Temporary downturn in activity heading into the start of the weekend

 Overview:

This morning's sounding continues to indicate extremely high moisture values and a relatively saturated atmosphere. While some cloud cover was in place this morning from Sonoran convection, this cloud cover was not as extensive as the previous 2 days, and a strong clearing trend is evident. This should allow decent instability to develop and storm intitation a long the higher terrain is possible, particularly in Santa Cruz and Cochise counties. Anvils from convection may be advected northward into the greater Tucson area limiting or decreasing the instability as we head into the evening. Over the next 48 h, a trough situated along the West Coast will slowly move inland which will result in a southwestward shift in the center of the upper-level high to near or south of the AZ/Mexico border. The result will be a shift in the upper-level flow to westerly, bringing in somewhat drier (but still at climo or above) air as we head into Friday. Enough low-level moisture should remain in place to support the threat of convection at least through Thursday, although storm trajectories will become decidedly east/northeastward by Thursday. 

This Morning:

The 12Z sounding from Tucson this morning continues to indicate extremely high precipitable water values of 1.75"

Given the relatively saturated nature of the sounding above 550 hPa, downdraft CAPE values this morning are less than 1000 J/kg although mid-level drying and surface heating this afternoon should create enough DCAPE to make gusty winds an issue with storms that do develop. The sounding analysis also indicates a forecast CAPE value of 1300 J/kg which is also enough instability to support stronger updrafts. The wind profile suggests little chance of convective organization so most updrafts should be relatively short lived. 

The 8 AM mesoanalysis indicates that moisture remains widespread and the relatively weak

nature of the moisture transport vectors means that this picture should not change appreciably over at least the next few hours.

Surface heating should not be an issue this morning as the visible satellite image indicates a strong clearing trend throughout southeastern Arizona

Mixed Layer CAPE values over eastern Pima county where already over 500 J/kg according to the mesoanalysis as of 9 AM this morning although wide spread convective inhibition is also present....

but should be overcome by surface heating.

High Resolution CAM ensembles means indicate surface based CAPE values peaking around 1 PM MST with values exceeding 1000 J/kg from central Pima county eastward....

with highest values of 1500+ J/kg near the AZ/Mexico border.

The UA WRF HRRR intialization from 12Z shows intitiation of convection in Coshise and Santa Cruz counties by 1-2 PM MST

and is consistent with the 15Z intialization. In both cases, the UA WRF does not bring the activity northward into the greater Tucson area. My suspicion is that this is at least in part due to anvil cloud debris moving into the area in the early evening as suggested by the reflectivities valid at 4:30 PM MST

From a SkewT perspective, instability at Tucson appears to peak around 1:30 PM MST

Interestingly, the wind profile from the surface to about 600 hPa is weakly backing, suggesting that there is some weak cold air advection occurring which also may be why activity n fails to spread into Tucson.

The general scenario of the activity being confined to Santa Cruz and Coshise counties is well supported by the ensemble probabilities,

Tomorrow:

By mid day tomorrow, the high-pressure system is expected to become situated near the AZ/Mexico border as west-southwesterly flow becomes predominate over Arizona.

While this is generally an unfavorable setup for monsoon activity and precipitable water values are expected to decrease to about 1.3-1.4" 

it should be noted that these moisture levels are still near or above the 90th percentile for this time of year. And with surface dewpoints remaing in the 55-60 F range

mean CAPE values in the HRRR ensemble still approach or exceed 1000 J/kg, particularly in the higher terrain in Sant Cruz.

Consequently, the UAWRF does indicate convection again firing up between 2-4 PM in this region

although given the upper-level flow, most of this activity will propagate into Cochise county. For Tucson to be impacted tomorrow, convection would have to initiate near Kitt Peak or the Tucson Mountains, which is currently a low-probability scenario.