August 1st, 2024: A Return of Outflow Dependent Storm Activity Today

 Synopsis

The upper level low over Central Baja will continue to move northward into Southern California with the 500mb anticyclone rebuilding into the Four Corners over the next 24 hours. Plenty of moisture and sufficient instability will promote a chance of showers and thunderstorms across mainly Southeast Arizona this afternoon and evening. However, without a pronounced synoptic scale forcing mechanism (inverted trough), storm coverage will be primarily dependent on outflow boundary trajectories and efficiency to produce lift. 

Current Conditions

At 9AM MST, mid level water vapor imagery overlaid with RAP 500mb analysis and GLM flashes displayed the 500mb anticyclone centered over the Southern Plains, an upper level low over Central Baja, and scattered showers/isolated thunderstorms across the lower deserts of Southwest Arizona and Southeast California.

Mid level water vapor imagery overlaid with RAP 500mb analysis and GLM flashes as of 9AM MST courtesy of College of Dupage.

This area of clouds, scattered showers, and isolated thunderstorms is associated with an embedded shortwave within the northeast quadrant of the low moving westward. 

Plenty of moisture across the region as indicated by the plume of moisture via mid level water vapor imagery and the 12z TUS sounding which measured 1.61 inches of precipitable water and a surface dew point of 67 degF.

12z TUS sounding courtesy of the SPC.

However, there is a lot of CIN this morning with minimal Mixed-layer CAPE. Also, there is a relatively dry layer near 850mb.

Today's Forecast

Synoptics

 
The aforementioned upper level low will move northward into Southern California, and the 500mb anticyclone will rebuild into the Four Corners region today. Southeast Arizona will be under the influence of subsidence during the morning and early afternoon hours in the wake of the inverted trough from last night/early this morning. 

Unlike yesterday, there isn't a pronounced inverted trough to provide significant, widespread synoptic scale forcing. However, the 12z GFS 500mb plane projection shows a subtle shortwave lifting northward into extreme SE AZ this afternoon as shown below. 

12z GFS 500mb cyclonic vorticity valid for 5PM MST this afternoon.

This feature could aid in convective initiation over the higher terrain this afternoon but the extent is somewhat unclear at this time. 

Besides the shortwave, Southern Arizona will be within the right entrance region of a weak upper level speed max associated with the upper level low moving northward into SoCal which is a relatively favorable region for synoptic scale ascent due to speed divergence aloft.

12z GFS 250mb wind speed valid at 5PM MST courtesy of Tropical Tidbits.

Overall, the synoptic pattern is marginally favorable for storms this afternoon and evening in Southeast Arizona but not as favorable as yesterday.

Thermodynamics

Will still have plenty of moisture to work with today as CAMs forecast precipitable water to remain between 1.25 and 1.50 inches and surface dew points in the 50s to low 60s into the late afternoon hours.

12z WRF-HRRR total precipitable water valid for 5PM MST this afternoon.

12z WRF-HRRR 2-meter dew points valid for 5PM MST this afternoon.

As mentioned above, Southeast Arizona will be under weak subsidence this morning and into the early afternoon hours in the wake of the inverted trough downstream. This will allow mostly clear skies and therefore sufficient heating of the surface to steepen low and mid level lapse rates into the late afternoon/early evening hours. The 12z KTUS WRF-HRRR sounding reflects this which forecasts between 500 and 700 J/kg of Mixed Layer CAPE late this afternoon.

12z KTUS WRF-HRRR sounding valid for 5PM MST this afternoon.

In addition, DCAPE is forecast to be a bit higher today (near 1300 J/kg), so there is a marginally increased threat for stronger outflow boundaries this afternoon and evening.

What to Expect/Impacts

There are some timing and coverage differences between morning CAM solutions. The 12z WRF-HRRR is more aggressive which shows scattered showers and thunderstorms across SE Arizona (including the Tucson vicinity) late this afternoon and into the early evening hours. The 15z WRF-HRRR forecasts more isolated storm activity remaining confined to the higher terrain as shown below.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 8PM MST this evening.

Tricky forecast today, but am leaning toward the 12z WRF-HRRR solution for a few reasons. First, the combination of the subtle shortwave moving northward into extreme SE AZ this afternoon and the weak upper level divergence will aid in convective initiation over the higher terrain across Southeast Arizona and near the International Border. Also, there will be several hours of insolation to steepen low and mid level lapse rates and combined with plenty of low level moisture will generate sufficient buoyancy by late this afternoon. 

Once convection initiates over the terrain, there is some uncertainty due to additional storm development in lower elevations dependent on outflow boundary trajectories and efficiency. The reason the 12z WRF-HRRR is more aggressive is due to it forecasting an outflow boundary to move through Eastern Pima County early this evening.

12z WRF-HRRR 10-meter winds valid for 8:30PM MST this evening.

This is backed up by the 12z HREF which has 50 to 70% probabilities of 4-hour max wind speeds in excess of 30 knots (where dbz > 20) in Eastern Pima County (including Tucson) this evening.

12z HREF 4-hour max wind speeds and probabilities of max wind speeds in excess of 30 knots where dbz > 20 valid at 8PM MST courtesy of the SPC.

One wildcard to keep an eye on is activity in Sonora as recent runs of the HRRR this morning have been consistently showing outflows initiating showers and storms through SE AZ between 8PM and midnight tonight. However, the issue with outflows later in the evening is they would need to be strong enough the breach increasing nocturnal CIN. 

Overall, expect scattered showers and thunderstorms across Southeast Arizona this afternoon and evening with coverage, timing, and intensity of convection dependent on outflow boundary trajectories and efficiency to produce ascent.

As far as potential impacts are concerned, any thunderstorm is capable of producing lightning, strong, gusty winds, blowing dust, and brief heavy rainfall. 

Tomorrow's Forecast

By tomorrow, the upper level low over central Baja will lift rapidly northward into California with the 500mb anticyclone rebuilding into the Four Corners region and mid level flow remaining southeasterly across Arizona. 

12z GFS 500mb cyclonic vorticity valid for 11AM MST tomorrow courtesy of Tropical Tidbits.

As the ridge rebuilds into the Intermountain West, 500mb heights will rise leading to the development of subsidence and increase high temperatures. The subsidence is reflected in the 12z KTUS WRF-HRRR sounding which shows a pronounced subsidence inversion near 500mb tomorrow afternoon.

12z KTUS WRF-HRRR sounding valid for tomorrow at 3:30PM MST.

It's unclear how much of an impact this subsidence inversion will have on convection tomorrow, but I would anticipate a decrease in storm coverage and a return to a lower grade monsoon pattern.