July 30th, 2024: Down Day Today with Increasing Storm Activity Tomorrow

Synopsis

A subtle shortwave embedded within the northeast quadrant of an upper level low over the Central Gulf of California produced overnight /early morning showers and thunderstorms across Southeast Arizona. This feature has moved northwest this morning leaving subsidence in its wake across the region. Convection will be suppressed across the area with only an isolated cell or two possible over the higher terrain and near the International Border this afternoon.

As the upper level low moves northward throughout the day today and into tomorrow, models forecast another, more potent shortwave embedded in the northeast quadrant of the low to move through Southern Arizona. CAMs forecast an increase in storm activity tomorrow, but the coverage and intensity of convection will be primarily dependent on the timing of the passage of the shortwave.

Current Conditions

The 12z GFS 500mb analysis this morning displays the subtropical anticyclone over the Southern Plains, an upper level low over the central Gulf of California, and a subtle shortwave trough over Southeast Arizona.

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

This shortwave was responsible for overnight/early morning showers and thunderstorms across Southeast Arizona, and will have a significant (negative) impact on storm development and coverage today.

The thermodynamic impact from the overnight/early morning activity is reflected in the 12z TUS sounding which shows a near saturated profile in the mid and upper levels, poor lapse rates, and minimal buoyancy (CAPE).

12z TUS sounding this morning courtesy of the SPC.

Today's Forecast

CAMs are in good agreement that storm development will be suppressed today primarily due to subsidence in the wake of the aforementioned shortwave. The 12z KTUS WRF-HRRR sounding forecasting minimal amounts of buoyancy (< 500 J/kg of CAPE) and a subsidence inversion in the mid levels this afternoon.

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

There is some instability (on the order of around 500 J/kg), but it will be difficult for updrafts to breach the strong mid level subsidence inversion. There should be enough instability for a few isolated cells over the highest peaks of the Sky Islands, near the International Border, and over the higher terrain south of Gila Bend. Other than that, it should be a relatively quiet day across the state.

12 WRF-HRRR simulated reflectivity valid for this afternoon at 5PM MST.

High temperatures will remain near to just slightly above normal with temps in Tucson near 100 degF and between 105 and 110 degF in the lower deserts between Phoenix and Yuma.

12z WRF-HRRR 2-meter temperatures valid for 4PM MST this afternoon.

Tomorrow's Forecast

The aforementioned upper level low over the central Gulf of CA moves northward tomorrow with another, more potent shortwave embedded within the northeast quadrant of the low is forecast to move through Southeastern Arizona.

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

This shortwave will provide some synoptic scale ascent in the form of cyclonic vorticity advection tomorrow. Models are in good agreement regarding this feature moving through Southeast Arizona tomorrow. 12z guidance this morning forecasts an active afternoon across Southeast and South-Central Arizona. The 12z GFS, HRRR, and our WRF-HRRR forecast the shortwave to move through during the afternoon hours which would be the best case scenario for storm coverage and intensity. The 12z WRF-HRRR KTUS sounding forecasts above 1.50 inches of precipitable water, between 500 and 1000 J/kg of Mixed-layer CAPE, and steep sfc to 3km lapse rates.

12z KTUS WRF-HRRR sounding valid for 2PM MST tomorrow afternoon.

In addition, the shortwave allows between 15 and 20 knots of vertical wind shear which would aid in organization of storms. With DCAPE forecast to be near 1000 J/kg, not anticipating much of a severe wind threat, but more of a heavy rainfall and localized flooding potential which is backed up by the 12z HREF. The 12z HREF forecasts a 50% probability of 3-hour rainfall accumulations exceeding one inch in Santa Cruz and the eastern half of Pima County (including Tucson) tomorrow afternoon.

12z HREF probabilities of 3-hour QPF exceeding one inch within 40km valid for 2PM MST tomorrow afternoon.

The 12z WRF-HRRR is quite aggressive with storm coverage tomorrow in the Tucson vicinity as shown below.

12z WRF-HRRR simulated reflectivity valid for tomorrow at 4PM MST.

The timing of the shortwave moving into SE AZ will have a critical impact on storm coverage and intensity tomorrow. The shortwave moving through later (early/mid afternoon) would allow several hours of surface heating during the morning hours and therefore increased buoyancy prior to the passage of the shortwave. An earlier arrival of the shortwave would mean less buoyancy and likely promote more of a low-topped, showery event. The devil is in the details as is typically the case during the monsoon, so will need to monitor CAM solutions closely this afternoon and evening. 

Overall, expect an increase in activity tomorrow with the coverage and intensity of storms dependent on the timing of the passage of the shortwave. Stay tuned!