Rare Early June Rainfall

                     Rare Early June Rainfall: June 1-4, 2025 

Atmospheric ingredients came together at just the right time to produce a very rare rainfall event across much of the Desert Southwest during the first few days of June. During the last few days of May, an area of low pressure was stationed near the northern Baja Peninsula and this feature combined with moisture remnants from Tropical Storm Alvin, which developed off the west coast of Mexico and moved northward before dissipating near the southern Baja Peninsula (Fig. 1). Precipitable water values (a measure of the amount of atmospheric moisture in a column of air) reached upwards of 300% of normal, or above 1.00”, which for early June is near record levels (Fig. 2).  The combination of the upper-level forcing from the low pressure as it migrated northeastward into the region and the tropical moisture surge resulted in widespread light to moderate rainfall activity across much of central and southern AZ on Sunday, June 1st. The activity began before daybreak and lasted through the evening hours before the bulk of the activity shifted northward into the northern half of AZ during the overnight and early morning hours on Monday, June 2nd. As the center of the low moved into south-central AZ during the overnight hours, a concentrated area of moderate to locally heavy rainfall activity developed and affected much of the northern half of the Phoenix metro area through the foothills and higher terrain areas to the north and east (Fig. 3). 

Fig. 1. GOES-16 Water Vapor (mid level) satellite image between 12 AM MST May 31 and 9 AM MST June 2, 2025

Fig. 2. Animation loop of the Precipitable Water Values between 11 PM MST May 31 and 5 AM MST June 4, 2025

Fig. 3. Radar reflectivity loop between 6:45 PM MDT May 31 and 9:45 AM MDT June 2, 2025

A secondary low pressure system originating from the northern Pacific traversed the region between Tuesday, June 3rd and Wednesday, June 4th (Fig. 4). Abundant low-level moisture was still present as precipitable water values ranged between 0.7-1.0” across the higher elevations to between 1.0-1.4” across the lower elevations and thus the combination of the lingering moisture and the upper-level forcing from the approaching low resulted in another round of widespread shower and thunderstorm activity. Thunderstorm activity developed during the afternoon hours on June 3rd across much of northern AZ through the southern Great Basin Region before expanding into portions of southeast CA during the evening hours and into portions of southwest and south-central AZ during the overnight hours (Fig. 5). 

Fig. 4. GOES-16 Water Vapor (mid level) satellite image between 12 PM MST June 2 and 5 PM MST June 4, 2025

Fig. 5. Radar reflectivity loop between 10:45 AM MDT June 3 and 9:45 AM MDT June 4, 2025

During the three-day period between June 1st and June 4th, most areas observed between 0.25-0.75” of rain with some localized areas observing rainfall amounts in excess of 1.00” where thunderstorm activity was more persistent (Fig. 6). The overall impacts from the multiple days of rain were generally minor with isolated flooding observed in the form of road closures where the more persistent rainfall activity fell (Fig. 7). This multi-day rainfall event did result in minor improvements to the overall drought conditions with the biggest improvements seen in the areas that observed the more significant rains (Fig. 8). Unfortunately,  most of the region still remains under severe to extreme drought conditions given the long term drought that has been in place since last summer as a result of the drier than normal 2024 Monsoon and 2025-2025 Winter.   

Fig. 6. Total rainfall amounts for the period between 12 AM MST June 1 and 10 AM MST June 4, 2025

Fig. 7. Spatial extent of the watches and warnings issued by the NWS offices in Phoenix, Tucson, Flagstaff, and Las Vegas. Locations where flooding was reported noted with the green dots.

Fig. 8. Depiction of drought conditions via the U.S. Drought Monitor on May 27 and June 10, 2025

How rare is this early June rainfall?

Since 1896 and prior to this year, during the time period between May 24th and June 7th, Phoenix has only observed measurable rain for 38 separate days while Yuma has only observed measurable rain on 11 separate days (Fig. 9)! Not surprisingly, given the rarity of rain in early June, both Phoenix and Yuma easily broke their daily rainfall record for June 1st at 0.24” and 0.10”, respectively, with the previous records being 0.02” and a trace. During the three days, Phoenix accumulated 0.45” of rain, setting the stage as the 2nd wettest start to June and the 7th overall wettest June on record (Fig. 10). 

Fig. 9a. The number of days each year from 1896 through 2025 that measurable precipitation (at least 0.01”) was observed in Phoenix, AZ between May 24 and June 7. Fig. 9b. Same as the top figure but for Yuma, AZ. 

Fig. 10. (Left) Total Precipitation Ranking for the month of June for Phoenix. (Right) Total Precipitation ranking for the first four days of June for Phoenix.