September 13th: Monsoon Farewell Event Day 3

 Synopsis

Very active night/early morning in Central Arizona with rounds of numerous showers and strong thunderstorms from Phoenix eastward and this activity is still ongoing this morning. Another round of showers and strong thunderstorms is expected this afternoon and evening across Southeastern Arizona (including Tucson). SPC has southeastern AZ in a marginal risk for severe storms once again with the primary threat being strong wind gusts and up to one inch hail. 

Current Conditions 

Visible imagery overlaid with GLM flashes this morning displays a cluster of showers and strong thunderstorms in Central Arizona just east of the Phoenix metro.

Visible imagery this morning courtesy of the College of Dupage as of 8:20 AM MST.

Very active evening/night/early morning in the Phoenix vicinity with a few severe thunderstorms producing damaging winds. The strongest storms were in the southeastern Phoenix area with a couple severe storms producing wind gusts that knocked down power lines and one cell (just southwest of Chandler) produced golf ball sized hail via SPC storm reports. The foothills to the east of Phoenix received the most rainfall by far with QPE values between 1 and 2 inches with even values as high as 3 to 4 inches near the Theodore Roosevelt Reservoir! 

24-hour Quantitative Precipitation Estimates courtesy of the NWS.

This mornings activity is associated with upper level divergence in the left exit region of a weak upper level speed max over Central Arizona via SPC 300mb analysis.

SPC 300mb analysis as of 15:00z this morning.

An impressive thermodynamic and vertical wind profile measured from the 12z TUS sounding which has 1.62 in of pwat, a surface dew point of 64 degF, MUCAPE of 1765 J/kg, and 52 kts of shear between the LCL and EL. 

12z TUS sounding courtesy of the SPC.

Today's Forecast


Synoptics

The overall upper level pattern will be conducive for widespread thunderstorm activity across southeastern Arizona this afternoon and evening. The weak, upper level speed max will continue to move eastward throughout the day today and by this afternoon Southeastern AZ will be located in the right entrance region of this feature. This will provide Southeastern AZ with synoptic scale ascent induced by upper level divergence. 

12z GFS 250mb wind speed and streamlines for today at 5:00 PM MST courtesy of Tropical Tidbits.

Via the 12z TUS sounding, weak low level veering as well as 52 kts of speed shear between the LCL and EL could support organized clusters of storms this afternoon and evening as well.

Moisture and Instability

Plenty of moisture to work with today with the 12z WRF-HRRR maintaining pwats between 1.25 and 1.50 in across Southeastern and South-central AZ into this afternoon.

12z WRF-HRRR total precipitable water for this afternoon at 3:00 PM MST.

Will have more than sufficient instability today considering the 1765 J/kg of MUCAPE from the 12z TUS sounding. Interesting to note that the 12z KTUS WRF-HRRR model sounding at 12z wasn't even close to initializing with the valid thermodynamic profile with only about 500 J/kg of MUCAPE compared to the 1765 J/kg observed from the 12z TUS sounding. 

12z KTUS WRF-HRRR sounding from the 12z initialization time.

The 12z KTUS WRF-HRRR model sounding forecasting just below 1000 J/kg of MUCAPE by the middle of this afternoon. 

12z KTUS WRF-HRRR sounding for this afternoon at 2:00 PM MST.

Even though the 12z WRF-HRRR did not initialize with the proper thermodynamic conditions, there should be a slight bit of low level mixing into this afternoon which will likely lead to slightly lower CAPE values by this afternoon compared to this morning. Albeit, am expecting CAPE to be between 1000 and 1500 J/kg across Southeastern AZ this afternoon and evening which is plenty sufficient enough for significant widespread storm development.

What to Expect/Potential Impacts

Overall, today is looking to be a very busy day across southeastern Arizona due to synoptic scale lift combined with sufficient moisture, thermodynamics, and impressive speed shear. The 12z WRF-HRRR has been performing well the past couple of days so am sticking with its guidance for today's forecast. Expect storms to initiate early this afternoon over the Tucson mountains and move rapidly eastward throughout the afternoon.  

12z WRF-HRRR simulated maximum reflectivity for this afternoon at 2:00 PM MST.

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

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

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

12z WRF-HRRR simulated maximum reflectivity for this evening at 6:00 PM MST.

SPC has Southern Arizona in a marginal risk for severe storms today once again with the primary potential impacts being strong, gusty winds and isolated one inch sized hail. Current CAM guidance suggests that the greatest chances for severe storms will remain mainly confined to Southeastern AZ this afternoon and evening.

SPC Day 1 Severe Weather Outlook.

Considering the synoptic scale support, could see a few rounds of storms in the Tucson vicinity which could lead to some localized flooding concerns. WPC has only a marginal risk for excessive rainfall today, but considering the ample amounts of low and mid level moisture and synoptic scale ascent, isolated flash flooding is definitely possible today.

WPC Day 1 Excessive Rainfall Outlook.

Another afternoon and evening of hazardous driving conditions, especially along the I10 corridor between the AZ/NM border and Phoenix. 

This will most likely be the last significant event of the 2023 Monsoon Season so enjoy the show today!