July 18th, 2024: Increased Storm Activity Today

 Synopsis

The 500mb anticyclone centered near Northeastern AZ this morning will begin to shift west toward the AZ/NV/UT Border throughout the next 24 hours or so. In response, mid level flow turns predominantly northeasterly across Arizona and in combination with plenty of moisture will promote chances for storm activity coming off the Mogollon Rim/White Mountains. CAMs forecast an increase in activity across the lower deserts this afternoon and evening due to outflow boundaries moving southwest off the higher terrain. 

Current Conditions

Radar and visible satellite this morning showed an area of scattered showers and isolated thunderstorms across Maricopa and Yuma Counties with nothing more than very light rain accumulations and a few lightning strikes. This activity will continue to dissipate and move west over the next couple hours or so.

The 12z TUS sounding this morning shows a messy vertical wind profile due to the closer proximity of the 500mb anticyclone as indicated by 12z GFS 500mb analysis.

12z TUS sounding courtesy of the SPC.

  

12z GFS 500mb analysis courtesy of Tropical Tidbits.

Plenty of moisture with the 12z TUS sounding measuring 1.51 inches and a surface dew point of 59 degF, but instability is marginal with only around 500 J/kg of CAPE and over -300 J/kg of CIN. 

Today's Forecast

Synoptics

The 500mb anticyclone is forecast to wobble around the Northern half of Arizona today as it begins to shift west toward the AZ/NV/UT Border over the next 24 hours or so. With the center of the anticyclone in closer proximity to our region, this promotes a relatively less favorable synoptic environment due to increased subsidence and weaker mid level flow. Even with this unfavorable setup, CAMs are in good agreement regarding increased storm coverage this afternoon and evening across mainly the eastern half of AZ. However, there's a subtle embedded shortwave in western New Mexico diving southwestward along the eastern flank of the 500mb anticyclone indicated by the 12z GFS.

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

Models forecast this shortwave to move into SE Arizona early this afternoon which would act as a synoptic scale mechanism for ascent. This could aid in storm development over the higher terrain of Southeastern Arizona.

Thermodynamics

Overall, expecting sufficient moisture to remain in place with surface dew points above 50 degF and pwats between 1 and 1.5 inches (increasing from east to west) across the state this afternoon and evening. Both the 12z and 15z KTUS WRF-HRRR model soundings forecasting less than 500 J/kg of CAPE late this afternoon and early evening.

12z KTUS WRF-HRRR sounding for this evening at 6:00PM MST.

15z KTUS WRF-HRRR sounding for this evening at 6:00PM MST.

Even with such limited buoyancy, the WRF-HRRR forecasting an impressive well-mixed profile from the surface to around 600mb this evening. With DCAPE forecast to be near 1500 J/kg this evening, this promotes an environment conducive for potent outflow boundaries. 

Phoenix is forecast to have more CAPE (likely closer to 1000 J/kg) due to warmer surface temperatures and higher surface dew points.

12z KPHX WRF-HRRR sounding for 6:00PM MST this evening.

The 15z KPHX WRF-HRRR is a bit less aggressive which shows less CAPE and more CIN this evening.

15z KPHX WRF-HRRR sounding for 6:00PM MST this evening.

What to Expect/Impacts

In general, the 12z and 15z WRF-HRRR as well as recent runs of the HRRR are in good agreement with increased shower and storm coverage across Arizona this afternoon and evening. The 12z and 15z WRF-HRRR as well as hourly runs of the HRRR this morning all show storms in Tucson this evening. The 12z WRF-HRRR is the most aggressive solution which shows outflow boundaries from the south and northeast converging in the area as shown below. 

  

12z WRF-HRRR 10-meter wind speed for this evening at 6:00PM MST.

The 15z solution is less aggressive, but still forecasts storms in the Tucson vicinity.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this evening at 9:00PM MST.

Considering relatively good consensus among CAMs, showers and thunderstorms are likely in the Tucson vicinity this evening.  

In Phoenix, the 12z WRF-HRRR and recent runs of the HRRR showing activity in the Phoenix area. Sufficient heating of the surface will lead to steep low and mid level lapse rates and in combination with dew points remaining in the mid to upper 50s will allow between 500 to 1000 J/kg of CAPE to develop this afternoon/evening. Therefore, if outflow boundaries can propagate off the Rim and into the metro, storms should have no problem developing. 

Since CAPE is on the lower end today, not expecting widespread strong/severe storms with main impacts being lightning, gusty winds, and brief heavy rainfall. Main concern will be gusty winds leading to blowing dust making driving conditions extremely hazardous. 

To summarize, greater chances of storms in the lower deserts this evening with coverage primarily dependent on outflow boundary trajectories and efficiency.

Friday through Sunday

The deterministic GFS and ECMWF as well as their ensemble members are in good agreement the 500mb anticyclone will shift west and become anchored near the AZ/NV/UT borders tomorrow through the weekend. A trough developing in the Central US will cause the mid level height gradient to tighten leading to brisk northeasterly winds aloft over Arizona.

12z GEFS 500mb cyclonic vorticity loop for 12z tomorrow through 12z Monday courtesy of Tropical Tidbits.

Models also forecast precipitable water to remain near to just above normal through the period allowing chances for afternoon/evening storm chances across the region. Overall, expect a mid grade monsoon pattern through at least early next week with day to day variability having an impact on storm coverage and intensity.