20200726

Discussion
This will be an abbreviated version today.  Initially, I thought I was off the hook for a discussion, but as so frequently happens during the monsoon season, the situation has changed, at least for southeastern Arizona. 

Generally, the longwave pattern remains the same with an intense anti-cyclone dominating much of the country.  500mb temperatures continue to increase, and I see a -2C over Texas this morning.  It's bad news for the southwest and into Mexico too, at 500mb temperatures are now -5 to -3C.  A weak trough continues at 500mb over the west coast resulting in mainly southerly steering flow at a somewhat favorable 10-15 knots. 

One way I like to define the peak of monsoon season is the presence of an easterly component to the winds above 500mb.  As seen below,  the center of the 300mb anti-cyclone is way over in Texas, and Arizona is under mainly dry southwesterly flow.  This results in dry mid and upper levels, which is often unfavorable as dry air from above is entrained into the mixed layer.

The Phoenix upper air data is extremely dry above 500mb, as well as an inversion due to this warm/dry mid-upper flow.

Tucson is less pronounced as they are right on the edge of the southwesterlies.   Based solely on the sounding, one would think that there will be no activity in SE Arizona today.

Day 1
It turns out that there is one of the occasional cases of moist air advection from southern NM into Arizona, through the so-called "Chiricahua Gap."  This is the lower elevation areas in far SW NM and SE Arizona and clearly seen in the 850mb plot at noon today.  The low-level easterlies over SE Arizona and the drier northwesterly flow will set up a weak area of low-level convergence, which can enhance thunderstorm development.  850mb dew-point temperatures of 12-14C are favorable for storms in SE Arizona while Tucson is right on the boundary. 

The model runs vary on the timing of the wetter and more unstable air as the WRFGFS has it in place by mid-day.  The WRFRR does by late afternoon.  Hopefully, later runs will able to shed some more light on this.  In any case, both have quite a bit of CAPE present, as high as 1500 J/kg.

Scattered deep convection initiates over the high terrain of far eastern and southeastern Arizona by later in the afternoon.

By early evening, southern Arizona storms develop/move westward towards eastern Pima County. 

Tucson has around 3-400 J/kg of CAPE by this time.  Will this be enough to support storms over the lower elevations?  The warm/dry layer at 400mb could be a problem. Note that one of the CAPE calculations is off and too high. 

The WRFRR moves storms into eastern Pima County with only weak activity over Tucson.

It looks like a strong outflow boundary moves through Tucson this evening, and a storm or two might form.

Day 2
CAPE is quite high over SE Arizona, so there is a risk of afternoon thunderstorms.

There is a strong inversion above 500mb, so despite a fair amount of CAPE, deep convection may be limited.  Westerlies are dominating the vertical profile.

Only scattered and relatively weak storms are present during the afternoon and early evening.

Another issue is that moist air advection is replaced by weak, dry air advection over SE Arizona.  The lame 2020 monsoon continues.  In fact, it looks downright grim in the extended range.