July 5th, 2024: Hot and Mostly Dry with Isolated Elevated Convection Possible in SE AZ This Afternoon

 Synopsis

The mid level anticyclone over the Western one-third of CONUS will remain centered near the California/Oregon Border today with dry northeasterly flow aloft over AZ. Temperatures will be a few degrees warmer today compared to yesterday with temps reaching near 110 degF in Tucson and near 115 degF in Phoenix. Some isolated areas in the lower deserts of Maricopa, Pinal, and Yuma counties could see temps near 117 to 118 degF this afternoon. Dry and mostly stable conditions will suppress convection across most of AZ, but a few elevated showers/storms are possible in extreme SE AZ late this afternoon/early evening. Any storm that develops could produce lightning, gusty winds, and localized blowing dust.

 Current Conditions

The 12z GFS 500mb analysis this morning displayed a 594 dm mid level anticyclone centered just off the coast of Northern California with dry northeasterly flow over AZ.

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

This hostile synoptic upper air pattern is reflected in the 12z TUS sounding this morning which shows a strong subsidence inversion near 500mb.

12z TUS sounding this morning courtesy of SPC.

There is a bit of instability though with around 1100 J/kg of CAPE but around -400 J/kg of CIN. Also, precipitable water is hanging around an inch with a surface dew point of 64 degF. This increase in low level moisture is from an overnight moisture surge from the Gulf of CA due to a decaying MCS in Sonora last night. 

As of 10:00AM MST, surface observations measuring dew points in the upper 50s to low 60s across the lower deserts of Southern AZ.

Surface observation as of 10:00AM MST courtesy of the NWS.

 Today's Forecast

Synoptics

The mid level anticyclone is expected to build eastward throughout the day and into tonight, eventually becoming centered over the California/Oregon Border.

12z GFS 500mb cyclonic vorticity for tonight at 11:00PM MST courtesy of Tropical Tidbits.

With the ridge building in from the west, mid level heights will rise a tad throughout the day, and in combination with anticyclonic vorticity advection will promote a hostile synoptic pattern for any storm development today.

Thermodynamics

The overnight low level moisture surge and warm surface temperatures has allowed between 500 to 1500 J/kg of CAPE to develop in SE AZ this morning. There is quite a bit of CIN though (between -200 to -400 J/kg) across the region. Sufficient heating of the surface should allow low and mid level lapse rates to steepen causing the layer of CIN to erode by late afternoon. In addition, the low level moisture will rapidly mix out with CAMs forecasting 2-meter dew points to drop in the upper 40s to low 50s by late this afternoon.

12z WRF-HRRR 2-meter dew points for this afternoon at 4:00PM MST.

The 12z KTUS WRF-HRRR sounding forecasting a textbook inverted-V sounding for late this afternoon with a relatively well-mixed surface to 600mb layer, around 500 J/kg of CAPE in the mid to upper levels, and 2100 J/kg of DCAPE.

12z KTUS WRF-HRRR sounding for this afternoon at 4:00PM MST.

What to Expect/Impacts

For the most part, expect a very hot day across the lowers deserts with temps (especially in South-central and Southwest AZ) reaching excessive heat criteria. High temps are forecast to be between 105 degF and 110 degF in the lower deserts of SE AZ (including Tucson), and between 110 and 115 degF in the lower deserts between Phoenix and Yuma. A few isolated areas in Pinal, Maricopa, and Yuma counties could see temps reach 117 to 118 degF by late this afternoon. 

12z WRF-HRRR 2-meter temps for this afternoon at 4:00PM MST.

Regarding storm chances in SE AZ, it's one of those situations of a tug of war between the hostile synoptic environment and localized thermodynamically driven circulations. As mentioned in the synoptics section above, rising mid level heights and anticyclonic vorticity advection will maintain subsidence across the region. This will be a major inhibition mechanism for any potential updraft. On the other hand, just enough lingering low level moisture and efficient surface heating will allow localized convergence (due to differential heating of terrain) to occur over the Sky Islands/higher terrain of SE AZ this afternoon. Our UA WRF-HRRR as well as recent runs of the HRRR continue to forecast a few elevated showers/storms in Cochise, Santa Cruz, and Southeastern Pima County later this afternoon/early evening. 

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for 5:00PM MST this afternoon.

Overall, expect the majority of activity to remain confined to the Sky Islands and the higher terrain near the International Border. However, I cannot completely rule out an isolated cell or two in the Tucson vicinity. The main concern with any shower/storm that develops is localized gusty winds due to the high potential for evaporative cooling in a deep, very dry sub cloud layer as DCAPE is forecast to be greater than 2000 J/kg this afternoon. This means that any cloud that attempts to precipitate has the potential to produce strong outflow winds and associated blowing dust. 

The 12z HREF has between a 30% and 50% chance of 4-hour max wind speeds in excess of 30 knots where dbz is greater than 20 in southeast Pima County and between 50% and 70% in Santa Cruz and southern Cochise Counties.

12z HREF conditional probabilities of 4-hour max wind speeds in excess of 30 knots where dbz > 20 at 5:00PM MST courtesy of SPC.

Traveling conditions could be hazardous in any location where a shower/storm develops with the main impacts being gusty winds and reduced visibility from blowing dust. Areas of most concern is the I10 corridor from Tucson eastward and the I19 corridor from Tucson southward.

In general, expect a very hot and mostly dry day across Arizona with a few isolated elevated showers and storms possible in SE AZ late this afternoon/early evening.