June 20th, 2024: First Round of Monsoonal Showers and Thunderstorms Today

 Synopsis

A combination of moisture from former Tropic Storm Alberto and outflow from yesterday's convection in New Mexico is moving through SE AZ this morning with dew points sky-rocketing into the upper 50s and low 60s across the region. Sufficient heating of the surface combined with the added low level moisture will lead to rapid destabilization across SE AZ into the middle of this afternoon. Surface convergence between easterly outflow winds and diurnally driven westerly winds will act as the primary lifting mechanism for convective initiation in the Tucson vicinity. In addition, a shortwave embedded in a broad trough over CA will provide synoptic scale support to maintain clusters of storms moving northeastward across the eastern half of the state. Brisk southerly/southwesterly steering flow should keep any excessive rainfall threat to a minimum with the primary hazards being gusty winds and lightning today. 



 Current Conditions

As of 9:30 AM MST, surface obs across SE AZ measuring dew points in the upper 50s to low 60s from Tucson eastward due to outflow from strong convection in New Mexico yesterday. Observations from the Tucson International Airport measure dew points increasing by 12 degrees in 15 minutes this morning!


Surface observations at Tucson International Airport (KTUS) this morning courtesy of the NWS.


The 12z TUS sounding displayed an unimpressive thermodynamic profile with no CAPE and pwat at 0.81 inches.


12z TUS Sounding courtesy of the SPC.


The 12z GFS 500mb analysis this morning displays a trough off the CA coast and an embedded shortwave over the Central Gulf of CA.


12z GFS 500mb analysis courtesy of Tropical Tidbits.





 Today's Forecast


Synoptics/Kinematic Forcing


Convective initiation will be primarily dependent on the location of a surface convergence boundary between the brisk easterly outflow winds and the weaker diurnally driven westerly winds. It's a bit tricky to pin point exactly where this convergence will occur this afternoon especially considering yesterday's 00z WRF-HRRR showed convection initiating just west of Tucson and today's 12z WRF-HRRR showing convection initiating just east of Tucson. 


00z (left) and 12z (right) WRF-HRRR simulated reflectivity for this afternoon at 3:30 PM MST.


The timing of convective initiation has been consistent in which different initializations agree on initiation around 3 PM MST this afternoon. Considering easterly winds have already move through the Tucson metro, I would expect convection to initiate over or just west of the Tucson vicinity by the middle of this afternoon. 


The upper air pattern today is quite atypical for a monsoon onset with a broad upper level trough off the CA coast and brisk southwesterly flow over AZ. The aforementioned embedded shortwave will lift northeast across the eastern half of the state this afternoon and evening providing synoptic scale support. 


12z GFS 500mb absolute vorticity for this afternoon at 5PM MST courtesy of Tropical Tidbits.


Once storms initiate along the convergence boundary, this shortwave will aid in not only maintaining storms, but potentially allowing some weak organization of storm clusters from Tucson northward. 



Thermodynamics


As of 18z, SPC mesoanalysis of Surface-Based CAPE indicating a strong gradient in instability across SE AZ.


SPC mesoanalysis of SBCAPE as of 18z this morning.


Latest CAM solutions (this morning) have been shifting convective initiation farther east compared to the 00z WRF-HRRR. Comparing the 18z SPC CAPE analysis to the 00z and 12z CAPE forecast at 18z shows that the 00z WRF-HRRR better represents thermodynamic conditions across the region compared to the 12z solution. Therefore, have opted to use the 00z solution for this discussion.



00z (left) and 12z (right) WRF-HRRR Max CAPE for this morning at 11:00 AM MST.


The 00z KTUS WRF-HRRR sounding forecasting around 500 J/kg of CAPE as well as around 1500 J/kg of DCAPE by the middle of this afternoon. 


00z KTUS WRF-HRRR sounding for this afternoon at 3:00 PM MST.

With that being said, sufficient instability will promote a chance for thunderstorms mainly from Tucson northeastward with some storms having the potential to produce very strong outflow boundaries.



What to Expect/Potential Impacts


Expect storms to initiate around 3PM this afternoon in the Tucson vicinity, and then will move rapidly northeastward into the higher terrain of Eastern AZ. As mentioned above storms will initiate along the convergence boundary between brisk easterly winds and weaker, drier westerly winds. Once storms initiate, the combination of outflow boundaries and a shortwave trough will maintain clusters of storms into the higher terrain northeast of Tucson. 


With brisk steering flow overhead, storms will move rapidly from southwest to northeast which should keep an excessive rainfall threat to a minimum. The primary impacts from storms will be lightning and gusty winds with some gusts potentially reaching severe criteria. The 12z HREF has 50 to 70% probabilities of wind speeds in excess of 30 knots with a few isolated areas having the potential of max wind speeds between 40 and 65 knots. 


12z HREF 4-hour max wind speeds and probability of wind speed greater than 30 kts courtesy of SPC.


Traveling conditions will be hazardous along I10 between the AZ/NM border and Tucson as well as between Tucson and Casa Grande. The Tucson evening commute could also be impacted with the main impact being reduced visibility due to blowing dust from outflow winds. 



 Tomorrow



The broad trough over the CA coast will lift northward tomorrow as an inverted trough associated with the remnant circulation of former TS Alberto moves westward toward AZ. As a result, mid level winds weaken significantly and begin to turn easterly across the SE AZ. 


12z GFS 500mb absolute vorticity for tomorrow at 5:00 PM MST courtesy of Tropical Tidbits.


This easterly wave will also transport a significant amount of deep moisture into AZ with forecast pwats around 1.50 inches by tomorrow afternoon.




12z WRF-HRRR precipitable water for tomorrow at 5:00 PM MST.


With added moisture and weaker steering flow, the threat for heavy rainfall and flash flooding increases, however a common issue with easterly waves (especially if associated with former tropical cyclones) is cloud cover. In fact, the 12z WRF-HRRR seems to be hinting at the potential for widespread mid and high clouds across SE AZ tomorrow.


12z WRF-HRRR simulated cloud top temperature for tomorrow at 1:00 PM MST.


Extensive cloud cover tomorrow could cause issues with sufficient insolation and therefore destabilization. With that being said, have opted to not go into detail with tomorrow's forecast and will need to keep an eye on model trends to see if cloud cover will be a significant issue for convective initiation tomorrow. 


















 









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