July 21st, 2025: ThunderSTORMS or ThunderSHOWERS in Tucson Today?

 Synopsis

Weak synoptic scale ascent combined with modest moisture and instability will generate scattered convection across Southeast Arizona (including Tucson) today. Main impacts from storms will be lightning, gusty winds, and brief heavy rainfall with localized flooding possible. Total precipitation is expected to be between 0.10 and 0.50 inches in the valleys of Southeast Arizona with higher amounts possible over the mountains.

Current Conditions

Visible satellite imagery overlaid with GOES GLM (Geostationary Lightning Mapper) flashes show scattered showers and isolated thunderstorms moving northward across Southeast Arizona this morning.

Visible satellite imagery overlaid with GLM flashes courtesy of College of Dupage.

The mid level low that has been anchored over extreme Southern California this weekend is finally lifting northeastward in response to a deepening mid latitude trough in the Pacific Northwest. In addition, the mid level anticyclone ("Monsoon High") is well to the east near Louisiana.

12z GFS 500mb analysis courtesy of Tropical Tidbits.

 

With the 500mb low to the west and a broad anticyclone to the east mid level southerly flow has slightly amplified.

Meanwhile, surface observations show dew points in the mid 50s to low 60s across extreme Southern Arizona thanks to an overnight gulf surge.

Surface observations as of 11:15AM MST courtesy of the NWS.

The 18z TUS sounding shows modest thermodynamics with 1.60 inches of precipitable water, but only around 100 J/kg of SBCAPE and unimpressive lapse rates with a near saturated 700 to 400mb layer.

18z TUS sounding courtesy of SPC.

However, this sounding is a bit contaminated from this morning's convection, so I anticipate at least a little recovery by later this afternoon.

Forecast Details

Synoptics/Dynamics

Morning convection is always a sign of dynamic forcing from either synoptic scale features or an MCV. In this case, much of this mornings activity is influenced by the synoptic scale pattern. Synoptic scale support will remain in place throughout the day today due to weak cyclonic vorticity advection (annotated in pink below) from the aforementioned mid level low lifting northeastward as well as subtle divergence near the tropopause.

12z GFS 500mb cyclonic vorticity valid for 11AM MST this morning courtesy of Tropical Tidbits.

12z GFS 250mb winds and streamlines valid at 11AM MST this morning courtesy of Tropical Tidbits.

This isn't very impressive synoptic scale forcing and quite subtle as well, but during the monsoon any little bit of synoptic scale support can be significant for storm coverage.

Moisture and Instability

There is quite a bit of uncertainty regarding thermodynamics today due to morning convection and mid level clouds. Even though synoptic scale dynamics assist in large scale ascent, it can sometimes cause problems with initiating convection during the morning hours and inhibit surface heating. The 12z suite of models seems to be too aggressive with thermodynamics since it did not resolve the morning convection. The 17z HRRR seems to be the most reasonable so will use that here. 

17z HRRR forecast sounding near Tucson valid at 2PM MST this afternoon courtesy of Pivotal Weather.

The amount of CAPE will be dependent on pockets of surface heating, so I am only expecting around 500 J/kg of CAPE by mid afternoon.

What to Expect

In general, expect an active day across Southeast Arizona. The main forecast uncertainty is going to be precipitation amounts especially near Tucson. All model solutions forecast measurable precipitation in Tucson this afternoon, but differ significantly on the amount. Rainfall accumulations across Southeast Arizona will really be dependent on how much instability can develop by this afternoon. Synoptic scale forcing will be sufficient enough for at least light to moderate rainfall, but strong thunderstorms capable of producing heavy rainfall is uncertain at this time. I used the 17z HRRR above for the likely thermodynamic environment by this afternoon, so will also show the 12 hour accumulated precipitation forecast (10AM to 10PM MST) below.

17z HRRR 12 hour accumulated precipitation from 10AM to 10PM MST today courtesy of College of Dupage.

Again, I am picking this solution because it picked up on the morning convection, however it is definitely possible for higher rainfall amounts as indicated by the 12z HREF which has 50% probabilities of 3 hour rainfall in excess of one inch in extreme Eastern Pima County as well Santa Cruz and Cochise County. There is even a 10% probability of 3 hour rainfall in excess of 3 inches near the border. 

12z HREF ensemble mean probabilities 3-hour rainfall and probabilities of one inch and three inches (red) or more valid at 5PM MST Courtesy of SPC.

Therefore, there is a chance for localized flooding and even flash flooding across the area today especially over the higher terrain near the border. 

Other impacts from storms include lightning and gusty winds with 12z HREF showing 50% probabilities of max wind speeds in excess of 30 knots this afternoon.

12z HREF 4-hour max wind speeds and probabilities of wind speeds in excess of 30 knots where dbz>20 valid at 5PM MST this afternoon courtesy of SPC.

The afternoon/evening commute in Tucson will likely be affected with slick roadways, localized flooding, and reduced instability being the primary hazards.