August 10th, 2024: Increased Storm Activity From Tucson Westward Today

 Synopsis

Today, Arizona remains in between a 500mb anticyclone over Western Texas and an upper level low over Northern Baja with southeasterly flow aloft across our region. An embedded shortwave moving across the International Border this morning is expected to continue to move northward through Southern Arizona and act as a synoptic scale lifting mechanism for storm initiation this afternoon. Storm coverage and intensity will be dependent on the amount of buoyancy that develops prior to the passage of the shortwave. Main impacts from storms today will be gusty winds, blowing dust, and brief heavy rainfall.

Current Conditions

At 9:45AM MST, visible satellite imagery overlaid with GLM flashes and RAP 500mb analysis displays an area of showers and thunderstorms near the International Border associated with an embedded shortwave moving northward into Arizona this morning.

Visible satellite imagery overlaid with glm flashes and RAP 500mb analysis as of 9:45AM MST courtesy of College of Dupage.

This feature will act as a dynamic focusing mechanism for convection from Tucson westward today.

Meanwhile, there is a very familiar moisture gradient across the Southern half of Arizona with SPC mesoanalysis showing a tight gradient in precipitable water with values as high as 1.7 inches in Yuma county and as low as 0.9 inches in Cochise County.

SPC mesoanalysis of precipitable water as of 9AM MST.

Thermodynamics aren't great but slightly better than yesterday with the 12z TUS sounding measuring 1.56 inches of precipitable water, a surface dew point of 58 degF, and MLCAPE near 200 J/kg.

12z TUS sounding this morning courtesy of the SPC.

There's quite a bit of CIN this morning (-262 J/kg of MLCIN), but heating of the surface should allow this region of CIN to erode as low and mid level lapse rates steepen. Also, there's some concerning inversions near 500 and 400mb associated with drier air moving in from the east.

 Today's Forecast

Synoptics/Dynamics

The broad synoptic scale upper air pattern is near identical today compared to yesterday with a 500mb anticyclone centered over Western Texas, an upper level low over Northern Baja, and southeasterly flow aloft across Arizona. 

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

However unlike yesterday, there's an embedded shortwave moving northward into Southern Arizona which will act as a dynamic focusing mechanism for storm initiation, maintenance, and coverage this afternoon and evening mainly from Tucson westward. Differential cyclonic vorticity advection ahead of the shortwave trough axis will allow storms to initiate early this afternoon across Santa Cruz, and the Eastern half of Pima County (will show forecast storm evolution in a later section). This feature is expected to continue to move northward into the late afternoon hours and could initiate storms in the lower deserts of Pinal, Maricopa, and Yuma Counties (including the Phoenix vicinity).

Overall, the synoptics/upper level dynamics are on the favorable side for convection across Southern Arizona mainly from Tucson westward today.

Thermodynamics

There are some significant differences between the 12z and 15z WRF-HRRR with the 15z being the more aggressive solution, and this is where things get tricky. As of 10:30AM MST, the sfc temp at the Tucson airport is 95 degF with a dew point of 61 degF. The 12z KTUS WRF-HRRR sounding forecasts a dew point of 53 degF and the 15z forecasts a dew point of 59 degF at 10:30AM MST as shown below.

12z (left) and 15z (right) KTUS WRF-HRRR soundings valid for 10:30AM MST this morning.

With that being said, the 15z seems to be more representative of the current thermodynamic environment. However, will still show the 12z forecast as it will represent a worst case scenario whereas the 15z will represent a best case scenario for storm coverage and intensity today.

CAMs agree that the greatest window of opportunity for storms in the Tucson vicinity is during the early and mid afternoon hours (between 1PM and 4PM MST). The 12z and 15z KTUS WRF-HRRR model soundings forecast the maximum in CAPE to occur around noon, but differ on the magnitude in which the 12z forecasts near 100 J/kg of MLCAPE and the 15z forecasts near 500 J/kg as shown below.

12z (left) and 15z (right) KTUS WRF-HRRR soundings valid for 12:30PM MST this afternoon.

 My opinion is the 15z seems more reasonable just considering it's more representative of the current conditions, so should have enough buoyancy in the Tucson vicinity for storm development and maintenance as the embedded shortwave moves through early this afternoon.

CAMs aren't overly excited with storm chances in the Phoenix metro this afternoon, but forecast thermos are near identical to the KTUS forecast soundings in which the 12z forecasts near 100 J/kg and the 15z forecasts near 500 J/kg of MLCAPE this afternoon via the KPHX forecast soundings as shown below.

12z (left) and 15z (right) KPHX WRF-HRRR soundings valid for 3:00PM MST this afternoon.

 

Also, both KTUS and KPHX model soundings forecast DCAPE in the 1200 to 1400 J/kg range, so there's an increased chance for strong outflow boundaries to induce additional lift in combination with the ascent induced by the embedded shortwave. However, storm development in Phoenix will be primarily dependent on the amount of lift that can be induced by the embedded shortwave as well as outflow boundary induced lift from convection to the south.

 
What to Expect/Impacts

Overall, expect increased storm activity from Tucson westward during the early and mid afternoon hours with increasing chances in Pinal, Yuma, and Maricopa Counties by late afternoon. As of 11AM, I'm seeing cumulus development over the higher terrain to the south and southeast of Tucson as my office window faces the south, so looks like convection is beginning to initiate in response to the approaching shortwave. 

Both the 12z and the 15z WRF-HRRR show the best chance for storms in the Tucson vicinity between 1 and 4PM MST as shown below. 

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 2PM MST this afternoon.

The differences in coverage and intensity in the simulated reflectivity between the 12z and 15z are directly correlated with the differences in the forecast buoyancy. Therefore, the coverage and intensity of storms from Tucson westward will be primarily dependent on the amount of CAPE that can develop prior to the maximum in differential CVA ahead of the passing shortwave. 

The same can be said farther north in Pinal, Yuma, and Maricopa Counties (including Phoenix) later this afternoon and into the early evening hours. CAMs differ on the coverage and intensity of storms in those areas which is correlated to the difference in forecast buoyancy by late this afternoon/early evening.

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

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 5PM MST this afternoon.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 7PM MST this evening.

Main threat from storms today is the risk of locally strong winds and blowing dust which the 12z HREF has a 70% probability of 4-hour max wind speeds in excess of 30 knots (where dbz > 20) in Pima, Pinal, and Santa Cruz counties this afternoon.

12z HREF 4-hour max wind speeds and probability of winds in excess of 30 knots where dbz > 20 valid for 5PM MST courtesy of the SPC.

Brisk steering flow should keep any excessive rainfall threat isolated today, but localized flooding is always a possibility especially near recent burn scars or in poor drainage areas.

Driving conditions could be hazardous along the I10 corridor between Tucson and Phoenix, the I19 corridor between Tucson and Nogales, and the I8 corridor between Casa Grande and Yuma this afternoon and evening. Main hazard is the risk of reduced visibility from blowing dust.