August 18th: Quick Update for Today

 Synopsis

Showers and Thunderstorms are still expected today, mainly from Tucson westward. A few storms could be severe with the main threat being strong, gusty outflow winds and blowing dust. 


 Current Conditions

Visible imagery this morning displayed numerous debris clouds from overnight/early morning convection across most of Arizona. 


Visible imagery this morning as of 8:50 AM MST courtesy of College of Dupage.

The 12z TUS sounding had plenty of moisture (pwat of 1.34 in) and some instability (CAPE of 523 J/kg). 


12z TUS sounding courtesy of the SPC.


Surface Observations as of 9:00 AM MST, measured dew points in the upper 50s to low 60s across southern AZ.


Surface Observations as of 9:00 AM MST courtesy of the NWS.


Today's Forecast

Debris clouds should clear out of southeastern Arizona over the next couple of hours with convective initiation likely sometime early this afternoon.


The 12z KTUS WRF-HRRR sounding is under-forecasting the surface dew point again. It forecast the surface dew point at 9:00 AM MST today to be 57 degF and currently (as of 9:00 AM MST) the dew point is at 64 degF. Other stations in the Tucson vicinity have surface dew points in the low 60s as well. With that in mind, am expecting higher CAPE and slightly lower DCAPE than the WRF is forecasting for this afternoon. The 12z forecasts CAPE of 306 J/kg and DCAPE of 1598 J/kg by the middle of the afternoon today.


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12z KTUS WRF-HRRR sounding for today at 3:00 PM MST.

 

My estimate would be CAPE around 600 to 800 J/kg and DCAPE around 1000 to 1400 J/kg by this afternoon. 


Moisture, instability, and DCAPE will be sufficient today for showers and thunderstorms, mainly Tucson westward with some storms capable of producing damaging winds. Below is the 12z WRF-HRRR forecast of the evolution of storms today. 


12z WRF-HRRR simulated maximum reflectivity at 3:00 PM MST.



12z WRF-HRRR simulated maximum reflectivity at 4:00 PM MST.


12z WRF-HRRR simulated maximum reflectivity at 5:00 PM MST.


12z WRF-HRRR simulated maximum reflectivity at 7:00 PM MST.


12z WRF-HRRR simulated maximum reflectivity at 9:00 PM MST.


12z WRF-HRRR simulated maximum reflectivity at 11:00 PM MST.


Phoenix will have similar thermodynamic conditions as Tucson this afternoon and there is a potential for vigorous outflow boundaries to move through the Phoenix metro, most likely later this evening. 


The main threat today will be strong, gusty outflow winds as well as blowing dust. Some of the strongest updrafts could produce nickel to quarter size hail but will remain isolated at best. Storm clusters are likely to move rapidly from southeast to northwest today and combined with a dry sub cloud layer will likely keep any threat for excessive rainfall at a minimum however localized flash flooding is always a possibility. 



See previous discussion for details regarding the potential impacts from Hurricane Hillary. 















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