August 3rd, 2024: Hot with Lower Desert Storm Coverage Dependent on Outflow Boundaries

Synopsis

The 500mb anticyclone will remain centered near the Four Corners with weak southeasterly flow aloft over Arizona.  A hot day is expected across the lower deserts today due to a strong subtropical ridge overhead. High temperatures are expected to reach between 105 and 110 degF in southeast Arizona and between 110 and 115 degF in South-central and Southwest Arizona this afternoon. 

Relatively unfavorable synoptic scale dynamics will confine storm activity mainly to the higher terrain and closer to the International Border where there is increased moisture this afternoon. CAM solutions forecast some storm activity along the I10 corridor between Tucson and Phoenix later today, so there is a chance in those areas this evening.



Current Conditions


At 9:45AM MST, mid level water vapor imagery and RAP 500mb analysis displayed the mid level anticyclone centered near the Four Corners, easterly/southeasterly flow aloft over Arizona, and a region of dry air moving into southeast Arizona this morning.


Mid level water vapor imagery overlaid with RAP 500mb analysis as of 9:45AM MST courtesy of College of Dupage.


Moisture has decreased considerably across the eastern half of Arizona over the past 24 hours as indicated by the 12z TUS sounding which measured 1.32 inches of precipitable water and a drying between the surface and 700mb.


12z TUS sounding courtesy of the SPC.


Instability has decreased considerably due to drying, and there is a lot of CIN (-417 J/kg of MLCIN). With that being said, it's going to take quite a few hours for surface heating to mix that out. 


Another sharp moisture gradient across Southern Arizona this morning with pwats increasing from east to west as indicated by current SPC mesoanalysis.


SPC mesoanalysis of precipitable water as of 10AM MST this morning.




Today's Forecast


Synoptics/Dynamics


Another day of convection dependent on thermodynamically driven circulations as the synoptic pattern remains marginally unfavorable. Even though the 500mb anticyclone remains near the Four Corners, the ridge is quite strong with 500mb heights above 594 dm today which is a recipe for subsidence. 


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


The 12z sounding didn't show any defined subsidence inversion, but I'm expecting one to develop by this afternoon due to drier air invading the mid levels from the east throughout the day today. 

In addition, we don't have any synoptic scale feature (i.e. inverted trough) to provide some upper level dynamics. So in general the synoptics are on the unfavorable side for storm development again today.


Thermodynamics


Overall, expecting a significant decrease in instability in SE AZ later today due to drier air moving in from the east. The 12z KTUS WRF-HRRR sounding forecasts between 100 and 300 J/kg of Mixed-layer CAPE this afternoon, surface dew points down to near 50 degF, and precipitable water between 1 and 1.25 inches.


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


With hot surface temps today, the boundary layer will become well-mixed and DCAPE will be between 1500 and 2000 J/kg across the lower deserts, so there is high potential for strong outflow winds in the vicinity of storms over the higher terrain. 


The best chance for showers and thunderstorms this afternoon in Southern Arizona will be over the Sky Islands as well as in Santa Cruz and Central Pima County due to better moisture and associated instability. 


12z WRF-HRRR total precipitable water valid for 3PM MST this afternoon.




What to Expect/Impacts


Both the 12z and 15z WRF-HRRR runs forecast scattered showers and thunderstorms over the Sky Islands of Southeast Arizona as well as in Santa Cruz County and Central Pima County. CAMs also in good agreement regarding storm activity over the higher terrain of Northern Arizona, especially near Flagstaff and Prescott this afternoon.


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


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



By later this evening, things get a bit more interesting and tricky from a forecasting perspective. Both the 12z and 15z WRF-HRRR as well as hourly runs of the HRRR this morning are showing storm activity along the I10 corridor between Tucson and Phoenix as well as along the I8 corridor between Casa Granda and Gila Bend after sunset. 


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


12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this evening at 9PM MST.


12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this evening at 11PM MST.


The reason for this is because CAMs forecast outflow from strong storms in Central Pima County to move into Eastern Pima and Pinal Counties allowing dew points to rise this evening as shown below.


12z WRF-HRRR 2-meter dew points valid for this evening at 10PM MST.


15z WRF-HRRR 2-meter dew points valid for this evening at 9PM MST.


My opinion: Scattered showers and thunderstorms are likely over the Sky Islands of SE AZ as well as in Santa Cruz County and in Central Pima counties this afternoon. With significant surface heating and hot temperatures, low and mid level lapse rates will become very steep by late afternoon, but buoyancy will be dependent on the amount of low level moisture. Outflow boundaries moving into the lower deserts between Tucson and Phoenix would need to be strong enough to breach nocturnal CIN later this evening, so if that can happen then there is a chance for storms in those areas. 


Considering DCAPE will be quite high, the main impacts from storms today will be locally strong wind gusts which is backed up by the 12z HREF which has a 70% probability of 4-hour max wind speeds in excess of 30 knots (where dbz > 20) in Santa Cruz and the Eastern half of Pima County this afternoon and evening.


12z HREF 4-hour max wind speeds and probabilities of 30 knots or greater (where dbz > 20) valid for this evening at 6PM MST courtesy of the SPC.


Considering the dry subcloud layer and high LCLs, excessive rainfall isn't likely, but isolated flash flooding is possible especially over the higher terrain and in Central Pima County where there is better moisture.


Traveling conditions could be extremely hazardous this afternoon and evening along the I19 corridor in Santa Cruz County, along the I10 corridor between Tucson and Phoenix, and along the I8 corridor between Casa Grande and Gila Bend. The main hazard would be reduced visibility due to blowing dust. 


Unfortunately, it will be a hot afternoon across the lower deserts with high temps between 105 and 110 degF in Southeast Arizona (including Tucson) and between 110 and 115 degF between Phoenix and Yuma. 


12z WRF-HRRR 2-meter temperatures valid for this afternoon at 3PM MST.



Tomorrow's Forecast


I'm hesitant to go into detail regarding tomorrow's forecast since in yesterday's discussion I wasn't expecting really any chances for storms in the lower deserts today. Overall, the synoptic pattern will remain near identical tomorrow, with storm coverage and timing dependent on the amount of moisture and associated instability as well as outflow boundary trajectories tomorrow afternoon and evening once again.







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