August 16th, 2024: Return of Southeasterly Flow To Provide Uptick in Storm Activity Through the Weekend

 Synopsis

The 500mb anticyclone has shifted toward New Mexico allowing southeasterly mid level flow to return across Arizona. Marginal wind shear in combination with high DCAPE and just enough buoyancy will allow scattered showers and thunderstorms across Southeast Arizona this afternoon and evening. Expect the majority of activity to remain over the higher terrain and near the International Border, but a few cells are possible in the Tucson vicinity later today. Main impacts from storms will be locally strong wind gusts and blowing dust.

CAMs are hinting at the potential for overnight elevated convection across Southern Arizona associated with storm activity in Sonora which could also have an impact on storm chances tomorrow.

Current Conditions

The synoptic scale upper air pattern has drastically changed over the past 24 hours with the 500mb anticyclone shifting to the east into New Mexico and southeasterly flow returning across Arizona.

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

There's also a trough approaching the West Coast as well as an upper level low/inverted trough off the coast of Baja associated with an MCC (Mesoscale Convective Complex) in Sonora last night (will discuss these features in more detail in the next section). 

This overnight convection in Sonora induced a Gulf of California moisture surge early this morning with surface observations across Southern Arizona reporting dew points in the upper 50s to mid 60s as shown below.

Surface dew point observations as of 9:45AM MST courtesy of the NWS.

Still have legacy subsidence as indicated by the 12z TUS sounding which measured an inversion near 500mb associated with the anticyclone overhead the past 48 hours.

12z TUS sounding this morning courtesy of the SPC.

There has been a slight increase in moisture with precipitable water increasing to 1.39 inches, but buoyancy isn't great with minimal CAPE, poor lapse rates, and a lot of CIN.

Today's Forecast

Synoptics/Dynamics

The synoptic scale pattern is expected to be quite busy, and become increasingly favorable throughout the day. Models forecast a trough off the West Coast to rapidly deepen throughout the day and in response the subtropical ridge will amplify downstream over the Rockies. In addition, the deepening mid latitude trough to the west will nudge the 500mb anticyclone to the east into New Mexico. At the same time, the deepening mid latitude trough to the west and amplifying ridge to the east will cause the upper level low/inverted trough off the coast of northern Baja to undergo stretching deformation and eventually become an embedded shortwave lifting northeastward across Southern California. These synoptic scale features will tighten the mid level height gradient across the Desert Southwest and allow brisk southeasterly flow across Arizona today.

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

In general, the synoptic scale pattern becomes relatively more favorable across Eastern Arizona today due to brisk southeasterly flow aloft. 

The main issue is the legacy subsidence from 48 hours of a strong anticyclone overhead. Am expecting the subsidence inversion to weaken throughout the day, but will still likely have an influence on updraft strength and lead to a delayed convective initiation.

 Thermodynamics and Vertical Wind Shear

Hot surface temperatures will allow steepening low and mid level lapse rates (from the surface to below the mid level subsidence inversion) across the region by late this afternoon. However, the air mass is still relatively moisture starved and expecting this morning's low level moisture to rapidly mix out throughout the day in Southeast Arizona. In fact, the 12z and 15z KTUS WRF-HRRR soundings forecasts around 1.25 inches of precipitable water, near 200 J/kg of Mixed-layer CAPE, and surface dew points in the low 50s by late this afternoon.

12z (left) and 15z (right) KTUS WRF-HRRR soundings valid for 4PM MST this afternoon.

Also, notice the subsidence inversion near 500mb and associated CIN (near -50 J/kg). 

However, DCAPE is forecast to be near 2000 J/kg promoting the threat for very strong outflow boundaries, and in combination with 15 to 20 knots of vertical wind shear and even a bit of veering could allow some storm activity to propagate off the higher terrain and into the lower deserts of Southeast Arizona this evening. 

What to Expect/Potential Impacts

Overall, am expecting the majority of storm activity to remain confined to the higher terrain and near the International Border in Southeast Arizona due to poor buoyancy. Both the 12z and 15z WRF-HRRR forecast convection to initiate over the higher terrain near the International Border in Santa Cruz and Cochise Counties by mid to late afternoon.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 3PM MST this afternoon.

Storm propagation will be brisk from southeast to northwest, and storm chances in the lower deserts of Southeast Arizona (including Tucson) will be solely dependent on outflow trajectories and efficiency to produce lift that can penetrate CIN and the mid level subsidence inversion. 

Both the 12z and 15z WRF-HRRR forecast isolated storm activity in the Tucson vicinity this evening that appears to be associated with strong outflows moving northward/northwestward from convection in Santa Cruz and Western Cochise Counties.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 6PM MST this evening.

12z (left) and 15z (right) WRF-HRRR 10-meter winds valid for 5:30PM MST this evening.

With DCAPE forecast near 2000 J/kg, brisk steering flow, and marginal wind shear, could see some strong outflow boundaries across Southeast Arizona and even near Tucson later today. 

Considering the thermodynamic ambiguity today, I think it's safe to say that storms in the Tucson metro are unlikely, but if strong outflow can make it into Tucson than an isolated cell or two is definitely possible.

Due to high LCLs, a dry sub cloud layer, and brisk steering flow, excessive rainfall is highly unlikely with the main threat today being strong gusty winds and blowing dust. The 12z HREF has between 50 and 70% probabilities of 4-hour max wind speeds in excess of 30 knots (where dbz > 20) in Santa Cruz, Western Cochise, and Southeastern Pima County (including Tucson) this afternoon and evening. 

12z HREF 4-hour max wind speeds and probabilities of wind speeds in excess of 30 knots where dbz>20 valid at 7PM MST this evening courtesy of the SPC.

With that being said, traveling conditions could be extremely hazardous along the I10 corridor between Wilcox and Tucson and especially along the I19 corridor between Tucson and Nogales with the main risk being reduced visibility from blowing dust.

CAMs forecasting another active evening in Sonora, and even are hinting at activity down there to impact Southern Arizona overnight into the early morning hours. Overnight shower/thunderstorm potential remains consistent with issues of thermodynamic ambiguity due to the atmosphere’s current state of attempting to overcome subsidence from the past few days. There are numerous factors that would support these chances this evening in central and southeastern Arizona (including Tucson and Phoenix), but they are largely dependent on outflows and likely an MCV associated with activity that has been frequenting Sonora recently. 

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 1AM MST tonight.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 3AM MST tomorrow morning.

Additionally, due to increasing steering flow and minimal instability, precipitation from convection that does develop will struggle to greatly impact a singular area due to swift movement of isolated cells and a dry subcloud layer. Overall, wouldn't expect anything more than a few lightning flashes and very light and spotty precip accumulations. 

Weekend Forecast

The broad synoptic scale upper air pattern remains relatively consistent through the weekend with a deep mid latitude trough off the West Coast, the 500mb anticyclone centered in New Mexico, and brisk southeasterly/southerly flow aloft across Arizona.

12z GEFS 500mb cyclonic vorticity loop valid between 12z Saturday and 06z Sunday courtesy of Tropical Tidbits.

Precipitable water is forecast to remain near to slightly above normal this weekend which should maintain a mid grade monsoon pattern through the period.

12z GEFS precipitable water normalized anomaly loop valid between 12z Saturday and 06z Sunday courtesy of Tropical Tidbits.

CAMs forecast an uptick in storm intensity and coverage tomorrow, but will hold off on providing details due to overnight convection potentially having an impact on timing of convective initiation and coverage of storms tomorrow. Stay tuned!!!