July 20th, 2024: Active Monsoon Weekend

 Synopsis

An active weekend is expected across Arizona due to a combination of northeasterly flow aloft and sufficient moisture and instability. The 500mb synoptic pattern today is characterized by a subtropical anticyclone anchored near the AZ/NV/CA Border region with weak northeasterly/easterly flow aloft over AZ. More sufficient thermodynamics today will promote a significant chance of showers and thunderstorms across mainly the eastern half of Arizona this afternoon and evening. 

 Current Conditions

As of 9:30AM MST, visible satellite imagery and lightning detection showing an area of showers and isolated thunderstorms in southwestern AZ moving slowly westward.

Visible satellite imagery overlaid with GLM flashes as of 9:30AM MST courtesy of College of Dupage.

This activity will continue to dissipate and move west toward SE CA over the next few hours. There are some mid level debris clouds scattered across the Eastern half of the state, but these should also begin to dissipate by midday.

Mid level flow has turned more northeasterly this morning as the 500mb anticyclone has become centered near the AZ/NV/CA Border region.

SPC 500mb analysis as of 12z this morning.

There's also an embedded shortwave over Northern AZ this morning which is evident from 500mb analysis and associated with the region of clouds/showers in NE AZ. 

Moisture is much more widespread today as indicated by SPC mesoanalysis and surface observations.

SPC mesoanalysis of precipitable water at 12z this morning.

Surface observations as of 9:45AM MST this morning courtesy of the NWS.

What really catches my attention is higher dew points over the Rim and White Mountains as well as the valley regions in Cochise and Graham Counties.

The 12z TUS sounding reflects the deeper moisture with a precipitable water at 1.58 inches and a surface dew point of 64 degF.

12z TUS sounding this morning courtesy of SPC.

There's a lot to like about this sounding with over 1000 J/kg of CAPE, decent low to mid level lapse rates, and a much weaker subsidence inversion above 500mb. There is quite a bit of CIN though which is to be expected, but CIN should erode relatively quickly as surface heating mixes out the boundary layer. There isn't a lot of vertical wind shear though, actually almost none.

Today's Forecast

Synoptics

The 500mb anticyclone will remain anchored near the AZ/NV/CA Border region today with predominantly weak northeasterly/easterly flow aloft across the eastern half of the state. With the ridge in relative close proximity, mid level flow will remain weak with mid level winds between 10 and 15 knots. As mentioned above, there's a subtle embedded shortwave in Northern AZ this morning. Based on current guidance, it appears this feature will become somewhat diffused by this afternoon, but may aid in convective initiation over the higher terrain. Other than that, it doesn't appear to have any real significant impact on storms today. Overall, the synoptics are somewhat neutral as there isn't any real focusing mechanism nor any inhibiting mechanism.

Thermodynamics

Storm initiation and maintenance will be primarily dependent on thermodynamically driven kinematics. As the higher terrain heats up (differential heating) this morning, the Mountain-Valley circulations will initiate convection over the Rim, White Mountains, and SE AZ Sky Islands by early afternoon. CAMs show between 500 and 1000 J/kg of CAPE early this afternoon over the Rim and White Mountains as well as in the adjacent valleys of Cochise and Graham counties.

12z WRF-HRRR Maximum CAPE for this afternoon at 1:00PM MST.

The 12z KTUS WRF-HRRR sounding (which is the most bullish solution) forecasts around 500 J/kg of CAPE, steep low and mid level lapse rates, and DCAPE around 1300 J/kg at 5:00PM MST this evening.

12z KTUS WRF-HRRR sounding for 5:00PM MST this evening.

If outflow boundaries can move through the Tucson metro there should be no problem initiating storms as thermodynamics will remain sufficient through the early evening hours.


CAMs forecast less instability in Phoenix with the 12z WRF-HRRR forecasting around 400 J/kg of CAPE but -69 J/kg of Mixed-layer CIN this evening.

12z KPHX WRF-HRRR sounding for 5:00PM MST this evening.

CAPE will likely be higher than what is being forecast by CAMs as observed CAPE from 00z soundings the last few days has confirmed this. Therefore, expecting CAPE to be between 500 and 1000 J/kg this afternoon in Tucson and Phoenix. 

What to Expect/Impacts

CAMs are in good agreement that storms will initiate over the Mogollon Rim, White Mountains, and Higher Terrain of SE AZ by early afternoon.

12z (left) and 15z (right) simulated reflectivity for this afternoon at 1:00PM MST.

By mid to late afternoon, outflow boundaries will move into the lower elevations of Graham, Cochise and Pima Counties. This is where there is some model disagreement and forecast uncertainty. Since there is a lack of a synoptic scale forcing mechanism and minimal wind shear, storm development in lower elevations is primarily dependent on outflow boundary trajectories. Even with marginally better thermodynamics, we are still in somewhat of a "pulse" storm mode as there isn't any mechanism to aid in organization. Both the 12z and 15z WRF-HRRR show storms in Graham, Cochise, and Santa Cruz counties by late afternoon as shown below.

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

Both the 12z and 15z WRF-HRRR show outflow boundaries moving into the Tucson metro along the I10 and I19 corridor this evening with the 12z being the more aggressive solution.

12z (left) and 15z (right) 10-meter wind speed for this evening at 8:00PM MST.

 

12z (left) and 15z (right) simulated reflectivity for this afternoon at 8:00PM MST.

The 12z forecasts a very strong outflow boundary to march northwestward along the I10 corridor into Pinal County and maybe even Maricopa County through the late evening hours.

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

CAMs are resistant to initiate storms in the Phoenix vicinity this evening likely due to the development of CIN.

12z (left) and 15z (right) simulated reflectivity for this afternoon at 10:00PM MST.

What concerns me is the potential for blowing dust or even a Haboob to develop along the I10 corridor from Tucson northwest into Casa Grande and maybe even Phoenix. Forecast DCAPE values aren't overly impressive (around 1500 J/kg), but CAMs seem to be hinting at this scenario. In fact, the 12z HREF has a 70% probability of 4-hour max wind speeds in excess of 30 knots (where dbz > 20) in Eastern Pima and Pinal Counties this evening.

12z HREF 4-hour max wind speeds and probabilities of wind speeds in excess of 30 knots where dbz>20 at 8:00PM MST this evening. 

Therefore, main threat from storms (which is typically the case) is the potential for strong winds and blowing dust. Not expecting much of an excessive rainfall threat, but isolated flash flooding is always a possibility. 

Overall, expect showers and thunderstorms across SE AZ this afternoon and evening with coverage and intensity in the lower deserts (including Tucson) dependent on outflow boundary trajectories and efficiency. Storms are unlikely in Phoenix, but cannot completely rule out an isolated short-lived cell or two.

 Tomorrow

The 500mb pattern transitions tomorrow as a trough digging into the Southern Plains strengthens the mid level height gradient and enhances northeasterly/northerly flow aloft.

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

 

CAMs are forecasting increased storm coverage tomorrow, especially for the lowers deserts (including Phoenix). Hesitant to go too much into detail as storms this evening may have an impact on coverage and timing of convective initiation tomorrow. For now, expect another active day tomorrow with potentially increased storm coverage across the region.