July 6th, 2024: Hot and Mostly Dry through Early Next Week

 Synopsis

Arizona is forecast to remain in the southeast quadrant of a 594 dm anticyclone centered over California today and tomorrow. Dry northeasterly flow aloft over AZ will maintain hot and mostly dry conditions with high temperatures reaching excessive heat criteria through early next week. Limited moisture and hostile synoptics is expected to suppress convection with only an isolated chance of showers/storms over the higher terrain near the International Border in SE AZ through early next week. The 500mb anticyclone will slowly shift east toward the Arizona/Utah Border Monday through Wednesday which will maintain hot, dry, and stable conditions until at least the middle of next week.  

Current Conditions

A few very short-lived thunderstorms developed along the foothills east of Oro Valley yesterday afternoon producing lightning, gusty winds, and light precip accumulations. Just enough low level moisture combined with thermodynamically driven circulations along the terrain won the battle against the hostile synoptics for a short period, but it didn't last long as those cells died rapidly once steering winds pushed them off the higher terrain. In fact, Eyad and I have never seen such a short life cycle of a thunderstorm with max dbz values near 60, so that was interesting. 

Anyway, 12z GFS 500mb analysis this morning displays a 594 dm anticyclone centered along the California/Oregon Border with northeasterly flow aloft over Arizona.

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

The strength of the high plus anticyclonic vorticity advection is a recipe for strong subsidence which is reflected in the 12z TUS sounding this morning.

12z TUS sounding this morning courtesy of SPC.

In addition, moisture continues to plummet with pwats well below an inch across the lower deserts this morning.

SPC mesoanalyis of precipitable water as of 9:00AM MST this morning.

An overnight MCC (Mesoscale Convective Complex) in Sonora induced another Gulf of California moisture surge late last night increasing surface dew points in the 50s to low 60s (increasing from east to west) across the lower deserts as shown by surface obs below.

Surface observations as of 9:30 AM MST this morning courtesy of the NWS.

This low level moisture is rapidly mixing out across the lower deserts in Southern AZ, and will continue to mix out throughout the day.

Today and Tomorrow


The mid level anticyclone will remain centered near Northern California today with dry northeasterly flow aloft over AZ. 

12z GFS 500mb cyclonic vorticity for this afternoon at 5:00PM MST courtesy of Tropical Tidbits.

This hostile synoptic pattern combined with limited moisture will suppress convection across most of AZ with a few isolated showers/storms possible over the higher terrain near the International Border in SE AZ this afternoon/evening. Main forecast highlight today is continued excessive heat with high temps between 100 and 110 degF in Southeast AZ (including Tucson) and between 110 and 115 degF in the lower deserts between Phoenix and Yuma.

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

By tomorrow, a shortwave trough moving into British Columbia will help chip away at the high amplitude 500mb ridge, and in response the center of the anticyclone will shift southeastward over Central California/Southwestern Nevada.

12z GFS 500mb cyclonic vorticity for tomorrow at 5:00AM MST courtesy of Tropical Tidbits.

Unfortunately, this pattern will make conditions even more hostile with increased subsidence due to a closer proximity of the center of the anticyclone, and continued drying due to persistent dry northeasterly flow aloft. CAMs forecasting precipitable water to remain well-below an inch across the lower deserts tomorrow afternoon.

12z WRF-HRRR total precipitable water for tomorrow afternoon at 3:00PM MST.

High temperatures are expected to reach excessive heat criteria again tomorrow with high temps in SE AZ between 100 and 110 degF and between 110 and 115 degF between Phoenix and Yuma. Some isolated areas in south-central and southwestern AZ could see temps near 117 to 118 degF tomorrow afternoon.

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

Early Next Week

Another shortwave trough moving into British Columbia early next week will slowly nudge the anticyclone eastward toward the Arizona/Utah Border Monday through Wednesday. 

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

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

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

Even with the anticyclone shifting to a better climatological location, the center of the anticyclone remains at 594 dm through Wednesday which would maintain large scale subsidence and excessive heat across AZ. Global models as well as the longer range runs of the UA WRF forecasting Monday and Tuesday to be the hottest days so far this summer with temps between 110 degF and 120 degF across the lower deserts. 

The deterministic GFS and ECMWF as well as their ensemble members are forecasting a gradual increase in moisture by the middle to late next week. There still is some uncertainty regarding exactly when and how much moisture will enter AZ, but it appears to be a gradual increase nonetheless. Even with gradual increasing moisture, it will likely take a few days next week for the atmosphere to make a full recovery from several consecutive days of a brutal synoptic pattern.