August 11th, 2024: Afternoon Convection Focused Mainly South and East of Tucson Today

 Synopsis

An MCV moving northwestward in Southern Arizona will leave subsidence in its wake and likely suppress afternoon/early evening convection west of Tucson today. Afternoon/evening storm chances will mainly remain confined to the higher terrain south and east of Tucson, but cannot rule out an isolated cell or two in the Tucson vicinity later today.



Current Conditions


Visible satellite imagery and lightning detection this morning displays an area of mid level clouds with embedded showers and isolated thunderstorms across Santa Cruz, Pima, and Yuma Counties.


Visible satellite imagery overlaid with glm flashes as of 10AM MST this morning courtesy of College of Dupage.


These clouds and embedded showers/storms is associated with an impressive MCV moving northwestward along the International Border. 


The broad synoptic scale pattern is characterized by a diffused region of high pressure across the Desert Southwest, an upper level low over Northern Baja, and southeasterly flow aloft across Arizona.


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


The moisture distribution is nearly the same as it was yesterday at this time with a tight gradient in precipitable water (increasing from east to west) across Southern Arizona.


SPC mesoanalysis of precipitable water as of 10AM MST courtesy of the SPC.


Buoyancy remains relatively marginal with the 12z TUS sounding measuring a precipitable water of 1.59 inches, a surface dew point of 63 degF, and 716 J/kg of Mixed-layer CAPE.


12z TUS sounding this morning courtesy of the SPC.


The sounding is a bit contaminated from this morning's convection as indicated by the poor mid level lapse rates.




Today's Forecast


Synoptics/Dynamics
 

The aforementioned MCV and associated clouds/showers will continue to move northwest throughout the day today leaving subsidence in its wake. CAMs in good agreement that this subsidence will suppress convection west of Tucson this afternoon and evening. Am also expecting a delayed initiation over the higher terrain of Santa Cruz and Eastern Pima County due to stubborn cloud cover remaining in the vicinity throughout the morning hours. In general, the dynamics are unfavorable for storm development today in Southern Arizona.



Thermodynamics


The coverage and intensity of storms in Southeast Arizona will be primarily dependent on the amount of buoyancy that can develop by this afternoon. As of 10:45AM MST, stubborn mid level clouds with embedded isolated showers remain across Santa Cruz and extreme Eastern Pima County (including Tucson). This will likely be an issue for storm chances in those areas this afternoon as I don't think there will be enough surface heating to erode CIN. The 15z WRF-HRRR as well as recent hourly runs of the HRRR seem to be picking up on this so have opted to use the 15z for this discussion.

The 15z KTUS WRF-HRRR sounding forecasts around 250 J/kg of MLCAPE, near -50 J/kg of MLCIN, and messy mid level lapse rates this afternoon.


15z KTUS WRF-HRRR sounding valid for 4PM MST this afternoon.


There still should be enough instability for shower and storm chances over the higher terrain to the south and east of Tucson, but convection will likely remain outside the metro due to poor thermodynamics. DCAPE is forecast to be around 1200 to 1400 J/kg, so could see some gusty winds and strong(ish) outflow boundaries. 



What to Expect/Impacts


Overall, am expecting somewhat of a "down day" across Southern Arizona due to subsidence in the wake of the MCV and relatively poor buoyancy. CAMs forecast convection to remain focused mainly in Cochise County this afternoon due to better surface heating generating steeper low and mid level lapse rates and associated increased buoyancy.  


15z WRF-HRRR simulated reflectivity valid for 3PM MST this afternoon.


CAMs forecast outflow from convection in Cochise County to initiate storms in Santa Cruz and extreme southeast Pima County by early evening.


15z WRF-HRRR simulated reflectivity valid for 6PM MST this evening.


I can't rule out an isolated storm or two in the Tucson vicinity, but seems unlikely due to poor buoyancy. Any storm activity in Tucson would be dependent on outflow boundary trajectories and efficiency to produce lift. 


Besides Southern Arizona, expect an active afternoon across the higher terrain of Northern Arizona with storms likely in Prescott and Flagstaff.


Main impacts from storms today will be locally strong and gust winds, blowing dust, and brief heavy rainfall. 


Traveling conditions could be hazardous along the I10 corridor between Tucson and the AZ/NM Border with the main hazard being reduced visibility from blowing dust. 







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