April 4th, 2025: Chances for Showers and Isolated Thunderstorms Across Eastern Arizona Today

 Synopsis

Weak ascent induced by an upper level low over Arizona combined with differential heating of terrain and modest buoyancy will be responsible for scattered showers and a few isolated thunderstorms across the eastern part of the state today. Greatest coverage of precipitation will be mainly confined to the higher terrain with decreased chances in the lower elevations. Showers and storms will be capable of producing brief heavy rain, small hail, gusty winds, and occasional lightning flashes. Rainfall amounts will be light and highly variable and dependent on storm frequency, coverage, and intensity with UA WRF forecasting a trace to 0.10 inch in the lower deserts and between a trace and 0.25 inches over the higher terrain. Snow levels will remain mainly above 7000 feet with a dusting to 2 inches possible, with highest amounts expected over the White Mountains.

Discussion

Visible satellite imagery this morning displays scattered, developing cumulus clouds across Eastern Arizona.

Visible satellite imagery as of 10:15AM MST courtesy of College of Dupage.

These cumulus clouds are associated with weak ascent associated with an upper level low anchored over Arizona combined with differential heating of terrain and modest CAPE.

SPC 500mb analysis valid at 10AM MST.

SPC mesoanalysis of Surface-based CAPE and CIN valid at 10AM MST.

Even though there will be relatively broad synoptic scale ascent induced by the upper low today, it looks to be quite weak as I don't see any significant embedded vorticity lobes in the analysis data or model outputs. As shown by the current visible imagery, convection appears to be mainly confined to the higher terrain as differential heating is likely acting as an additional and stronger lifting mechanism. 

Another issue today is the lack of deep moisture leading to only modest buoyancy across our region. In fact, the 12z TUS sounding only measured a measly 0.44 inches of precipitable water.

12z TUS sounding courtesy of the SPC.

The 15z KTUS UA WRF-HRRR sounding forecasts precipitable water to remain at or below 0.5 inches today.

15z KTUS UA WRF-HRRR sounding valid for 1PM this afternoon.

It's unfortunate we don't have just a little more moisture as low and mid level lapse rates are expected to steepen into the afternoon, but maximum CAPE values will likely remain between 100 and 250 J/kg. 

CAMs (including UA WRF ensemble members) forecast the majority of convection to remain confined to the Rim, White Mountains, and higher terrain of Southeast Arizona this afternoon which seems reasonable due to the lack of any dominant synoptic scale forcing. 

In addition, since Arizona will remain beneath the center of the upper level low, vertical wind shear is almost non-existent, and steering winds look to remain below 15 knots throughout the afternoon. This will make it quite challenging for showers over the higher terrain to propagate into the lower elevations (especially Phoenix). 

Also, I'll be using the 15z UA WRF-HRRR ensemble member as it seems to be the most reasonable solution due to a more representative initialization of this morning's observations. 

The most interesting aspect of today's conditions, in my opinion, is the potential for weak, localized outflow winds in Southeast Arizona which is evident by the high LCLs, inverted-V profile, and near 500 J/kg of DCAPE on the model sounding shown above. The 15z shows some gusty outflow winds reaching the Tucson vicinity later this afternoon.

15z UA WRF-HRRR 10-meter winds valid for 3:30 PM MST this afternoon.

These localized outflow boundaries could make things a bit interesting in the Tucson area later today and may help generate convective showers and even an isolated thunderstorm. The 15z forecasts the best chance for precipitation in Tucson between 4 and 6PM this afternoon as shown below.

15z UA WRF-HRRR simulated reflectivity valid for 4PM MST this afternoon.

A few isolated showers are possible in Phoenix this afternoon, but will be highly dependent on activity propagating off the higher terrain. 

As mentioned above, a few isolated thunderstorms are possible, especially over the higher terrain. Thunderstorms will be capable of producing brief heavy rainfall, gusty winds, small hail, and lightning. 

Rainfall amounts will be very light with a trace to 0.10 inches expected in the lower deserts and between a trace and 0.25 inches over the higher terrain. 

15z UA WRF-HRRR accumulated precipitation valid from 15z this morning to 12z tomorrow morning.

 

Snow levels will remain mainly above 7000 feet with a dusting to 2 inches of accumulated snow possible with highest amounts expected over the White Mountains.

15z UA WRF-HRRR accumulated snowfall valid from 15z this morning to 12z tomorrow morning.

Overall, don't expect any significant precipitation today, but I would still anticipate some localized impacts from gusty outflow winds. Reduced visibility, especially in Southeast Arizona, could make traveling conditions hazardous. Also, I would expect the evening commute in Tucson to be impacted by mainly light rainfall and gusty winds. 

-Tyler