August 9th, 2024: Decreased Storm Activity Today Due To Overworked Thermodynamics

 Synopsis

Arizona will remain between an upper level low over the northern Gulf of CA/Northern Baja and an anticyclone over western Texas with southeasterly/easterly flow aloft across the state today. The synoptic upper air pattern is relatively favorable, but overnight convection and drier air moving in from the east will limit instability today. The majority of storm activity will remain confined to the higher terrain, but some isolated shower/storm activity is possible in the Tucson vicinity this afternoon and evening. 


Models show an uptick in storm coverage and intensity tomorrow due to an embedded shortwave forecast to move northward into the region. 



Yesterday's Cliff Notes


The Tucson metro finally broke its August Thunderstorm Losing Streak (at least that's what I'm calling it) as storms rolled through the metro yesterday afternoon producing heavy rain and gusty winds. The Tucson airport reported a wind gust of 70 mph as well as 0.92 inches of precipitation, and there were a couple additional, isolated reports around the metro (via SPC storm reports) of downed power lines and minor damage to trees. 


The Phoenix area received some light precip accumulations with only a trace reported at Sky Harbor and between a few hundredths to around 0.25 inch in some areas to the northwest.


Current Conditions 


Visible satellite imagery this morning displays multiple MCVs and associated mid level clouds and isolated showers across Arizona.


Visible imagery overlaid with gem flashes as of 9:30AM MST courtesy of College of Dupage.


Southern Arizona remains in a relatively favorable synoptic upper air pattern with a weak anticyclone centered well to our east and an upper level low to our southwest allowing southeasterly flow aloft across the region.


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


Meanwhile, the thermodynamics have been overworked as indicated by the 12z TUS sounding this morning which shows minimal CAPE, quite a bit of CIN, and poor low and mid level lapse rates.


12z TUS sounding courtesy of the SPC.


In addition, moisture has decreased compared to yesterday with precipitable water down to 1.42 inches, ankle deep low level moisture, and a dry slot between just above the surface and 700mb.



Today's Forecast


As mentioned above, the general synoptic scale upper air pattern remains relatively favorable with southeasterly flow aloft and even a bit of cyclonic curvature in the 500mb wind field. 


The main issue today is the thermodynamics due to overnight storm activity and drier air moving in from the east. Low level water vapor imagery as of 10AM MST shows a region of dry air in New Mexico and Western Texas.


Low level water vapor as of 10AM MST courtesy of College of Dupage.


The 12z WRF-HRRR (and the 15z) forecast easterly low level winds to transport this drier air into Southeast Arizona this afternoon as shown below.


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


Both the 12z and 15z KTUS WRF-HRRR soundings reflect this drier air which forecast surface dew points in the low to mid 50s and minimal buoyancy (on the order of 100 to 200 J/kg of MLCAPE) this afternoon.


12z (left) and 15z (right) KTUS WRF-HRRR soundings valid at 2PM MST this afternoon.


There's also some mid level warming occurring near 500mb, so that will have an impact on cumulus growth and potential updraft strength. Should get enough clearing across Southeast Arizona for the surface to heat and allow low and mid level lapse rates to steepen below the region of warming near 500mb. As illustrated above, expecting inverted-V soundings by this afternoon with DCAPE reaching near 1200 J/kg.

The only real good thing from these model soundings is the vertical wind profile which shows 15 to 20 knots of vertical wind shear this afternoon associated with the marginally favorable synoptic scale upper air pattern.


Even with such poor thermodynamics, both the 12z and 15z WRF-HRRR forecast some weak activity in the Tucson vicinity late this afternoon and early evening.


12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 4PM MST this afternoon.


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



It appears that the relatively favorable synoptic upper air pattern combined with some marginal wind shear and just enough moisture and instability will promote a chance for scattered showers and isolated low-topped thunderstorms across Southeast Arizona (including Tucson) this afternoon and evening. Not expecting any cells that develop to be particularly strong due to minimal CAPE, but could see some locally gusty winds and blowing dust. The 12z HREF has a 70% probability of 4-hour max wind speeds in excess of 30 knots (where dbz >20) in Western Cochise, Eastern Pima (including Tucson), and Eastern Pinal Counties late this afternoon/early evening.


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



The 12z WRF-HRRR does show some activity in Phoenix this evening but the 15z disagrees. Any shower/storm development in Phoenix would be dependent on outflows from storm activity in Southeast Arizona and/or over the Mogollon Rim. 


In summary, expect a decrease in storm coverage and intensity across Arizona today, but still a chance for scattered showers and isolated thunderstorms possible this afternoon and evening mainly in Southeast Arizona and over the higher terrain.




 

Tomorrow's Forecast


The synoptic scale upper air pattern remains nearly unchanged tomorrow with the 500mb anticyclone centered to our east in Western Texas, the upper level low over Northern Baja, and southeasterly flow aloft across Arizona.


12z GFS 500mb cyclonic vorticity valid for tomorrow at 5AM MST courtesy of Tropical Tidbits.


CAMs forecast an uptick in storm intensity and coverage tomorrow afternoon, which could be associated with an embedded shortwave (likely from overnight activity in Sonora) forecast to move northward into Southeast Arizona. 


12z WRF-HRRR simulated reflectivity valid for 3PM MST tomorrow afternoon.


Taking this solution with a grain of salt as the extent of low level moisture as well as the timing of the embedded shortwave will have a significant impact on the coverage and intensity of convection tomorrow. Stay tuned!








 

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