22, 2024, several thunderstorm clusters developed over the greater Phoenix area producing damaging microburst winds. This resulted in downed power lines as well as several large uprooted trees that caused damage to light posts, vehicles, and concrete fencing structures. In addition to the microburst wind damage, a large dust storm was observed in the Phoenix East Valley, reducing visibility to as low as a quarter of a mile at times. A full event writeup can be accessed here: August 22: Phoenix Metro Severe Thunderstorms and Dust Storm 

4. NWS Weather Warnings During the 2024 Monsoon

The number of Flash Flood Warnings issued by the local NWS offices across the southwestern U.S. on average was on the low side with just over 600 warnings issued regionally compared to over the 1000 total warnings issued during the more active 2021 and 2022 seasons (Fig. 13-14). In particular, areas from the western half of Arizona through California and the southern Great Basin observed the least number of warnings whereas areas such southeastern Arizona through New Mexico did observe more warnings. This conceptually matches with the overall rainfall anomaly pattern observed during the season with the least number of warnings occurring in areas that observed below normal rainfall and the greatest number of warnings occurring in areas that observed near to above normal rainfall. In terms of overall hotspots for Flash Flood Warnings, it was focused mainly over southeast Arizona, near the Tucson area, and over New Mexico, especially in the burn scar areas, with the rest of the region observing very little in the way of warnings (Fig. 15). 

Fig. 13. Number of Flash Flood Warnings issued per NWS office between June 15-September 30, 2024. Source: Iowa Environmental Mesonet

Fig. 14a. Number of Flash Flood Warnings issued per NWS office between June 15-September 30, 2021. Fig. 14b. Same as the top figure but for 2022. Source: Iowa Environmental Mesonet

Fig. 15. Monsoon 2024 Flash Flood Warning (FFW) Spatial Frequency.

The total number of the Severe Thunderstorm Warnings, on the other hand, was on the high side with over 900 warnings issued regionally during the season (Fig. 16). Similar to the Flash Flood Warning distribution, the highest number of warnings was observed mainly across southeast Arizona through New Mexico. Similar to previous years, the hot spot for Severe Thunderstorm Warnings was in the lower elevation areas adjacent to the higher mountain chains. This includes some of the major metropolitan areas such as Phoenix and Tucson as well as across the New Mexico High Plains. (Fig. 17). 

Fig. 16. Number of Severe Thunderstorm Warnings issued per NWS office between June 15 - September 30, 2024. Source: Iowa Environmental Mesonet

Fig. 17. Monsoon 2024 Severe Thunderstorm Warning (SVR) Spatial Frequency.

With the overall lack of significant rainfall across the majority of the Desert Southwest, extreme heat was once again a very prominent hazard in 2024 across the lower elevation areas with a total of over 40 Excessive Heat Warning Days across south-central and western Arizona (Fig. 18). Across Maricopa County, where the Greater Phoenix Metropolitan area is located, there were a total of 45 Excessive Heat Warning Days. This exceeds the 42 total warning days issued in 2023 and is second most to 2020, which had a total of 49 Warning Days. What made this year’s extreme heat unique was that it started in early June and was very persistent throughout the summer months and even into the early fall through early October. The early fall heat episode, which was unprecedented, included a 21 consecutive day stretch that saw Phoenix either tying or breaking a daily high temperature record as well as observing its first ever 110+ degree high temperature in the month of October. Despite the very prolonged heat stretch, the total number of heat-related fatalities in Maricopa County for 2024 so far is lower compared to 2023. According to the Maricopa County Department of Public Health, as of late October, the total confirmed number of heat-related fatalities was 466 vs. 569 confirmed by the same time period in 2023 with the caveat that there are approximately 191 deaths still under investigation. Nevertheless, the lower number of heat fatalities reported so far for 2024 most likely can be attributed to significant mitigation efforts by the Heat Relief Networks and especially given the prolonged extreme heat stretch observed throughout the summer and early fall. 

Fig. 18. Total number of Excessive Heat Warning Days per County in Arizona in 2024. Source: Iowa Environmental Mesonet