August 20th, 2024: Active Day Expected Across Southeast Arizona

 Synopsis

An inverted trough approaching our region from the southeast will influence storm chances across Southern Arizona today and tomorrow. Favorable dynamics combined with sufficient buoyancy will promote a chance of storms across much of Southeast Arizona (including Tucson) this afternoon and evening and potentially overnight. Main impacts from storms today is gusty winds, blowing dust, and brief heavy rainfall.


The aforementioned inverted trough is forecast to lift northward across   Arizona tomorrow, but there is quite a bit of uncertainty regarding the timing and extent of the maximum in dynamic forcing.



Current Conditions


12z GFS 500mb analysis this morning displays a 594 dm anticyclone over New Mexico, a mid latitude trough off the coast of the Pacific Northwest, and an inverted trough in Northwestern Mexico.


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


This inverted trough will be the primary driver of storm chances across Southern Arizona over the next 48 hours.


Don't have a 12z TUS sounding this morning, so will use SPC mesoanalysis to show current moisture distributions. Precipitable water remains modest across the region with values between 1 and 1.5 inches across Pima County (increasing from east to west).


SPC mesoanalysis of precipitable water as of 9AM MST.



Today's Forecast


Synoptics/Dynamics


With the inverted trough approaching from the south/southeast, and the 500mb anticyclone centered in New Mexico, the mid level height gradient has tightened allowing brisk southeasterly mid level flow across Southern Arizona. By this afternoon, models forecast Southeast Arizona to be within a region of differential cyclonic vorticity advection ahead of the inverted trough axis.



12z WRF-HRRR 500mb temperatures valid for 2PM MST this afternoon.


It appears the best synoptic scale forcing will remain closer to the International Border this afternoon. 


By later this evening, models forecast the inverted trough axis to shift farther north and west toward Arizona.


12z WRF-HRRR 500mb temperatures valid for 8PM MST this evening.


This shifts the region of differential CVA and associated ascent farther north into Tucson, and CAMs forecast shower and storm activity in the vicinity. Also, with a favorable synoptic pattern in place tonight, could see some scattered nocturnal convection occur almost anywhere across the lower deserts of Southern Arizona.



Thermodynamics & Wind Shear


Buoyancy is expected to be much better today compared to yesterday due to weaker subsidence and marginally increased low level moisture. Mostly clear skies throughout the day will allow the surface to heat freely with low and mid level lapse rates becoming quite steep by mid to late afternoon across Southeast Arizona. The 12z and 15z KTUS WRF-HRRR soundings forecast between 500 and 700 J/kg of Mixed-layer CAPE by late this afternoon as shown below.


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


However, there still does appear to be some subsidence near 500mb, and around -50 J/kg of MLCIN which is the only unfavorable aspects of the thermodynamic profile in Tucson today.


DCAPE is forecast to be between 1500 and 1700 J/kg today, so this should allow strong outflow boundaries to develop across the region. 


Regarding wind shear, the forecast vertical wind profile is impressive, and probably the best I've seen thus far this season. Veering winds with 20 to 25 knots of effective shear between the surface and mid levels is really good especially in an easterly flow regime. The SPC RAP forecasts between 20 and 30 knots of effective shear across much of Southern Arizona this afternoon as shown below.


RAP effective shear forecast valid for 3PM MST this afternoon courtesy of the SPC.


Considering the background shear environment, I wouldn't be surprised to see some of the strongest updrafts achieve short-lived, modest rotation especially near the International Border.  



What To Expect/Impacts


It's looking like this afternoon, evening, and even overnight will be quite active across Southern Arizona due to increasing favorable dynamics and sufficient instability. 

Both the 12z and 15z WRF-HRRR forecast convection to initiate over the higher terrain in Santa Cruz, Eastern Pima, and Western Cochise Counties by early afternoon.


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


I expect convection to remain confined mainly to the higher terrain near the International Border through the majority of the afternoon hours due to the maximum in dynamic forcing remaining in that region during the period. 


By late afternoon/early evening, both the 12z and 15z WRF-HRRR as well as morning runs of the HRRR forecast showers and storms in the Tucson vicinity.


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


CAMs are forecasting some really strong outflow winds across Pima and Pinal Counties this evening as well.


12z WRF-HRRR 10-meter winds valid for 5PM MST this afternoon.


12z WRF-HRRR 10-meter winds valid for 6PM MST this evening.


12z WRF-HRRR 10-meter winds valid for 7PM MST this evening.


If this were to verify (which isn't unreasonable), this could lead to some widespread blowing dust concerns and hazardous traveling conditions along the I10 corridor between Tucson and Casa Grande as well as along the I8 corridor between Casa Grande and Gila Bend.


The 12z HREF isn't quite as bullish as the UA WRF, but still has 50 to 70% probabilities of 4-hour max wind speeds in excess of 30 knots (where dbz >20) in Santa Cruz and Pima Counties 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 for 7PM MST this evening courtesy of the SPC.


With that being said, main impact/hazard from thunderstorms today is strong wind gusts and associated blowing dust. 


Excessive rainfall appears unlikely at this time due to a relatively dry subcloud layer and brisk steering flow. However, isolated flash flooding/localized flooding is possible in regions with high soil moisture, near recent burn scars, or poor drainage areas. 


As mentioned above in the synoptics section, there may be some persistent overnight activity across Southern Arizona as the region of dynamic forcing shifts northward across the region. CAMs only show scattered, weak convection across the area, but definitely something to look out for tonight.



Tomorrow's Forecast


The aforementioned inverted trough is expected to rapidly lift northward across Arizona tomorrow, but there's too much uncertainty regarding the timing and extent of the maximum in dynamic forcing for me to have enough confidence to provide details at this time.  




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