May 31st, 2025: Showers and Thunderstorms Across Arizona Through Sunday Night

Synopsis

An upper level cutoff low will remain anchored off the coast of Northern Baja California today before lifting rapidly northward across Arizona tomorrow. This feature is drawing tropical moisture northward from former EastPac TC Alvin into the Southwest CONUS. The majority of shower and storm coverage will remain in Mexico and extreme Southern California through this evening, but by tomorrow precipitation chances will shift into Arizona as the aforementioned low lifts northward across our region. Models forecast impressive precipitable water values near 1.50 inches tomorrow which would be record breaking for June 1st in AZ. A combination of synoptic scale ascent, deep moisture, and modest instability will generate numerous showers with embedded thunderstorms across the state tomorrow afternoon and evening. Total rainfall will be highly variable and dependent on the frequency and intensity of showers and storms. In general, expect 0.1 to 0.5 inches across the lower deserts with higher amounts likely near the International Border in Pima and Santa Cruz Counties. 

Current Conditions

Northern Mexico Regional visible satellite imagery as of 11:45AM MST this morning displays post-tropical storm Alvin just south of Cabo San Lucas as well as showers and thunderstorms moving northwestward into extreme Southern California.

GOES-19 visible imagery valid at 11:45AM MST courtesy of College of Dupage.

Moisture from the remnants of Alvin is being drawn northward thanks to a closed low off the coast of Northern Baja.

12z GFS 500mb analysis this morning courtesy of Tropical Tidbits.

SPC mesoanalysis of precipitable water shows moisture increasing from the south, but it is still quite dry across Arizona.

SPC mesoanalysis of total precipitable water as of 12PM MST.

Before I get into the forecast, let's take a moment to admire this weather pattern as it really takes a synoptic miracle to get significant precipitation this time of year in this part of the country. First, the EastPac decides to spawn a TC at the end of May and a cutoff low decides to park itself in the perfect spot to transport moisture from that TC into the area. Weather truly is amazing. Anyway, let's get into the forecast details.

Forecast: Today and Tonight

I'm not going to spend too much time on the forecast for today through tonight as most of the activity should remain mainly south of the border. The only thing to expect through tonight is scattered mid and high clouds with maybe an isolated cell or two near the Border. 

Forecast: Tomorrow

Synoptic Scale Pattern & Dynamics

By tomorrow, the closed low off the coast of Baja will lift northward across Arizona in response to a Pacific trough deepening along the West Coast. As the upper level low lifts northeastward, synoptic scale ascent induced by differential cyclonic vorticity advection (CVA) will occur across the state.

12z GFS 500mb cyclonic vorticity valid at 11AM MST tomorrow courtesy of Tropical Tidbits.

In addition, this pattern will be conducive for deep tropical moisture to be transported into Arizona with the UA WRF-GFS showing 700-300mb IVT on the order to 200 to 300 g/kg m/s tomorrow afternoon.

12z UA WRF-GFS 700-300mb IVT valid for 4PM MST tomorrow courtesy of the UA Power Forecasting Group.

Overall, the synoptic scale pattern is conducive for large-scale ascent and moisture transport across Arizona tomorrow.

Thermodynamics

Moisture will definitely not be an issue with models forecasting total precipitable water between 1.25 and 1.75 inches across Southern Arizona tomorrow.

12z HRRR total precipitable water valid for tomorrow at 5PM MST courtesy of Pivotal Weather.

These moisture values would most definitely break records for June 1st based on Tucson sounding climatology shown below.

Tucson sounding climatology of precipitable water courtesy of the SPC.

The model soundings for KTUS and KPHX show an incredibly moist troposphere with surface dew points in the low to mid 60s!

12z HRRR KTUS sounding valid for 5PM MST tomorrow courtesy of Pivotal Weather.

12z HRRR KPHX sounding valid for 5PM MST tomorrow courtesy of Pivotal Weather.

The main issue for tomorrow is the messy low and mid level lapse rates which are typical with deep moisture. In addition, models show significant cloud cover during the morning and early afternoon hours which would inhibit surface insolation and therefore mixing of the boundary layer.

12z HRRR total cloud cover valid for tomorrow at 11AM MST courtesy of Pivotal Weather.

Albeit, there is still plenty of low level moisture for MUCAPE values on the order of 500 to 1000 J/kg, but SBCAPE and MLCAPE will remain below 500 J/kg due to poor low level lapse rates. 

Timing, Impacts, and Rainfall Amounts

Based on HRRR, UA WRF, and HREF guidance, bands of showers will begin after midnight tonight in extreme Southern Arizona with coverage gradually increasing after sunrise and throughout the day. The greatest coverage of showers and scattered thunderstorms in Yuma and Maricopa Counties (including Phoenix) will be during the late morning through late afternoon hours. Southeast Arizona (including Tucson) will experience the greatest coverage of precipitation during the mid afternoon through the late evening.

Bands of showers could have a few embedded thunderstorms, but I wouldn't expect anything more than occasional lightning, locally gusty winds, and brief heavy rainfall. To be honest, tomorrow should be a good soaker for a state that is in a pretty bad drought currently. Flash flooding is unlikely, but localized flooding is definitely possible especially in poor drainage areas. Periods of heavy rainfall are possible especially in Southeast California where the 12z HREF ensemble mean shows 30 to 50% probabilities of 3-hour rainfall exceeding one inch.

12z HREF 3-hr mean QPF and probability of QPF in excess of one inch valid for 8PM MST tomorrow courtesy of the SPC.

There could also be periods of brief heavy rainfall in Santa Cruz and eastern Pima County (including Tucson) as the 12z HREF ensemble mean shows 0.10 to 0.25 inch per hour rain rates tomorrow evening.

12z HREF ensemble mean 1-hour QPF valid for 8PM MST tomorrow courtesy of the SPC.

 

Am using the 12z HREF ensemble mean 48-hour total precipitation here as it seems to be the most reasonable as opposed to individual CAM solutions. I would expect between 0.1 and 0.5 inches across much of the lower deserts with the highest amounts expected in Southeast California as well as Pima County (including Tucson). 

12z HREF ensemble mean 48-hour total precipitation forecast valid from 12z this morning though 12z June 2nd.