Review of the 2025 Monsoon Across the Southwest U.S.
1. Overview
For the third consecutive year, a significant portion of the Desert Southwest experienced below normal rainfall during the 2025 North American Monsoon (June 15 - Sept 30). Below normal rainfall was particularly evident in the Great Basin and Four Corners area, extending through northern and eastern Arizona, and the western third of New Mexico. Conversely, central and eastern New Mexico, as well as southern California through south-central Arizona, including the metropolitan areas of Phoenix and Yuma, recorded above-normal rainfall (Figs. 1-2). The above normal rainfall observed across these areas can be tied to a couple of intense rainfall events that occurred mostly during the month of September. Consistent with the long term trend, temperatures were well-above normal across much of the region (Fig. 3).
Fig. 1. Precipitation ranking for June - September 2025. Source: WestWide Drought Tracker.
Fig. 2. Percent of average precipitation between June 15th and September 30th, 2025. Source: CLIMAS/OSU PRISM.
Fig. 3. Temperature ranking for June - September 2025. Source: WestWide Drought Tracker.
Lightning data is one way of assessing how active a particular monsoon was. Not surprisingly, given the mostly negative rainfall anomalies, lightning activity for 2025 was below average across Arizona. Data from the Vaisala National Lightning Detection Network (NLDN) shows the 2025 Monsoon having the third fewest total lightning strikes across Arizona since 1990 with just over 200,000 total strikes (Fig. 4). Approximately two-thirds of the total lightning activity across Arizona occurred from the middle of August through the end of September, coinciding with the most active portion of the 2025 Monsoon Season (Fig. 5).
Fig. 4. Total lightning strikes across Arizona for the monsoon season for each year between 1990 - 2025. Source: NLDN.
Fig. 5. Cumulative day-to-day total lightning strikes across Arizona for the monsoon season for each year between 1990-2025. Source: NLDN
In contrast to the 2022-2023 and 2023-2024 winter seasons, which experienced above normal precipitation, the 2024-2025 Winter Season registered well-below normal precipitation across much of the region. Combined with a very dry 2024 fall, drought substantially worsened across much of the southwestern U.S. with many areas falling under severe to extreme drought conditions (i.e. D2 or greater). Areas that registered below-normal monsoon rainfall observed unchanged drought conditions or worsened, whereas, areas that registered above-normal rainfall observed a slight improvement. (Fig. 6).
Fig. 6. 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. 7). As it drifts northward during the course of the summer months, it causes 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 and thus an increase in showers and thunderstorms.
Fig. 7. Animation showing the typical development, northward migration, and ultimate demise of the “monsoon high” through the summer. The black contours and red/blue colorfill represent the average height (in meters) of the 500 mb pressure level, representing the vertical middle of the atmosphere. The blue/magenta/cyan lines represent precipitable water (PWAT, the depth of water if water vapor in a vertical column of the atmosphere above a specific location is condensed into liquid) values of 1.0/1.5/2.0”. The inset trace is for Phoenix, AZ, showing the seasonal PWAT trend. Graphic produced by Paul Iñiguez, former Science and Operations Officer at NWS Phoenix. Data source: NCEP NARR.
The Monsoon season offers a temporary respite from the intense heat prevalent across much of the Desert Southwest during the summer months. The influx of moisture increases cloud cover, leading to periodic showers and thunderstorms. This helps limit surface heating and spread rain-cooled air to the ground, resulting in a temporary reduction in surface temperatures. For most of the Desert Southwest, approximately 50% of the annual precipitation occurs during the June through September period. Overall, the percentage of annual rainfall 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. 8).
